Publishing type：Research paper (scientific journal)  

Since grain refinement such as nano twin and nano crystalline in a metallic material is possible to improve the mechanical property, surface modification to generate such nanostructure is demanded. This study aims to develop ultrahigh velocity impact testing using microparticle to vary metallic microstructure due to severe plastic deformation with high strain rate. Regarding an impact test, this study developed Laser Induced Particle Impact Test (LIPIT), in which laser ablation induces supersonic ejection of microparticle. We first proposed a measurement method of particle velocity using mechanoluminescence (ML). It is found that a particle collision with the ML emits light due to mechanical stimulus. The light is detected using the photomultiplier tube (PMT), so that timing of particle arrival is determined, and then the particle impact velocity is estimated. This measurement method was verified using a high-speed video camera. In addition, to investigate the light emission response of ML, mechanical stimulus using pulsed laser irradiation was applied to the ML. It is found that the peak intensity and decay time in the emission response were dependent on the irradiated laser energy. By converting the irradiated laser energy to impact force using elastic plate theory of transverse impact, the relationship between emission response and impact force was discussed. Finally, our developed LIPIT was applied to a pure copper, producing nano crystalline structure on the surface around the impact indentation crater. In contrast, no deformation twinning was observed in the quasi-static indentation crater. Therefore, the developed LIPIT may realize a new surface modification technique, because this induces severe plastic deformation with ultrahigh strain rates, resulting in microstructural changes.

DOI： 10.2472/jsms.73.343

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Publishing type：Research paper (scientific journal)  

This study develops a surface treatment based on the laser induced particle impact test (LIPIT) to improve the tribological performance of a material surface. Using pulsed laser ablation, LIPIT enables us to conduct hypervelocity microparticle projectile impacts on a material surface. With LIPIT, microparticles impact the material surface with a velocity of 400 m/s to 750 m/s depending on particle mass. This results in a deep impact crater whose cross-sectional profile becomes textured surfaces which may improve tribological performance. A finite element method simulated elastoplastic deformation by a single particle impact. It is shown that LIPIT produces a deep impact crater, compared with general shot peening (SP). Next, LIPIT was conducted repeatedly to create a textured surface for a wide area of SUS316 stainless steel. To compare this with other surface treatments, SP and SP with polishing were additionally conducted. It is revealed that the LIPIT crater has a higher aspect ratio (deep dimple structure), producing a unique LIPIT treated surface. To investigate tribological performance, this study conducted a ball-on-plate type wear test with reciprocating sliding. The wear test was also conducted for non-treatment, LIPIT, SP, and SP with polishing. It is shown that the LIPIT treated specimen is the low friction coefficient. In addition, the onset of seizure wear significantly delays owing to LIPIT. These results indicate that LIPIT mitigates frictional resistance since the dimple textured surface produced by LIPIT enhances lubricant retention.

DOI： 10.1016/j.mtcomm.2024.108213

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Authorship：Corresponding author  

DOI： 10.1016/j.actamat.2023.119467

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jpowsour.2023.233442

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.polymertesting.2023.108161

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Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/00218464.2023.2236028

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10965-023-03616-7

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Other Link： https://link.springer.com/article/10.1007/s10965-023-03616-7/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.mtcomm.2023.106237

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Authorship：Corresponding author  

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.polymertesting.2021.107124

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

In this study, we have proposed an indentation method to characterize a surface degraded layer due to UV light irradiation of polymers. Here, we assume that the degraded layer has a homogeneous and isotropic nature, and that the target material is a bilayer system (i.e., the degraded layer is formed on the substrate of the non-degraded base material). This method enables the quantitative measurement of mechanical properties (Young's modulus E and yield strength σY) and thickness of the degraded layer. In the first step, we conducted a shallow indentation in order to ignore the effect of substrate (non-degraded base material). This method deduces the mechanical property of degraded layer itself. The second step is deep indentation (i.e. indentation depth is deeper to capture substrate effect) for measuring the thickness of the degraded layer. To establish these methods, reverse analysis with the dimensionless function was newly proposed. We conducted a parametric study using the finite element method, in which the material cases cover various polymers, degradation properties, and layer thickness. Experimental verification was finally carried out, and the degraded layer due to UV light for polycarbonate (PC) was successfully characterized. Because this method is simple, convenient, and non-destructive, it might be useful for characterization of weathering degradation in other engineering polymers.

DOI： 10.1016/j.mtcomm.2020.101873

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Nature.  

DOI： 10.1007/s10921-020-00733-x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

DOI： 10.1039/D0SM00122H

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

Direct bonding of metal-resin plays a critical role in jointing of dissimilar materials and the adhesion strength is known to be dependent on strain rate (loading rate) due to the strain rate sensitivity of polymeric resin. This study evaluated adhesion strength and adhesion durability against repetitive loading for the interface between aluminum alloy and epoxy resin. For experiment, a pulsed YAG laser was used to generate strong elastic waves, resulting in interfacial fracture. This method is called Laser Shock Adhesion Test (LaSAT), which enables us to evaluate impact strength of interfacial fracture. This study prepared two types of specimens with different curing temperature (20°C and 100°C). It is found that the specimen with higher temperature curing shows larger adhesion strength. Subsequently, repetitive LaSAT experiments (cyclic loading tests) were conducted to evaluate adhesion durability. This reveals that adhesion strength showed cyclic fatigue characteristics and higher curing temperature improves fatigue strength. To elucidate this mechanism at molecular level, molecular dynamics (MD) simulation was conducted for the interfacial material with epoxy resin. This study created allatomistic model of Al2O3/epoxy resin interface, and repetitive tensile deformation was applied until delamination. It is found that the number of loading cycles to delamination was increased when the applied tensile stress was lower. It is also found that the 400K curing model showed larger adhesion strength than that of the 300K curing model. This trend is very similar with the results of LaSAT experiments. Our comprehensive study with LaSAT experiments and MD simulations evaluates adhesion strength of Al/epoxy interface and reveals its fracture mechanism.

DOI： 10.1115/IMECE2020-23777

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study investigated the crack growth behavior of porous polymer membranes, experimentally and numerically, in order to clarify the criterion of crack growth. A notch was introduced into the membrane as an initial crack (pre-crack) and a uniaxial loading was applied for the stable crack propagation. During the test, crack propagation behavior was observed using a CCD camera and Digital Image Correlation (DIC) method. The strain around the pre-crack tip at the onset of crack propagation was measured experimentally using DIC method. It was clarified that large-scale yielding developed before the onset of crack growth. The stable crack propagation was observed for all tensile tests. In parallel, a homogenized model that mimicked porous polymer membrane was created using finite element method (FEM) in order to investigate stress/strain distribution around the crack tip. This study employed the yield criterion proposed by Deshpande and Fleck. The computed strain distribution was compared with that of experiment, showing a good agreement each other. By using strain distribution from DIC method and FEM computation, J-integral value was calculated to investigate the criterion of crack growth. Regardless of the initial crack length, it is found that the J-integral value at the initiation of crack growth becomes constant for all tests. It is concluded that we successfully determined the criterion of crack propagation of porous polymer membrane. Therefore, our study using DIC experiment and FEM computation is useful to clarify the crack growth behavior of porous polymer membrane and determines the criterion of crack propagation.

DOI： 10.1115/IMECE2020-23456

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley Online Library  

DOI： 10.1002/smll.201903213

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Society for Metals  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP science  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Society for Metals  

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Language：English   Publishing type：Part of collection (book)   Publisher：Springer International Publishing  

Indentation is a convenient method to evaluate mechanical properties of materials as well as to simulate contact fracture with locally plastic deformation. Indentation experiment has been widely used for brittle solids, including ceramics and glass, for evaluating the fracture properties. With the aid of computational framework, simulation of crack propagation (for quasi-static and dynamic impact) is conducted to characterize 'brittleness' of materials. In this review, we explore the applicability of indentation method for hydrogen embrittlement cracking (HEC). HEC is an important issue in the development of hydrogenbased energy systems. Especially high-strength steels tend to suffer from HE cracking, which leads to a significant decrease in the mechanical properties of the steels, including the critical stress for crack initiation and resistance to crack propagation. For such materials integrity for HEC, convenient material testing is necessary. In this review, the first part describes new indentation methodology to evaluate threshold stress intensity factor KISCC, and the latter one is investigation into HEC morphology due to residual stress produced by indentation impression. Our findings will be useful for predicting KISCC for HE instead of conventional long-term test with fracture mechanics testing. It will also indicate the stress criterion of HE cracking from an indentation impression crater, when the formed crater (for instance due to shot peening or foreign object contact) is exposed to a hydrogen environment.

DOI： 10.1007/978-3-319-58729-5_23

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study systematically investigates the uniaxial and biaxial tensile deformation behavior of polytetrafluoroethylene (PTFE) membranes which are used for water purification. The present PTFE membrane has micron size pores with open cell structure, and the pore is anisotropic shape. During the uniaxial tensile test, the membranes undergo elastic deformation and plastic deformation with strain rate sensitivity (i.e. time-dependent deformation behavior). In addition, it strongly demonstrates anisotropic deformation, i.e. deformation behavior is different along longitudinal direction and transverse ones. To clarify the microscopic deformation mechanism, in-situ SEM observation is carried out during tensile loading. It is found that the anisotropic deformation behavior appears due to the inherent pore structure. Next, to investigate deformation behavior under biaxial loading condition, small punch test using a spherical indenter is carried out. The membrane undergoes elastic and plastic deformations. Finally, crack nucleates around the indenter contact and indenter completely penetrates through the membrane. It is also found that the membrane demonstrates anisotropic out-of-plane deformation behavior. To clarify these mechanisms, FEM computation is carried out, such that experimental results of force-displacement curve and out-of-plane deformation behavior are compared with the computational ones. The present FEM model enables the prediction of the membrane’s deformation behavior under biaxial loading.

