Published papers

To improve the fatigue life of electronic packages, it is necessary to clarify thermal fatigue characteristics of underfill resin materials. In this study, a new setup for measuring thermal fatigue crack growth was developed, and the effects of thermal cycle conditions and silica filler contents on fatigue crack propagation were investigated. The results show that the likelihood of crack initiation increased with the filler content and the temperature of the thermal cycles. Comparing the results with those from mechanical fatigue tests, crack initiation and propagation occurred at lower ΔK and the crack growth rates were higher for thermal fatigue. Observations of the fracture surface revealed that the cracks propagated inside the underfill resin, which resulted in smaller crack growth resistance than that in mechanical fatigue. The addition of fillers introduced a crack propagation path at the interface between fillers and resins and enhanced the crack propagation resistance in thermal fatigue.

In this study, a simple strain based damage model is applied to simulate the ductile crack growth in thin 590 MPa class steel plates with notch and hole. The model depends only on the stress triaxiality and has two parameters. Determination of two parameters is straightforward by analyzing the tensile test results of flat plate specimen and plate specimen with sharp notches. The determined damage model is then applied to simulate the crack growth in other two types of test specimens containing plates with hole and notch, and a good agreement between the simulated and experimental results is obtained. It is found that the simulation results are primarily affected by the in-plane element size in the crack growth direction. On the other hand, the out-of-plane element size does not affect simulation results. It is found that the use of an in-plane element size of 0.1 mm or less is appropriate.

© 2018 The Japan Society of Mechanical Engineers. Severe head injuries-and even deaths-have been reported in the practice of judo, often resulting from inexperience in using the appropriate falling technique (ukemi). In this study, we developed a novel head protector that protects the head and suppresses the neck extension during impact with the mat. The head protector was fabricated for this purpose and consists of a head support structure and shock-absorbing materials. The effectiveness of the head protector was tested experimentally with an apparatus with head and neck components that were based on a Hybrid III dummy. Several configurations of the support structure characteristics and the shock-absorbing materials were evaluated. The proposed head protector was capable of reducing the head angular velocity variation range-which is an important evaluation index for the onset of acute subdural hematoma -by 29% using the devices alone, by 21% using shock-absorbing materials alone, and by 47% using a combination of both. These results show that the proposed structures are highly effective in suppressing extensions at the cervical-thoracic joint (C7-T1), whereas the shock-absorbing materials are highly effective in suppressing rotations at the atlanto-occipital joint (O-C1). It is concluded that wearing the proposed head protector-which combines both effects-can reduce the risk of severe head injuries, even those resulting from severe impacts such as those associated with the judo ukemi practice.

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Structural acrylic adhesives are of special interest because those adhesives are cured at room temperature and can be bonded to oily substrates. To use those adhesives widely for structural bonding, it is necessary to clarify the methodology for predicting strengths of bonding structures with those adhesives. Recently, cohesive zone models (CZMs) have been receiving intensive attentions for simulation of fracture strengths of adhesive joints, especially when bonded with ductile adhesives. The traction-separation laws under mode I and mode II loadings require to estimate fracture toughness of adhesively bonded joints. In this paper, the traction-separation laws of an acrylic adhesive in mode I and mode II were directly obtained from experiments using Arcan type adhesively bonded specimens. The traction-separation laws were determined by simultaneously recording the J-integral and the opening displacements in the directions normal and tangential to the adhesive layer, respectively.

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. In this study, the tensile shear and bending tests of adhesively bonded single lap joints with the acrylic adhesive was evaluated experimentally and numerically. In the previous paper, the traction-separation laws in mode 1 and mode 2 for an acrylic adhesive were directly obtained from the observation of failure process using Arcan type adhesively bonded specimens: simultaneous measurements of the J-integral and the opening displacements in the directions normal, δ n and tangential to the adhesive layer, δ s respectively. The experimental results were compared with numerical simulations conducted in ABAQUS including cohesive damage model. The cohesive laws obtained in the previous paper were simplified to trapezoidal shape from the experimentally obtained ones which were indicated in the previous paper. A good agreement was found between the experimental and numerical results. Then, to investigate the damage evolution in the adhesive layer for some lap joints, microscopic video observation was conducted near the end of the adhesive layer, and the video image have been compared with the contours of damage variable obtained by FEM corresponding to the video images. The observed damage evolution also agrees with the trend of damage variable.