DOI： 10.1115/IMECE2019-11099

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study investigates the deformation behavior of porous polymer materials with 3D random pore structure. The test sample has sub-micron-sized pores with an open cellular structure, which plays a critical role for water purification. The base polymer is PVDF (polyvinylidene difluoride). First, the surface and cross section of the sample are observed using FESEM to investigate the microstructure (cell size and geometry of the cell ligament, etc). Next, uni-axial tensile loading is carried out for polymeric membrane and it is found that the membranes underwent elasto-plastic deformation. In order to establish a numerical model, finite element metod (FEM) is employed. Using a software of Surface Evolver, 3D random pore structure is created in the representative volume element (RVE). The established computational model can predict both elastic deformation and plastic deformation. Furthermore, viscoplastic deformation behavior (i.e. time-dependent deformation and creep deformation) is investigated, experimentally and numerically. In particular, creep compliance is measured, and we investigate the effect of applied loading on creep deformation behavior. Using the time–temperature–stress superposition principle (TTSSP), we obtain a new master curve, which covers higher stress level, and successfully establish an FEM model of creep deformation of the test sample. The present model enables the prediction of the macroscopic and microscopic deformation behavior of the porous materials, by taking into account of 3D random pore structure.

DOI： 10.1115/IMECE2019-11143

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study aims to evaluate the adhesion strength and durability of hard coatings in a non-contact manner by using Laser Shock Adhesion Test (LaSAT). Both heat-treated and as-plated (non-heat-treated) Ni-P coatings deposited on carbon steel substrate by electroless plating method were prepared as specimens for this study. LaSAT uses strong elastic waves induced by pulsed laser irradiation in order to apply tensile stress to the coating/substrate interface so that interface delamination occurs. The out-of-plane displacement is also measured simultaneously by using a laser ultrasonic interferometer, and the interface delamination is detected by the change of the out-of-plane displacement waveform. Furthermore, computation of the elastic wave propagation using Finite Difference Time Domain (FDTD) is carried out to estimate the interfacial tensile stress. In addition, the adhesion durability is investigated by repetitive pulsed laser irradiation, and the relationship between the adhesion strength and the number of laser irradiation cycles until delamination was obtained. Finally, we discussed the effects of heat treatment on the adhesion strength and durability. This method of LaSAT is a quick measurement for the adhesion durability, and then it may shed some light on quality control of surface coating.

DOI： 10.1115/IMECE2019-11145

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study evaluates the interfacial fracture strength and toughness of an epoxy resin/aluminum alloy based on two different tests of impact loading and quasi-static loading. In the impact test, we carried out the Laser Shock Adhesion Test (LaSAT). This method uses a strong ultrasonic wave induced by pulsed laser irradiation to induce interfacial fracture. Due to the grease layer ablation caused by laser irradiation, strong elastic wave generates and propagates. The tensile stress acts on the Al/epoxy resin interface, inducing an interfacial delamination. This delamination is further extended by an additional laser irradiation in order to evaluate the interfacial fracture toughness. By simulating the experimental delamination progression in FEM (Finite Element Method), we evaluated the dynamic fracture toughness of the Al alloy/epoxy resin interface. On the other hand, a quasi-static test for the toughness evaluation was conducted using a uniaxial tensile test. Before the tensile test, we produced an initial crack (initial delamination) at the interface by using laser ablation. Subsequently, this sample having initial crack is loaded by uniaxial tension. It is found that the interfacial crack progresses, resulting in unstable interfacial fracture. Furthermore, we conducted FEM simulation, in order to estimate the stress distribution near the delamination. By deriving a stress intensity factor from the stress distribution, we evaluated the quasi-static fracture toughness. To compare the dynamic and quasi-static fracture toughness of Al alloy/epoxy resin interface, we clarified the loading rate dependency of interfacial fracture toughness.

DOI： 10.1115/IMECE2019-11081

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Society of Mechanical Engineers (ASME)  

This study develops a new experimental system to investigate surface buckling patterns that emerge due to the biaxial compression of elastomeric bilayer spherical shell. The sample used in this study is a thick PDMS (polydimethylsiloxane) substrate and a thin PDMS film. In order to induce biaxial strain to the surface of PDMS substrate, air pressured device like bulge test is first developed. This system enables a flat PDMS plate to isotropically expand and become a hemisphere due to air pressure. In other words, the surface of PDMS substrate is stretched and subjected to biaxial loading. Subsequently, a thin PDMS film is covered on the hemisphere substrate. By releasing the air pressure, the pre-stretch in the substrate is released gradually and the outer film is subjected to biaxial compression. During the biaxial compression, various film buckling patterns are observed. It is found that the surface buckling patterns are wrinkle formation that is associated with film delamination. This formation pattern is dependent on the surface strain distribution, i.e. radial strain and circumferential strain. In order to control the pattern of buckling-delamination, we systematically changed the material parameters, such as the film thickness, Young’s modulus, interfacial fracture toughness, etc. Our experiment is capable in reproducing various wrinkle patterns with film delamination on an elastomeric bilayer spherical shell.

DOI： 10.1115/IMECE2019-11141

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

Polycarbonate (PC) is an important engineering polymer that has been used as the structural material in a wide range of applications. Using coarse-grained (CG) molecular dynamics (MD), this work systematically investigates the elastoplastic deformation of PC under two tension scenarios. To drastically improve computational efficiency, a new CG force field is developed for PC based on full-atom MD simulations. Effectiveness of the force field is verified by the calculations of glass transition temperature, Young's modulus, strength, and distribution functions of bond stretching, bending and torsion. Using the newly developed force field, PC is found to undergo elastic deformation followed by the onset of yielding and plastic flow, in both uniaxial tension and constrained tension. The onset of yielding and plastic deformation are correlated with structural evolution using MD snapshots. Energy variations associated with bond stretching, bending and torsion are further analyzed to provide insights into their respective contributions to the deformation process. MD results are further compared with Argon's model, which does not only accurately predict the mechanical strength of PC, but also provides molecular-level insights into the deformation mechanisms.

DOI： 10.1016/j.commatsci.2018.01.001

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer New York LLC  

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.

DOI： 10.1007/s10921-017-0456-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study establishes a computational model for contact fractures in concrete mortar subjected to both monotonic and cyclic contact loadings. We perform experiments using monotonic and cyclic indentation tests with a ball indenter (i.e., cyclic spherical indentation). During the tests, the acoustic emission (AE) method is utilized to detect micro-scale damage in contact fractures. For monotonic contact loading, we determine the critical contact load when the radial crack initiates. It is found that radial cracks occur outside the contact zone (outside of the impression crater). Next, cyclic contact loading is performed, and it is found that the crack propagates with the increasing number of cycles. This fatigue crack growth can be captured by the AE method. In addition, a finite element method (FEM) is used to elucidate the stress distribution of crack initiation and crack propagation during both indentation loadings. FEM computation with a damage-based cohesive zone model (CZM) is carried out to simulate crack growth during cyclic indentation tests. The cohesive model follows a linear damage-dependent traction-separation relationship coupled with a damage evolution equation. It is found that our computation successfully simulates fatigue crack growth. By using the comprehensive experimental/computational framework, the nucleation process (mechanism) of such a complicated crack system is clarified. This methodology may be useful for examining crack propagation of other brittle materials under both monotonic and cyclic contact loadings.

DOI： 10.1016/j.engfailanal.2017.10.004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

This study investigated the fracture properties of wrinkled metallic films on a polydimethylsiloxane (PDMS) soft substrate. In particular, the crack density of the wrinkled film during tensile deformation was examined. In order to achieve better deformability of metallic thin films, a method to fabricate a wrinkled thin film on a PDMS soft substrate was first established. The copper (Cu) nano-film fabricated in this study possessed a wrinkled geometry, which plays a critical role in determining the extent of large elastic deformation. To create the wrinkled structure, wet-etching with a polymeric sacrificial layer was used. A sacrificial layer was first deposited onto a silicone rubber sheet. During the curing process of the layer, a compressive strain was applied such that the hardened surface layer buckled, and a wrinkled form was obtained. Subsequently, a PDMS solution was used to cover the layer in order to form a wrinkled PDMS substrate. Finally, the Cu film was deposited onto the wrinkled PDMS, such that the wrinkled Cu film on a soft PDMS substrate was fabricated. The use of uni-axial tensile tests resulted in film crack generation at the stress concentration zone in the wrinkled structure of the films. When the tensile loading was increased, the number of cracks increased. It was found that the increase in crack density was strongly related to the inhomogeneous nature of the wrinkled structure. Such a trend in crack density was investigated using FEM (finite element method) computations, such that this study established a simple mechanical model that may be used to predict the increase in crack density during tensile deformation. This model was verified through several experiments using various wrinkle patterns. The proposed mechanical model may be useful to predict the crack density of a wrinkled metallic film subject to tensile loading.

DOI： 10.1088/1361-6463/aa95cb

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

Low-density porous materials and foams have been widely used for a variety of applications, such as light structural components, impact energy absorption, thermal insulation and sound absorption. The macroscopic deformation of such materials is strongly dependent on their inherent micro-cellular structure. This study investigated the compressive anisotropic deformation behavior of low-density polymeric foam by using X-ray computed tomography (CT) and the finite element method (FEM) in order to understand both the microscopic and macroscopic deformation behavior. The foams used in this study have a closed cell structure, with pores that are elliptical in shape. Three different types of expansion ratios were employed. The porosities of these materials were 93.5, 95, and 96%. From the observations using the X-ray CT method, the averaged pore heights were 1 mm and the aspect ratios were 2, 2.5, and 2.25, respectively. The foam demonstrated anisotropic deformation, dependent on the uni-axial compression direction. It was found that the deformation rigidity in the longitudinal direction was larger than that in the transverse direction. By using the X-ray CT method in situ, the microscopic deformation behavior when subjected to compressive loading was observed. Deformation and collapse of pores was observed for both directions during the loading. In conjunction with this, FEM computations were carried out to elucidate how such pore geometry undergoes elastoplastic deformation and leads to macroscopic deformation behavior. The FEM-created three-dimensional spatial structures were based on elongated rhombic dodecahedrons. It is revealed that the FEM computation shows relatively good agreement with the experimental results. Thus, our experimental and computational models may be useful for microstructural design using anisotropic cellular materials. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.polymertesting.2017.09.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASME: The American Society of Mechanical Engineers  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASME: The American Society of Mechanical Engineers  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

We investigated the compressive deformation behavior of hollow-strut cellular materials. The present cellular structure consists of pentagon and/or hexagonal shaped cellular network, where the individual strut cross-section is hollow triangular prism. The porosity of entire cellular material network is 96.5%. Uni-axial compressive test was applied, and both macroscopic (network-level) and microscopic (strut-level) deformation behaviors were investigated. The macroscopic nominal stress-strain curve showed a linear relationship during elastic deformation, and then a stress plateau region was observed, followed by the gradual increase in plastic flow stress. Next, by using X-ray micro-CT technique, the strut geometry was quantitatively identified, and based on which finite element method (FEM) was carried out to elucidate the relationship between the strut geometry and the microscopic/macroscopic elastoplastic deformation. A three dimensional spatial structure unit model was established to mimic the present open-cell structure, where the employed strut material properties were obtained from micro-indentation experiments. The FEM computational result agrees reasonably with experimental one of the macroscopic Young's modulus and yield stress. It also suggests that the stress concentration occurs in the minimum cross-section of strut, and then plastic deformation starts at this local point. Such a local yield phenomenon becomes a trigger of buckling for strut, leading to macroscopic plastic deformation characteristics. Furthermore, strain rate effect of the present cellular material was investigated numerically. The obtained result showed reasonable deformation behavior, but this may be improved in the future. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.matdes.2016.05.048

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The tensile deformation behavior of polyvinylidenedifluoride (PVDF) symmetrical microfiltration hollow-fiber (HF) membranes was studied. The membranes had submicron pores with a three-dimensional open-cell structure. The surface and cross section of the porous membranes were observed by FESEM (field emission scanning electron microscope) to investigate the microstructure of the cell, namely, its size and ligament geometry. During uniaxial tensile tests, the membranes underwent elastic deformation and plastic deformation. Large deformation induced pore growth along the tensile direction, resulting in an increase in water permeability. In order to establish a mechanical model for tensile deformation, the finite element method (FEM) was employed. In this model, the Kelvin polyhedron (truncated octahedron structure) was used to mimic a three-dimensional open-cell structure. A one-unit cell based on this structure was created, and a periodical boundary condition was employed for the FEM computation. The FEM model could reproduce the overall elastoplastic deformation behavior of the porous membrane and provide useful insight into the fabrication of porous membranes and reliable operation of water purification. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.memsci.2015.09.048

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

This study aims to evaluate delamination characteristics of surface coating by using strong ultrasonic wave induced by pulsed laser irradiation. Strong tensile stress wave is induced by pulse laser irradiation, and delamination of the coating layer/substrate interface (or spallation) is produced by confined silicone grease breakdown (i.e. ablation). Upon various levels of energy of irradiation laser, the coating delamination and displacement are measured in-situ. Parallel computation of elastic wave propagation using FDTD (finite difference time domain) yields the wave propagation in the specimen, and estimates the interfacial strength with stress wave distribution. The delamination area is visualized by laser ultrasonic wave scanning technique after the laser spallation test. The interaction between displacement waveform and delamination is explored. The present technique may shed some light on various coating depositions for assessing the coating adhesion quality. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.surfcoat.2015.12.026

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The present study proposes a new indentation method to predict the viscoplastic properties of polymeric materials utilizing three different indentation experiments. Numerical experiments with finite element method (FEM) are first carried out to simulate response of materials with various elastic and viscoplastic properties. The strain rate is featured by a loading rate of indentation, and the hardening rate can be captured through variations of the indenter shape. Next, a parametric FEM study is conducted to build the relationship between indentation load-depth curves and material parameters. Dimensionless analysis is employed to represent the relationship with simple formulae, through which, a reverse algorithm is proposed to extract the viscoplastic properties. Finally, both numerical and experimental investigations are performed to verify the proposed method. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2015.08.033

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Hard brittle films and coatings are often employed as a protective coating for metallic ductile substrates. In use, such coatings are generally subjected to cyclic/repeated contact loading and sliding over long periods of time. This study investigated the monotonic and cyclic contact fracture mechanism of hard coatings on ductile substrates (an electroplated Ni-P coating on a stainless steel substrate, SUS304) in order to evaluate their mechanical durability. In the experiment, both monotonic and cyclic indentation tests using a ball indenter with large contact force were performed. The fracture nucleation process was identified using the acoustic emission method. For monotonic contact loading (single indentation), coating cracks are produced by the excessive plastic deformation of the substrate, itself caused by contact loading, which makes the bending curvature of a coating a critical moment. Subsequently, cyclic contact loading (cyclic indentation) was applied to the coating in order to investigate the cycle number of film cracking. It was found that the critical contact force for coating fracture decreases, compared with that of monotonic loading. This critical force is dependent on the number of loading cycles. This may be due to the fact that cyclic contact loading encourages large plastic deformation of the SUS304 substrate owing to cyclic plasticity. Therefore, the cyclic plastic deformation behavior of the substrate was investigated using cyclic microindentation tests and the finite element method. In the computation, the Chaboche model was employed to compute the cyclic plastic deformation of the substrate, since it simulates cyclic plasticity. We clarified the cyclic contact fracture mechanism of electroplated Ni-P coating on an SUS304 substrate. Based on this, we finally predicted the coating lifetime (i.e., mechanical durability) under cyclic contact loading. Therefore, the present study is useful for obtaining information about film/coating fracture properties under both monotonic and cyclic contact loadings. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfailanal.2015.07.039

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASME  

This study investigated the hydrogen embrittlement (HE) cracking behavior produced by local contact loading of high-strength steel. When a spherical impression was applied to a hydrogen-absorbed high-strength steel, HE induces contact fracture, where radial cracks are initiated and propagated from the indentation impression. The length of the radial crack was found to be dependent on the hydrogen content in the steel as well as the applied contact force. A combined experimental/computational investigation was conducted in order to clarify the mechanism of hydrogen-induced contact fracture. In the computation, crack propagation was simulated using a cohesive zone model (CZM) in finite element method (FEM), in order to elucidate stress criterion of the present HE crack. It was found that the normal tensile stress was developed around impression, and it initiated and propagated the HE crack. It was also revealed that the hydrogen content enhanced contact fracture damage, especially the resistance of crack propagation (i.e., threshold stress intensity factor, Kth). The findings may be useful for countermeasure of contact fracture coupled with hydrogen in high-strength steel. Such phenomenon is potentially experienced in various contact components in hydrogen environment.

DOI： 10.1115/1.4029530

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

This study proposed a method to evaluate the residual stress and plastic strain of an austenitic stainless steel using a microindentation test. The austenitic stainless steel SUS316L obeys the Ludwick's work hardening law and is subjected to in-plane equi-biaxial residual stress. A numerical experiment with the finite element method (FEM) was carried out to simulate an indentation test for SUS316L having various plastic strains (pre-strains) and residual stresses. It was found that the indentation force increased with increasing pre-strain as well as with compressive residual stress. Next, a parametric FEM study by changing both residual stress sigma(res) and pre-strain epsilon(pre) was conducted to deduce the relationship between the indentation curve and the parameters epsilon(pre) and sigma(res) (which were employed for the FEM study). This relationship can be expressed by a dimensionless function with simple formulae. Thus, the present method can estimate both epsilon(pre) and sigma(res), when a single indentation test is applied to SUS316L.

DOI： 10.1007/s11665-014-1280-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

This study investigated the mechanism of hydrogen embrittlement (HE) cracking, produced by residual stress that develops around an indentation impression crater. A spherical indenter was used to indent a high strength steel, forming locally compressive plastic deformation (i.e., an impression crater). Subsequently, hydrogen was charged into the specimen with the impression. It is found that HE cracking occurs around the impression, and propagates radially. The radial crack morphology was changed by varying the applied indentation force and time of hydrogen charging. In fact, two types of cracks were observed, short cracks (less than 50 mu m in length) and long ones. The impression due to a small indentation force produced only short cracks, while a large force yielded both short and long cracks. In order to clarify the mechanism of crack morphology formation, the finite element method (FEM) was used to compute the residual stress field surrounding the impression. It was found that circumferential stress with a tensile component was developed around the impression, leading to initiation and propagation of HE cracks. For the smaller indentation force, the potential area for crack initiation is at the rim of the impression (on the material surface). However, a larger force induced crack initiation additionally beneath the surface leading to longer crack formation. Crack propagation was simulated using a cohesive zone model (CZM) in FEM in order to elucidate the crack morphology showing both short and long cracks. Multiple crack propagation was simulated using two CZM elements in order to investigate the interaction between two cracks. It was found that the crack length becomes shorter when the distance between the two cracks is closer (the crack angle is small). This is due to the fact that the stress as the driving force decreases owing to neighboring crack generation. On the contrary, when the distance between the two cracks is large (the cracks are away from each other), the lengths of the two cracks are identical, since there is no interaction between them. This study also investigated the relationship between crack growth resistance (i.e., threshold stress intensity factor, K-th) and simulated crack length. From the length of the long crack, the HE crack growth resistance, K-th, can be estimated. It was found to be strongly dependent on hydrogen content, while there is little influence of the indentation force. This study thus established the computational framework for the prediction of HE damage (crack morphology) surrounding the indentation impression for various applied indentation forces and hydrogen content. The findings will be useful for predicting HE cracking from an indentation impression crater, when the crater formed, for instance, due to shot peening or foreign object contact, is exposed to a hydrogen environment. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2014.11.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Mg17Al12 is one of intermetallic compounds in die-cast magnesium alloy (AZ91D) and will affect the macroscopic mechanical properties of AZ91D. The mechanical properties of Mg17Al12 phase is extracted by an instrumented indentation. Indentation tests on both eutectic beta-Mg17Al12 phase and alpha-Mg matrix demonstrate that the yield stress and flow stress of Mg17Al12 phase are much larger than that of Mg matrix. In particular, its yield stress can be twice higher than that of the Mg matrix, and is more than 300 MPa. Besides, the capabilities of plastic deformation of both phases are enhanced at high temperatures. The mechanical properties of both phases as a function of temperature are obtained, and their temperature effect on the macroscopic deformation behavior is further discussed. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2014.07.106

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：材料試験技術協会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The fracture mechanism of porous silicon carbide (SiC) was investigated by using the acoustic emission (AE) technique. Four point bending tests were conducted at different loading speeds, and integrated with AE monitoring. The results revealed that the quasi-static fatigue behavior had a critical role on the degradation of fracture strength, and fracture was caused by the microcracks of binder. For the AEs detected during the bending tests, source wave analysis of AE was carried out in order to quantitatively estimate the volume of microcrack. It was found that the crack volume was slightly smaller than that of binder fracture observed by a scanning electron microscope (SEM), thus suggested that the small crack propagates in the binder and produces AEs. It was also found that the longer the testing time, the smaller the rnicrocrack occurred at the lower stress level. Therefore, the fracture mechanism of the porous SiC was found to be controlled by particle binder fracture, which is time dependent. In addition, AE is testified as a useful technique to monitor the small crack of binder, which plays a very important role in material strength and filtering function of porous SiC. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2014.02.079

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IWA PUBLISHING  

The tensile deformation behavior of polytetrafluoroethylene (PTFE) hollow fiber membranes is studied. PTFE membranes at present have sub-micron pores with an open cell structure, which plays a critical role in water purification. One of the main challenges in water purification is that the pore structure becomes covered with biofouling, leading to blocked pores. To maintain the capacity for water purification, physical cleaning along with mechanical deformation is usually conducted. Thus, it is crucial to understand the mechanical properties, in particular the deformation behavior, of the membrane fibers. Using uniaxial tension experiments, we established a fundamental discrete model to describe the deformation behavior of a porous structure using a finite element method. The present model enables the prediction of the macroscopic deformation behavior of the membrane, by taking into account the changes of pore structure. The insight may be useful for porous membrane fabrication and provide insights for the reliable operation of water purification.

DOI： 10.2166/wst.2014.364

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

The contact fracture property and mechanism of electroplated Ni-P coating on stainless steel substrate were investigated using ball indentation testing, through a comprehensive experimental and numerical approach. First, the elastoplastic properties of both coating and substrate were evaluated using micro-indentation tests. Next, ball indentation test with large contact force was performed, such that the brittle coating on ductile substrate suffers from cracks, including ring crack (propagates circumferentially) and radial cracks (propagates radially), owing to the coating bending effect. The fracture nucleation process was investigated using the acoustic emission technique (AET). In addition, finite element method (FEM) with cohesive zone model (CZM) was carried out to compute stress field and simulate crack initiation around the impression during the test. By using the comprehensive experimental/computational framework, the nucleation process (mechanism) of such a complicate crack system was clarified. The present technique and fracture mechanism may be applicable to the analysis of structural integrity of other brittle coatings. © 2012 Elsevier B.V.

DOI： 10.1016/j.msea.2012.11.054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer  

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Language：English   Publishing type：Research paper (scientific journal)  

Various wrinkle patterns can be formed due to the buckling of a stiff thin film on a compliant substrate. However, most wrinkled films previously reported were fixed on a large deformable substrate and thereby the potential deformability of the film was mechanically constrained by the substrate. In this study, we developed a technique for forming various wrinkled structures on the surface of a sacrificial resin layer. Since the sacrificial layer can be subsequently removed with a solvent, freestanding wrinkled films are created using the sacrificial layer. We found that a wrinkled structure is formed on the surface of the layer by applying a compressive strain to the resin layer at the appropriate moment during the hardening process. The wrinkle pattern depends on the curing time and the timing of the straining in two in-plane orthogonal directions. In addition to conventional stripe and labyrinth patterns by simple uniaxial and equi-biaxial strains, respectively, it was found that independent biaxial strains induce interesting structures, such as an orthogonally ordered wrinkle pattern and a nonsymmetrical buckling structure, in which the stripe array produced by the first straining remains and many finer wrinkles appear in each stripe by the second straining in the orthogonal direction. We conducted tensile experiments for 300-nm-thick freestanding Cu films having these wrinkled structures. The wrinkled nano-films have a variety of mechanical properties: the stripe structure has extremely high deformability (more than 10% strain) and reversibility, the labyrinth structure shows planar isotropic deformation, and the nonsymmetrical buckling structure has an anisotropic modulus and strength. Finite element analysis on the wrinkle structures revealed that the local stress concentration dominates the fracture limits. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4807579

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

An evaluation method for out-of-plane anisotropic plastic properties is proposed using single spherical indentation. For a plastically anisotropic material, whose properties (i.e. yield stresses) are different among orthogonal directions (e.g. in-plane vs. out-of-plane direction), the proposed method is shown effective for distinguishing the anisotropic properties. Upon spherical impression, the indentation curves are dependent on plastic anisotropy (i.e. the yield stress ratio in different directions). By bridging the indentation response with impression geometry through extensive finite element analyses, dimensionless functions are established in order to deduce anisotropic plastic properties. The effectiveness of the method was investigated by numerical experiments. The validity and future issue were discussed. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2013.05.020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Indentation method was employed to evaluate the local mechanical properties of welded stainless steel at a high temperature of 320°C. The welding process often changes the mechanical properties (in particular the plastic properties), and make the material property inhomogeneous. An indentation method was proposed to effectively evaluate the stress-strain relationship (approximated by the Ludwick-type hardening law) in the welded SUS316L at 320°C. Functional relationship between the indention response and plastic property was established using finite element method (FEM). The dimensional function was deduced based on sufficiently-deep indentation, so that it can directly estimate plastic properties up to large uni-axial strain (about 20%). Spherical indentation tests applied to welded SUS316L may enable the evaluation of the distribution of plastic properties, such as the yield stress, plastic strain and tensile strength. The properties around the welded area were estimated to be higher than the base material of SUS316L, owing to the local plastic deformation from welding-induced thermal expansion and construction. Parallel test was conducted to validate the model. The proposed indentation technique can quantitatively evaluate the local mechanical properties at high working temperature, and supply useful information on inhomogeneous property distribution in materials. © 2013 Elsevier Ltd.

DOI： 10.1016/j.matdes.2013.06.015

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Chinese Society of Theoretical and Applied Mechanics  

The contact fracture property and mechanism of electroplated Ni-P coating on stainless steel substrate were investigated using ball indentation testing, through a comprehensive experimental and numerical approach. First, the elastoplastic properties of both coating and substrate were evaluated using micro indentation tests. Next, ball indentation test with large contact force was performed, such that the brittle coating on ductile substrate suffers from cracks, including ring crack (propagates circumferentially) and radial cracks (propagates radially), owing to the coating bending effect. The fracture nucleation process was investigated using the acoustic emission technique (AET). In addition, finite element method (FEM) with cohesive zone model (CZM) was carried out to compute stress field and simulate crack initiation around the impression during the test. By using the comprehensive experimental/ computational framework, the nucleation process (mechanism) of such a complicate crack system was clarified. The present technique and fracture mechanism may be applicable to the analysis of structural integrity of other brittle coatings. Copyright © (2013) by International Conference on Fracture.

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We developed an experimental technique for evaluating fatigue crack propagation properties of freestanding nano-films and conducted tensile and fatigue crack propagation experiments for about 500-nm-thick freestanding copper (Cu) films. We employed a sacrificial etching method for fabricating freestanding metallic nano-film specimens. A piezoelectric-actuator with long stroke and a load cell for small load were used for applying cyclic loading to nano-films. We developed original jigs for handling and aligning the nano-film specimens. Focused ion beam (FIB) was employed to introduce a single side-edge-notch in the nano-film specimens for fatigue crack propagation experiments. The results of tensile experiments revealed that the nano-films have resistance to plastic deformation comparable to cold-rolled Cu bulk, but have lower ductility. The results of fatigue crack propagation experiments revealed that a fatigue crack stably propagates in the nano-film by a uniaxial cyclic loading with constant maximum stress before unstable fracture. The fatigue crack propagation rate da/dN of the Cu nano-films is higher than that of Cu bulk counterpart in higher stress intensity factor range,ΔK. Fatigue crack started to propagate from the notch tip below the threshold value of Cu bulk with coarse grains, and the Cu nano-films show similar fatigue crack propagation properties to Cu bulk with ultrafine grains in lower ΔK. Morphology of the fracture surface transited from microstructure-sensitive rough surface to microstructure-insensitive ductile surface or chisel point fracture as fatigue crack propagated. This suggests that the dominant fracture mechanism changes from the accumulation of cyclic deformation to the monotonic tensile-dominant fracture. © 2012 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaia.78.808

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

We developed an experimental technique for evaluating fatigue crack propagation properties of freestanding nano-films and conducted tensile and fatigue crack propagation experiments for about 500-nm-thick freestanding copper (Cu) films. We employed a sacrificial etching method for fabricating freestanding metallic nano-film specimens. A piezoelectric-actuator with long stroke and a load cell for small load were used for applying cyclic loading to nano-films. We developed original jigs for handling and aligning the nano-film specimens. Focused ion beam (FIB) was employed to introduce a single side-edge-notch in the nano-film specimens for fatigue crack propagation experiments. The results of tensile experiments revealed that the nano-films have resistance to plastic deformation comparable to cold-rolled Cu bulk, but have lower ductility. The results of fatigue crack propagation experiments revealed that a fatigue crack stably propagates in the nano-film by a uniaxial cyclic loading with constant maximum stress before unstable fracture. The fatigue crack propagation rate da/dN of the Cu nano-films is higher than that of Cu bulk counterpart in higher stress intensity factor range, ΔK. Fatigue crack started to propagate from the notch tip below the threshold value of Cu bulk with coarse grains, and the Cu nano-films show similar fatigue crack propagation properties to Cu bulk with ultrafine grains in lower ΔK. Morphology of the fracture surface transited from microstructure-sensitive rough surface to microstructure-insensitive ductile surface or chisel point fracture as fatigue crack propagated. This suggests that the dominant fracture mechanism changes from the accumulation of cyclic deformation to the monotonic tensile-dominant fracture.

DOI： 10.1299/kikaia.78.808

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

To clarify the effects of film thickness on the creep properties of nano-films we conducted tensile creep experiments on freestanding aluminum films with thickness values in the range similar to 100-800 nm at room temperature. The nano-films showed typical creep behavior comprising transient, steady-state, and accelerated creep stages. The steady-state creep exponents of the 100-800 nm thick specimens were 0.84-2.7 in the stress range 30-120 MPa, which are close to the value for diffusion creep (1). Creep deformation clearly shows a thickness effect: the steady-state creep rate increases as the thickness decreases from 800 to 400 nm, shows a peak in the range 400-200 nm, and then decreases in the 200-100 nm thickness range. The creep experiments under a small stress of 1 MPa show a negative strain rate, indicating the presence of a driving force to reduce the surface area due to surface tension. The explanation for the thickness effect is as follows. Since the ratio of surface and grain boundary area to volume increases with decreasing thickness, diffusion creep along these paths is enhanced, resulting in an increase in the creep rate. As the thickness decreases to 200-100 nm, however, the surface tension effect to reduce the surface area becomes dominant, decreasing the creep rate. In addition, the creep rate of the nano-films is about two or three orders of magnitude smaller than that of the bulk material dominated by the dislocation creep mechanism. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2012.04.036

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

The present study investigated hydrogen embrittlement (HE) cracking upon indentation of high-strength steel. When an indentation test was applied to high-strength steel that included absorbed hydrogen, radial cracks initiated and propagated at the corners of the indentation impression, whereas the as-received steel with no hydrogen absorption exhibited no cracking. In addition, the length of the hydrogen-induced indentation cracks was found to be dependent on the hydrogen content in the steel. A combined experimental and computational investigation was conducted for a mechanistic study of Vickers indentation cracking caused by hydrogen embrittlement. Crack propagation was simulated based on a cohesive zone model in the finite element method and the stress intensity factor for the crack was evaluated. It was found that the simulated crack initiates at loading, and then propagates upon unloading. The value of the stress intensity factor at the indentation crack was in agreement with the threshold resistance to crack propagation for HE cracking (threshold stress intensity factor, K-ISCC). This suggests that the length of the cracks can be used to evaluate the susceptibility of the steel to HE cracking. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2011.10.049

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We propose a multiscale shape-assisted self-assembly that combines the growth of ordered nanocolumns by glancing angle deposition (GLAD) and the assembly of colloidal nanoparticles by utilizing capillary force. In this work, in the first self-assembly, a SiO2 nanocolumn array was grown by GLAD wherein the column size and interval were regularly controlled using pre-patterned nuclei on a substrate. In the second self-assembly, SiO2 spherical nanoparticles were assembled into the nanocolumn array using an isopropyl alcohol (IPA) or a water colloidal suspension, creating a column-particle nanocomposite. The location and quantity of the assembled nanoparticles could be controlled by choice of the suspension (IPA or water) and the particle concentration. Loading experiments with a diamond flat punch revealed that the initial stiffness and fracture strength of the nanocomposite are significantly increased compared with the nanocolumn array without nanoparticles. The assembled nanoparticles between the nanocolumns effectively constrained the deformation and fracture of the nanocolumns. The multiscale shape-assisted self-assembly thus enabled the synthesis of column-particle nanocomposites with excellent mechanical properties as well as large surface areas.

DOI： 10.1088/0022-3727/45/2/025302

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

In this study, a method to predict residual life of low-cycle fatigue in austenitic stainless steel (SUS316NG) was proposed based on indentation test. Low-cycle fatigue tests for SUS316NG were first conducted based on uniaxial tensile-compressive loading under the control of true strain range. Applied strain ranges were varied from about 3 to 12%. Their hysteresis loops of stress and strain were monitored during the fatigue tests. Plastic deformation range in hysteresis loop at each cycle could be roughly expressed by bi-linear hardening rule, whose plastic properties involve yield stress and work-hardening coefficient. The cyclic plastic properties were found to be dependent on the number of cycles and applied strain range, due to work-hardening. We experimentally investigated the empirical relationship between the plastic properties and number of cycles for each applied strain range. It is found that the relationship quantitatively predicts the applied strain range and number of cycles, when the plastic properties, or yield stress and work-hardening coefficient were known. Indentation tests were applied to the samples subjected to low cycle fatigue test, in order to quantitatively determine the plastic properties. The estimated properties were assigned to the proposed relationship, yielding the applied strain range and the cycle numbers. The proposed method was applied to the several stainless steel samples subjected to low cycle fatigue tests, suggesting that their residual lives could be reasonably predicted. Our method is thus useful for predicting the residual life of low-cycle fatigue in austenitic stainless steel.

DOI： 10.1299/kikaia.77.1859

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

In order to investigate the mechanics of interfacial fracture in structured nano-elements grown by glancing angle deposition (GLAD), fracture experiments were conducted on oblique Ti nanocolumns grown on a Si substrate using a micro-brick specimen. Two types of specimens, a Forward specimen (loading with the column tilt direction) and a Reverse specimen (loading against the column tilt), were prepared to clarify the effect of an asymmetric nanostructure on the interface strength. The specimens fractured at the interface or near the interface between the Ti nanocolumns and the Si substrate for both specimens. The critical force and displacement at fracture in the Forward specimens were about 2.0 times and 1.6 times as large as those in the Reverse specimens, respectively, showing clear anisotropy in the interface strength. The local stress distribution along the interface in the single nanocolumn at fracture was analyzed by finite element analysis. While the stress singularity in the Reverse specimen was greater than that in the Forward specimen, the normal stresses in a region 1-3 nm near the interface edge were almost identical regardless of the loading direction, suggesting that intensified stress in the nanoscale region dominated fracture. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfracmech.2011.08.005

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

This paper discusses the corrosion fatigue property of a high-strength cold-rolled eutectoid steel in deionized water to investigate the countermeasures against corrosion fatigue. The effects of temperature, stress cycle frequency and dissolved oxygen concentration on the corrosion fatigue strength in deionized water were investigated. In the case of smooth sample, lower stress cycle frequency, higher temperature and higher dissolved oxygen concentration decreased the fatigue strength in deionized water compared with that in dry air. Note that the corrosion fatigue strength decreased in the deionized water with a dissolved oxygen concentration of even 0.05ppm. The morphology of corrosion pit depended on the specimen orientation relating with a rolled pearlitic microstructure. Moreover, the addition of inhibitor increased the corrosion fatigue strength up to that in dry air because of the surface corrosion protection. It is more effective countermeasures for corrosion protection to add the inhibitor than to decrease the dissolved oxygen. However, even in ion-exchanged water with inhibitor, the fatigue strength of a sample having an artificial crevice was decreased compared with that of sample without the crevice. The corrosion pit formed inside of the crevice caused a decrease in fatigue strength. These results suggest that the corrosion protection effect becomes insufficient inside the crevice because the nitrous ion depleted due to the interruption of diffusion and convection.

DOI： 10.2472/jsms.60.1023

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Other Link： https://jlc.jst.go.jp/DN/JALC/00382654617?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

We conducted fracture toughness experiments on freestanding copper films with thicknesses ranging from about 800 to 100 nm deposited by electron beam evaporation to elucidate the size effect on fracture toughness in the nano- or submicron-scale. It was found that initially, the crack propagated stably under loading, and then the crack propagation rate rapidly increased, resulting in unstable fracture. The fracture toughness K(c) was estimated on the basis of the R-curve concept to be 7.81 +/- 1.22 MPa(1/2) for the 800-nm-thick film, 6.63 +/- 1.05 MPa m(1/2) for the 500-nm-thick film and 2.34 +/- 0.54 MPa(1/2) for the 100-nm-thick film. Thus, a clear size effect was observed. The fracture surface suggested that the crack underwent large plastic deformation in the thicker 800-nm and 500-nm films, whereas it propagated with highly localized plastic deformation in the thinner 100-nm film. This size effect in fracture toughness might be related to a transition in deformation and fracture morphology near the crack tip. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2011.07.071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

To fulfill the promises of MgB2 and MgB2-xCx in superconducting electrical transportation, the components made by these materials need to undergo plastic work process, for which the knowledge of their mechanical behavior (i.e. elastoplastic and strength properties) is critical. This study proposes the use of a convenient method based on indentation to evaluate both the elastoplastic and strength properties of MgB2 and MgB2-xCx. Through the reverse analysis using dual sharp indenters, we first extract the yield stress and work hardening exponent quantitatively. Next, the critical stress for crack nucleation (fracture strength) is estimated from a combination of experiment and numerical simulation of indentation stress field using the deduced elastoplastic properties. The critical fracture strain of MgB2 is evaluated to be reasonably close to that from conventional tensile test. We further show that in MgB2-xCx, the material stiffness, yield stress and fracture strength are degraded due to carbon additive, while the work hardening exponent is slightly higher. Therefore, the addition of C does not improve the mechanical properties of MgB2. The findings shed some useful light on the workability of MgB2, and may become a basis for understanding the feasibility of fabrication and usage of superconductor (where the mechanical and electrical properties are coupled). (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.matchemphys.2010.10.021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We propose a technique for producing freestanding films having a wrinkled structure. In an experiment, we discovered that a wrinkle pattern can be made on the surface of a sacrificial resin layer by applying compressive strain under the appropriate conditions. Using this phenomenon, we made a freestanding wrinkled film by depositing a film on the wrinkled resin surface and then removing the resin layer with an organic solvent. Uniaxial tensile tests for 300 nm thick freestanding copper wrinkled films revealed that the films have superior deformability: the fracture elongation is more than ten times larger than that of its straight film counterpart. (c) 2011 American Institute of Physics. [doi:10.1063/1.3549203]

DOI： 10.1063/1.3549203

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

In this study, an evaluation method for anisotropic plastic properties is proposed using dimensional analysis of single spherical indentation. The method considers a plastically anisotropic material, whose properties are different among orthogonal directions (e.g. longitudinal and transverse direction). In other words, the stress-strain curves along both directions can be simultaneously determined by the present method. When a spherical indenter is impressed against a plane cut along the longitudinal direction (i.e. indentation was conducted along the transverse direction), the impression geometry was found to change depending on the orthogonal axis. That is, the pile-up height in the transverse direction becomes higher than that in the longitudinal one. Using this characteristic and the indentation curve, dimensional functions were established in order to deduce anisotropic plastic properties. Extensive finite element analysis was first carried out to compute the indentation response (indentation curve and impression geometry). Subsequently, dimensionless functions connecting the anisotropic plastic properties to the indentation response were established. The effectiveness of the method was verified by numerical experiments. Finally, an indentation experiment was carried out on an SiC whisker-reinforced aluminum alloy to deduce the anisotropic plastic properties. The estimation results were found to be reliable. The present method is thus useful for determining the plastic properties of various anisotropic materials, such as cold-worked steel, composites and coatings.(C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2010.08.014

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Self-organized titanium oxide (TiO2) nanotube arrays were formed by an anodic oxidization technique. Nanoindentation experiments were conducted using three different indenters (i.e. flat punch, cone and triangular pyramid) to explore the mechanics governing the strength of the TiO2 nanotubes. The force-displacement responses were strongly dependent on the tip geometry and size of the nanotubes. A stress-based fracture model considering graded, porous and discrete structures of the nanotube arrays is proposed, and fracture stress was estimated based on the model. The experimental fracture stresses obtained using different indenters were almost the same, regardless of the indenter shape, which indicates the validity of the model. The evaluated compressive strength of the TiO2 ligament was approximately 3 GPa, and no clear size effects were observed for the 55-110 nm diameter nanotubes. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2010.05.029

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Nanoindentation experiments were conducted for 30 nm-thick Au films on two types of substrates, polyimide (compliant) and glass (stiff), to clarify the dominant mechanics of incipient plasticity from the interface. A high resolved shear stress T(r) could be effectively applied to the Au/polyimide interface due to the compliant substrate, and plastic deformation was initiated at the interface. The critical resolved shear stress T(crss) at the interface was determined to have a value of 0.4 similar to 0.5 GPa. On the other hand, in Au/glass, T(r) peaked within the Au film, and the maximum values were 1.1 similar to 2.2 GPa depending on the tip radius, whereas the values of T(r) at the Au/glass interface were almost identical at 0.5 similar to 0.7 GPa. Therefore, plastic deformation might be initiated from the Au/glass interface. The values of T(crss), for heterogeneous nucleation at the interfaces were smaller than that for homogeneous nucleation in the Au films. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2010.04.069

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IMPERIAL COLLEGE PRESS  

This study proposes a method of reverse analysis to estimate the anisotropic plastic properties of engineering steels by spherical indentation. The method takes into consideration materials that obey the work-hardening law and show in-plane anisotropic yield stress. Finite element analysis was first carried out to compute the indentation behavior of such materials, showing that a permanent impression exhibited an anisotropic shape which was strongly dependent on the orthotropic axis. Based on the anisotropy of the impression geometry, we developed a simple approach to determine the yield stress, work-hardening exponent and yield stress ratio. The approach consists of several functions related to the parameters of two impression geometries, produced by dual spherical indentations with different indentation forces. Since the present method uses only two impression geometries and does not necessitate indentation force - displacement curves (indentation curves), it is a particularly useful technique to evaluate "indistinguishable materials" which are special sets of materials with distinct plastic properties, yet yield almost identical indentation curves.

DOI： 10.1142/S1758825110000536

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

We developed a noble technique of micro mechanical tensile testing to evaluate local mechanical properties in cold-worked materials. The specimen used here was a cold drawn austenitic stainless steel (Type 316L) rod. A cold work generally varies the mechanical properties (especially plastic properties), and thus leads the property to nonhomogeneous distribution in the material. To evaluate local mechanical properties in such cold-worked materials, we developed the technique of micro tensile test to examine a stress-strain curve of a limited area (smaller than 1mm^2) of interest. The specimens with gage length of 5mm and cross section of 1.2×0.7mm rectangle were machined from the rod using wire-electrical discharge machining to achieve little formation of work hardened surface layer during the fabricaiton process. For the as-received material, the stress-strain curve obtained by the present technique exhibited excellent agreement with that from the general tensile test based on JIS Z 2241. When the technique was applied to the cold-worked material, it was found that near the surface of worked rod, yield stress, tensile strength and residual plastic strain were higher, while fracture strain was lower. In parallel with the experiment, finite element analysis was carried out to compute the distribution of residual plastic strain after cold drawn, indicating the good agreement with that obtained from the present technique. It is thus revealed that the developed technique can quantitatively evaluate the local mechanical properties, and thus supply us rich information on nonhomogeneously-distributed properties in plastic worked materials.

DOI： 10.1299/kikaia.76.493

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The effect of hydrogen on the interface fracture toughness of two nano-film/substrate structures, Ni/Si and Cu/Si, were evaluated using four-point bend specimens with and without hydrogen charging. Hydrogen typically decreases the fracture toughness of materials. However, we found in this study that the interfacial toughness between the Ni film and the Si substrate increased due to the presence of hydrogen, while that of Cu/Si decreased. Nanoindentation experiments for the Ni and Cu films revealed that local plasticity in the Ni and Cu films is promoted by the charged hydrogen. The critical stress intensity at the Ni/Si interface crack considering the plasticity of Ni, namely the true fracture toughness, is scarcely influenced by the existence of hydrogen. The apparent increase in fracture toughness of the Ni/Si interface is due to the large stress relaxation near the crack tip caused by softening due to the presence of hydrogen. Although the promotion of plastic deformation of Cu relaxes the stress intensity at the Cu/Si interface crack, the apparent interfacial toughness still decreases because of the significant decrease in the true toughness due to the presence of hydrogen. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfracmech.2009.11.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The present study investigates hydrogen embrittlement cracking (HEC) upon Vickers indentation in a high-strength steel. When an indentation test is applied to the high-strength steel, i.e., steel that has absorbed hydrogen, several cracks appear around the impression, whereas as-received steel with no hydrogen absorption does not produce any cracks. An experimental/computational framework is used to elucidate the mechanism of such indentation cracks caused by hydrogen embrittlement. We use the acoustic emission technique (AET) to clarify at which point during the test the crack initiates. In parallel with the experiment, finite element analysis (FEA) is carried out in order to compute the stress field around the impression. Based on the combined results, we discuss the mechanism of crack initiation and the critical stress required to nucleate the crack. The findings of the present paper may be useful for characterizing local contact fracture properties, which are often seriously deteriorated by hydrogen embrittlement. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.mechrescom.2010.01.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Hard thin films are often employed as protective coatings for metal substrates and their fatigue/fracture property (especially that under contact) needs to be sufficiently understood. In this study, we present a combined experimental/computational framework for exploring the fracture characteristics of hard thin films upon both monotonic and cyclic contacts. The techniques of acoustic emission and corrosion potential fluctuation are combined to monitor the mode and initiation of crack, and numerical simulations based on the finite element method provide further information on the criterion of film fracture. For a model system of a TiN film physical vapor deposited onto a stainless steel substrate, ring cracks are produced when the stress in the film exceeds Critical - such a critical moment arrives when the substrate undergoes excessive deformation, which makes the curvature of film bending critical just outside the contact zone. Since cyclic contact loading encourages large plastic deformation of substrate due to ratcheting plasticity, it is found that the critical contact force degrades, compared with monotonic loading. The present experimental/computational methodology is useful for obtaining the information of film fracture property under both monotonic and cyclic contact loads. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2009.07.199

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

The elastoplastic properties and fracture strength of thick diamond like carbon films (with thickness over 10 mu m) are studied An indentation-based framework is outlined where the dual sharp shallow micro indentation method is employed to measure the elastoplastic properties of the film and the substrate; next. integrated acoustic emission and corrosion potential fluctuation techniques are employed to characterize the ring cracks formed due to deep spherical indentation The fracture strength of the DLC film is obtained from parallel numerical simulations with the identified elastoplastic properties. It is found that the thick DLC film prepared by using the plasma-based ion implantation method has better fracture property than thin DLC films in previous studies. (C) 2009 Elsevier B.V. All rights reserved

DOI： 10.1016/j.diamond.2009.10.019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Indentation fatigue, where a cyclic load is applied on the sample via an indenter, emerges as an alternative approach for measuring the fatigue properties of materials. We have carried out indentation fatigue tests on a poly(vinyl chloride) (PVC) bulk material, as well as on TiN and NiP films/coatings deposited on SUS304 steel substrates, and demonstrate that a simple power-law relationship can be established between the indentation load amplitude and number of cycles to failure. Such a law is very similar to the conventional fatigue strength law obtained from uniaxial tests. The agreement between the fatigue stress exponents obtained by uniaxial and indentation fatigue tests suggests the potential applicability of the indentation fatigue technique for extracting the fatigue properties of materials.

DOI： 10.1080/09500831003662495

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In this study, a method to evaluate the in-plane anisotropic plastic properties of engineering steels by single spherical indentation is proposed. This method deals with materials that obey the work hardening law and have in-plane anisotropy of the yield stress in orthogonal directions. The method thus estimates the yield stress sigma(Y), the work hardening exponent n, and the ratio of the yield stresses m. Computations were first carried out by the finite element method (FEM) in order to quantitatively study the deformation surrounding the impression, when the spherical indenter was pressed against the anisotropic plastic materials. The permanent impression that remained after fully unloading the indenter was found to be non-uniform. The pile-up height in the direction of lower yield stress becomes higher than that in the direction of high yield stress. Such differences in height were found to become larger when m is higher. Based on this characteristic, extensive FEM calculations were carried out in order to propose the method to identify the anisotropic plastic properties for various materials whose mechanical properties are available for many engineering steels. The method involves several functions that relate the indentation response (indentation curve and geometry of impression) to the anisotropic plastic property of the material. Numerical experiments were conducted in order to verify the effectiveness of the proposed method. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2009.10.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The crystallographic orientation dependence of surface morphology of indentation creep on a nickel-based single crystal superalloy is investigated by using crystal plasticity slip theory with a three-dimensional (3D) finite element model. The numerical results show that the pile-up patterns developed around the indentation imprint exhibit four-, two-, and threefold symmetry on the surfaces of [001]-, [011]-, and [111]-oriented single crystals, respectively. The evolution of radial and hoop stresses around the crater provides important information for possible radial crack nucleation, whose critical locations depend on crystallographic orientations. These characteristics can be well explained in the viewpoint of crystallographic anisotropy. The findings may shed some light on understanding of the crystal structures and its time-dependent deformation mechanisms with the indentation method. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2009.03.004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Indentation is widely used to probe the fracture toughness of brittle materials such as ceramics or glass. In use, a hardened surface layer is inevitable in ceramics due to grinding and polishing; the surface layer also exists during sample preparation for indentation test. We study the mechanics of indentation induced lateral cracking in TiB(2) (titanium di-boride) ceramics, and show that the surface hardened layer has a pronounced effect on the initiation position of lateral crack, thereby affecting the wear resistance rate. The occurrence of lateral crack underneath the surface is detected in situ by an integrated acoustic emission (AE) and corrosion potential fluctuation (CPF) technique. By computing the detailed stress distribution, we discuss the mechanics of lateral crack formation in ceramics under the influence of surface hardening. It is found that when the maximum contact force is low, the lateral crack is confined within the surface layer, whereas when the maximum indentation force is high, the crack prefers to nucleate at the interface. The findings in this paper may be useful to optimize the surface polishing process and improve wear properties of ceramics. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2008.12.001

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Classification of corrosion-AEs from noises is attempted for the source location of acoustic emissions (AEs) from storage tank in service. As the AEs are monitored by resonant-type sensors with center frequencies lower than 50 kHz and amplified by 60dB, the system detects both the corrosion-AEs and noises. Noises are generally minimized by changing the threshold value of the monitoring system or interrupting the monitoring during strong winds and rain drops, but not be separated based on the quantitative waveform analyses. We first studied waveform characteristics of AE events monitored by sensors mounted on annular plate or side wall of the tank, and then separated corrosion-AEs from noises. Percentage of corrosion-AEs to the total events are found to be 30∼50% of the total events monitored by the sensors on the annular plate in windless day, but less than 10% at the monitoring on windy day. © 2008 The Society of Materials Science.

DOI： 10.2472/jsms.57.1101

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

We study the detailed mechanism of formation and propagation of spiral cracks upon spherical indentation on a thick diamond-like carbon (DLC) film deposited on a ductile steel substrate. Unlike the conventional ring cracks appeared just outside the impression, the non-axisymmetric spiral cracks are formed within the crater. Integrated acoustic emission (AE) and corrosion potential fluctuation (CPF) techniques are applied to monitor the crack process in situ. It is found that spiral cracking is possible only within a relatively narrow range of maximum indentation force, below which there is no fracture and above which the ring cracks are formed. Detailed stress analyses show that the spiral cracks are nucleated below the indenter when the equivalent stress is above a critical level, which then extends during unloading. The proposed theory agrees well with experimental observations and it is expected such mechanism can be extended to similar brittle film/ductile substrate systems. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2008.04.069

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：日本金属学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society Publishing  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

This paper discussed the fracture mechanism of diamond like carbon (DLC) film subjected to contact loading. The DLC films with over 10μm thick were deposited onto stainless steel (SUS304) by Radio Frequency (RF) plasma-based ion implantation method with different applied voltage of-20, -15 and -10kV. Rockwell indentation tests with simultaneous monitoring of acoustic emission (AE) and corrosion potential fluctuation (CPF) techniques were applied to the DLC films to clarify the mechanism of contact fracture. The DLC films were found to produce two types of crack morphology. One is general ring crack produced by tensile stress developed around contact area of indenter under loading process. The other indicated spiral crack morphology. This crack was found to be produced under unloading process detected by AE and CPF analyses, which could determine the indentation force, Fu, at the onset of spiral crack. The values of Fu are strongly dependent on the maximum indentation force and coating process. Finite element analysis suggested that the DLC deformed plasticaly prior to the tensile fracture under loading process. Due to this plastic strain, the film buckling occurred under unloading process and resulted in the delamination and the spiral crack. Rockwell indentation method with AE and CPF monitoring system could determine the onset of film fracture, which enabled us to discuss the detailed fracture mechanism. © 2008 The Society of Materials Science.

DOI： 10.2472/jsms.57.474

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本機械学会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY  

We proposed an improved method to determine the plastic properties of a bulk material with only one sharp indenter. This method uses the data of loading curvature in an indentation curve to obtain the solution of the plastic properties. When the method yields two solutions, the indentation unloading data is employed to select the unique one. To verify the effectiveness of the proposed method, the plastic properties of four engineering materials were estimated on an experimental basis.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

Fracture strength of CVD diamond films deposited by the hot-filament chemical vapor deposition method on sintered SiC substrate was estimated using the indentation method and FEM analysis. The authors determined the critical indentation force to cause ring crack in the diamond film correctly utilizing both of the acoustic emission (AE) and corrosion potential fluctuation (CPF) method during indentation of Rockwell indenter of natural single crystal diamond with radius of 400 p.m. The polarity distribution of the first arrival symmetric mode of the Lamb wave AEs was utilized for the classification of the Mode-I ring crack and the Mode-II interfacial exfoliation. The CPF measurement in the acidified solution was possible due to weak electrical conductivity of CVD diamond and SiC. The critical indentation force for the micro-meter size polycrystalline diamond (MCD) and nano-meter size diamond (NCD) films with different thicknesses were accurately determined by AE and/or CPF. Fracture strength of these films were estimated by FEM using the critical indentation forces. The fracture strength of the MCD was estimated as 6.4 GPa for 21 p.m thick MCD and 4.0 GPa for 70 pm thick MCD film. Fracture strengths of these films were supposed to be determined by the transngranular cleavage crack, while that (6.1 GPa) of the NCD film was determined by weak graphite phase along grain boundaries.

DOI： 10.1299/kikaia.73.57

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Other Link： https://jlc.jst.go.jp/DN/JALC/00289895083?from=CiNii

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

Evaluations of elasto-plastic properties and fracture strength were studied utilizing indentation technique. The elasto-plastic properties of nine monolithic metals were estimated using dual indentation method and compared to the properties under tensile loading test. This method could predict stress - strain curve and was found to determine ultimate tensile strength. For brittle coating, fracture strength of diamond -like carbon (DLC) film was estimated by hybrid indentation with acoustic emission (AE), corrosion potential fluctuation (CPF) and finite element method (FEM). AE and CPF data provided information on the timing of film cracking and its mechanism. Fracture strength of the films could be evaluated using Vickers and Rockwell indentation.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

Evaluations of elasto-plastic properties and fracture strength were studied utilizing indentation technique. The elasto-plastic properties of nine monolithic metals were estimated using dual indentation method and compared to the properties under tensile loading test. This method could predict stress - strain curve and was found to determine ultimate tensile strength. For brittle coating, fracture strength of diamond -like carbon (DLC) film was estimated by hybrid indentation with acoustic emission (AE), corrosion potential fluctuation (CPF) and finite element method (FEM). AE and CPF data provided information on the timing of film cracking and its mechanism. Fracture strength of the films could be evaluated using Vickers and Rockwell indentation.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acoustic Emission Working Group  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acoustic Emission Working Group  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Corrosion damage of floor plates of a naphtha storage tank was monitored by acoustic emission (AE). AEs from the fracture of rusts were monitored by AE sensors of 30 and 50 kHz resonant frequencies, mounted on the terrace of annular plates and the side wall. AE signals were separated from noise by the waveform inspection. The signal counts from the sensors on the terrace were larger than those from the side wall sensors, and submitted to the location estimation of corrosion zone. The ratio of AE signals to total AE events were as low as a few percent. Source locations of the Lamb wave AEs detected by the AE sensors on the terrace of annular plates were estimated by three methods and ranked into three levels depending on the determination accuracy of the arrival times. Four portions in the third quadrant of the tank bottom were located as corrosion zones. They appear to be spreading from the zone with relatively large reduction of wall thickness measured in 2003. This paper also discusses some future problems of condition monitoring of huge steel structures by AE.

DOI： 10.3323/jcorr.55.406

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：腐食防食協会  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

This paper discusses the mechanism of environmental assisted cracking (EAC) of an austenite/ferrite duplex stainless steel in a 35 mass % MgCl2 solution at 363 K by measuring both the acoustic emission (AE) and corrosion potential fluctuation (CPF We utilized a new fracture toughness testing device and measured the growth characteristics of macroscopic crack. The duplex stainless steel was found to suffer delayed fracture through the ferritic grains in this solution rather the active path corrosion (APC) type SCC in austenitic grains. We detected no clear CPF but strong AEs during the test AE sources were located at the zone 1-3 mm from of crack tip or at the zone with high hydrostatic pressure at which micro crack of delayed fracture took place. Source wave analysis of AE revealed the fast brittle micro fractures. Fractographic study supported the brittle fracture of ferritic phase by hydrogen and subsequent ductile fracture of the austenitic phase.

DOI： 10.3323/jcorr.55.364

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Science  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：腐食防食協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publisher：Society for Experimental Mechanics  

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Language：English   Publisher：Society for Experimental Mechanics  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：腐食防食協会  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Both the acoustic emission (AE) and corrosion potential fluctuation were monitored for chloride SCC of sensitized Type-304 steel plate under bi-axial stress state. Branched SCCs were produced from rectangular-shaped corrosion pits initiated by falling-off of surface grains and filled with chromium oxide in 30mass% MgCl 2 solution (363K). Both the AE and potential fluctuation were simultaneously detected at corrosion pit formation and SCC propagation. Two types of AE (Type-I and Type-II) were monitored. Type-I AEs with higher frequency components were detected during the pit growth and supposed to be produced by falling-off of surface grains due to intergranular attack, while a number of Type-II AEs (about 12 500 counts) with low frequency components were detected at SCC progression and supposed to be produced by cracking of the oxides. Though the AEs detected during SCC test are not the primarily signals from the SCC itself, secondary AE can be usefully utilized to monitor the SCC initiation and propagation as well as the corrosion potential fluctuation.

DOI： 10.2472/jsms.55.211

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TRANS TECH PUBLICATIONS LTD  

Both the acoustic emission (AE) and corrosion potential fluctuation were monitored for chloride stress corrosion cracking (SCC) of sensitized Type-304 stainless steel plate under bi-axial stress states. Branched SCCs were produced from rectangular-shaped corrosion pits initiated by falling-off of surface grains and filled with chromium oxide in 30 mass% MgCl2 solution (363K). Both the AE and potential fluctuation were simultaneously detected during pit formation and SCC growth. Two types of AE (Type-I and Type-II) were monitored. Type-I AEs with higher frequency components were detected during the pit growth and supposed to be produced by failing-off of surface grains due to intergranular attack, while a number of Type-II AEs (approximately 12,500 counts) with low frequency components were detected during SCC propagation and supposed to be produced by cracking of the chromium oxy-hydroxides. Though the AEs detected during SCC test are not always the primary AEs from the SCC itself, secondary AEs can be usefully utilized to monitor the SCC initiation and propagation as well as the corrosion potential fluctuation.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

This paper discusses the mechanism of chloride SCC of sensitized and non-sensitized Type-304 stainless steel based on the data obtained by simultaneous monitoring of AE and corrosion potential fluctuation (CPF). The steel produced transgranular SCC via corrosion pits filled with corrosion product (chromium oxy-hydroxides) in 35 mass % MgCl2 solution. Both the pitting corrosion and transgranular SCC did not produce primary AEs due to anodic dissolution, while they generated rapid drop (RD) type CPF. We, however, detected the secondary AEs from hydrogen gas evolution and fracture of corrosion products. Hydrogen gas was found to produce AE with single frequency component, while the fracture of corrosion products produced AEs with broad frequency components. The sensitized steel produced primary AEs due to the failing-off of grains as well as the secondary AEs. Microscopic progression of SCC was consistently interpreted by the timing and time-lag of AEs and CPFs.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TRANS TECH PUBLICATIONS LTD  

Both the acoustic emission (AE) and corrosion potential fluctuation were monitored for chloride stress corrosion cracking (SCC) of sensitized Type-304 stainless steel plate under bi-axial stress states. Branched SCCs were produced from rectangular-shaped corrosion pits initiated by falling-off of surface grains and filled with chromium oxide in 30 mass% MgCl2 solution (363K). Both the AE and potential fluctuation were simultaneously detected during pit formation and SCC growth. Two types of AE (Type-I and Type-II) were monitored. Type-I AEs with higher frequency components were detected during the pit growth and supposed to be produced by failing-off of surface grains due to intergranular attack, while a number of Type-II AEs (approximately 12,500 counts) with low frequency components were detected during SCC propagation and supposed to be produced by cracking of the chromium oxy-hydroxides. Though the AEs detected during SCC test are not always the primary AEs from the SCC itself, secondary AEs can be usefully utilized to monitor the SCC initiation and propagation as well as the corrosion potential fluctuation.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acoustic Emission Working Group  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acoustic Emission Working Group  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acoustic Emission Working Group  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Korean Society for Nondestructive Testing  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：腐食防食協会  

CiNii Books

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本実験力学会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Micro-Vickers indentation tests in conjunction with acoustic emission (AE) monitoring were performed for TIN films deposited on three kinds of substrates, soft and hard steel by physical vapor deposition (PVD) method. We aimed to evaluate the intrinsic strength of thin films. In this study, three types of substrate with different hardness were employed. One was a WC-Co cermet, denoted as hard substrate, while the others were mild steel (SS400) and metastable austenitic stainless steel (SUS304), denoted as soft substrate. Equidistance step-wise terrace, or rippled cracks were observed and detected AEs only for soft substrates materials. Detected AE waveforms were analyzed using the polarity pattern in order to classify the cracking behavior. Most AEs detected were produced by Mode-I cracking with opening vector in the direction parallel to the film surface. Internal stress in the films was analyzed by the finite element method (FEM) to study the influence of substrates on the fracture mechanism. It was revealed that bending stress was developed by indenter penetration and it induced rippled crack initiation for the soft substrate materials. It was also found that the rippled cracks initiated at a certain distance of spacing. Using the FEM and the value of cracking space (L c), we proposed the method to determine the bending strength of the thin films.

DOI： 10.2472/jsms.54.1030

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：腐食防食協会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料試験技術協会  

CiNii Books

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

Locations of localized corrosions of the steel floor plate of a cylindrical storage tank 32.93 m in diameter were estimated by AE analysis and correlated with the localized wall-reduction zone measured by ultrasonic test. We monitored Lamb wave AEs for one hour twice using eight 50 kHz AE sensors mounted on the terrace of the annular plate. Sources of 84 AE events were located from the first 1hr AE monitoring and those of 75 events from the second 1hr monitoring. Here, the source locations were estimated by two location schemes, i.e., virtual source scanning method (VSS) and conventional time of flight method (TOF). The VSS methods located 80% of AE events into a specified zone in the third quadrant of the floor plate which agrees quite well with the wall reduction zones measured by the ultrasonic method. AE sources located near the annular plate agree fairly well with the wall reductions of the annular plate. Some wall reduction zones of the annular plate could not be detected by AE due to the weak signals of some sensors.

DOI： 10.11396/jjsndi.54.318

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

CiNii Books

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Language：English   Publisher：Elsevier Science  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Corrosion Engineering  

We propose a new hybrid method for evaluating the fracture strength of the hard films deposited on soft substrate metal. The method utilizes acoustic emission (AE), corrosion potential fluctuation (CPF) data and Finite Element Method (FEM) analysis. Both the AE and CPF were simultaneously detected during the Vickers indentation test into the hard film (TIN) in a buffer solution (pH:4.0 phthalic acid) and used for the determination of both the number and distance of ripple cracks. AE system detected Lamb wave signals from the Mode-I ripple cracks due to the tensile stress induced by the bending of the soft substrate (SUS 304), Event counts of rapid drop (RD-) type CPF with a large shift rate and AE signals agreed well with the number of ripple cracks. Non-linear FEM analysis revealed the Mode-I crack generation in the film at the slight outside of the contact boundary of the indenter and can predict the fracture strength of the film.

DOI： 10.3323/jcorr1991.54.532

Scopus

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Corrosion Engineering  

Acoustic emission (AK) from external stress corrosion cracking (ESCC) on a butt-weld 4 inch SUS 304 steel pipe of 4 mm thickness was monitored as a function of internal hydrostatic pressure. AEs were detected by the resonant sensors with center frequencies of 50 kHz and 450 kHz and analyzed using the signal processing system developed. Among three types of AEs detected by the 50 kHz sensors. Lamb wave AEs with broad frequency band were located near the ESCCs and found to be generated during pressure rise and reducing. Radiation pattern analyses of zero-th order symmetric Lamb mode suggested AEs from surface friction, crack of the rust in ESCC and falling-off grains of IG-ESCC. AEs monitored by the sensors with 450 kHz center frequency were possibly produced by the falling-off of grains along IG-SCC. Though the primarily AEs from TG-SCC can hardly be detected by conventional AE system, we can detect secondary AEs associated with ESCC. This paper also discussed how to detect the secondary AEs from ESCC in process plants.

DOI： 10.3323/jcorr1991.54.329

Scopus

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料試験技術協会  

CiNii Books

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料試験技術協会  

CiNii Books

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Society for Experimental Mechanics  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Society for Experimental Mechanics  

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Language：English   Publisher：Elsevier Science  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Society for the Advancement of Material and Process Engineering  

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Language：English   Publisher：日本非破壊検査協会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

Static and cyclic fatigue strength in porous SiC and Cordierite ceramics were evaluated using quasi-static and cyclic tests, i. e. applied force, P, increasing and applied force range, ΔP, increasing tests, respectively. The evaluation method used Weibull distribution, in which the fracture parameters are stress at failure, σ_f, for quasi-static loading and maximum stress at failure, σ_<max, f>, for cyclic loading. The results revealed that the static fatigue behavior had an important role on the degradation of fracture strength and that cyclic component of loading do not influence on the fracture strength. Statistical characteristics of the fatigue strength were expressed by P-S-N curve estimated by the results of ΔP-increasing tests. During the fatigue experiments, crack nucleation was detected using acoustic emission (AE), and the fracture surfaces were observed by SEM. Based on these results, fracture mechanisms of the porous SiC and Cordierite ceramics were controlled by particle binders, and the binder fractures have a great variety of magnitude.

DOI： 10.2472/jsms.51.9Appendix_116

CiNii Books

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Other Link： https://projects.repo.nii.ac.jp/?action=repository_uri&item_id=243841

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Elsevier Science  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本材料学会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本非破壊検査協会  

CiNii Books

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本セラミックス協会  

In order to evaluate statistical characteristics of both static and cyclic fatigue strength in HIP and normal sintered silicon nitrides, namely toughened Si_3N_4 and SN1, load, P-increasing(quasi-static loading)test and load range, ΔP-increasing(cyclic loading)test were conducted in the room air. Acoustic emission(AE)method was attempted during the fatigue tests for the toughened Si_3N_4. The SN1 exhibited static fatigue behavior, while the static strength of the toughened Si_3N_4 was independent of time, i. e., no static fatigue. However, cyclic fatigue behavior appeared in the both materials. ΔP-increasing tests with different initial value of maxmum stress, σ_<max, i> enabled to evaluate cyclic loading condition under which fatigue damage nucleated. The result suggested that σ_<max, i> influenced on the maximum stress at failure, σ_<max, f> for statistically weak specimens in the both materials, and thus fatigue damage was nucleated at lower stress levels for these specimens. Although this behavior could not be confirmed by AE analysis, AE waveform simulation revealed that the crack volume nucleated under the cyclic loading were smaller than those under quasi-static loading. Based on the above results, observation of fracture surfaces and finite element analysis, the fracture mechanisms of static and cyclic fatigue were discussed in detail.

DOI： 10.2109/jcersj.108.1261_842

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00069265648?from=CiNii

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