© Published under licence by IOP Publishing Ltd. This paper presents a study on fracture mechanisms of polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) (50/50) blends with different ABS types under a drop weight impact test (DWIT) using a circular sheet specimen. Formation of secondary crack indicated by a stress-whitening layer on the mid-plane of scattered specimens and secondary surface of fracture perpendicular to primary fracture surface were captured under scanning electron microscope (SEM). Although the both blends finally failed in brittle modes, SEM observation showed that their secondary fracture mechanisms were completely different. Observation through the thickness of the etched PC/ABS specimen samples using SEM also clearly showed that PC and ABS were immiscible. The immiscibility between PC and ABS was indicated by presence of their layer structures through the thickness of the blends. It was revealed that layer of ABS structure was influenced by size of rubber particle and this latter parameter then affected microstructure and fracture mechanisms of the blends. Impact-induced fracture mechanisms of the blends due to such microstructures are discussed in this paper. It was also pointed out that the secondary cracking was likely caused by interface delamination between PC and ABS layers in the core due to transverse shear stress generated during the impact test.

2018 The Society of Materials Science, Japan Structural adhesives are used for the purpose of weight saving and rigidity improvement in recent automotive development. In this study, fracture behaviors of a cohesive zone under mixed-mode conditions were focused on, and an evaluation method for the parameters of J-integral using the crack tip opening displacement was proposed. The method is an application of the Dugdale model which is known as an idea to deal with localized plasticity region of a crack tip for adhesive joints. Validity of the method was verified using finite element analysis. Furthermore, the method was applied for adhesive joint on galvannealed steel and its fracture mechanism was investigated.

© 2019 The Society of Materials Science, Japan Structural adhesives have been used to reduce weight and increase rigidity in recent automotive developments. This study focuses on the fracture behaviors of the cohesive zone under mixed-mode conditions, and a method for evaluation of the J-integral parameters using the displacement of the crack tip opening is proposed. The method is an application of the Dugdale model, which is known as an ideal model to work with a localized plasticity ahead of the crack tip. The validity of the method is verified through finite element analysis. Furthermore, the method is applied to adhesive joints on galvannealed steel, and its fracture mechanism is investigated.

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Acute subdural hematoma (ASDH) sometimes occurred in judo because of the bridging veins’ failure by rotation of the brain in the skull. However, the relationship between intracranial behaviour and the motion of the body on occiput impact has not yet been clarified. In this study, we developed an intracranial mechanical model based on multibody dynamics and compared it with experimental results. The results show the importance of modelling bridging veins and cerebral spinal fluid to the relative displacement between brain and skull. The proposed model will contribute to accident analyses or the optimum design of supporting devices.

© 2020 by the Japan Society for Computational Engineering and Science. In the last decade, one of the gradient damage models called “phase-field fracture model” has been receiving attention. The model for brittle fractures has acquired great success, but that for ductile fractures is still under development. In this context, we propose a novel phase-field model with a variable regularization length parameter that is defined as an increasing function of the measure of the plastic strain. Since the regularization length parameter l representing the width of a diffuse crack is known to determine the damaged region around the crack, it has been modified so as to increase depending on the amount of plastic deformation. Thanks to this variable regularization length parameter l∗, the phase-field parameter d is calculated in consideration of the damage due to plastic deformation. The conventional elasto-plastic model with J2 theory in plasticity is employed and couple to our proposed phase-field model. Several numerical examples are conducted to demonstrate the capability of our proposed model and validate the introduction of l∗ in comparison with the experimental result.

© 2020 Hydrogen Energy Publications LLC Hydrogen production by water splitting is expected to be a significant power source in future because of its cleanness and environment-friendliness. In this study, an actuator driven by hydrogen produced by water splitting is studied and a hydrogen-induced stress model for a bimorph-type hydrogen actuator is developed. From the examination of the soft actuator having electrodes made of polyvinyl chloride with palladium electrode (Pd-PVC), the stress and bending moment induced by hydrogen absorption during water splitting are quantitatively evaluated. The results show that the thickness of the palladium electrode affects the mechanical performance of the Pd-PVC actuator, with the thinner palladium electrode showing a larger bending moment and tip displacement. A simple mechano-electro-chemical coupling equation is proposed to describe the relationship between bending moment, stress, and the electric charge used for water splitting. A palladium electrode soft actuator can work as a hydrogen generator/storage mechanism and as a mechanical actuator, and it can be useful for the power source of small systems or micro- or nano-devices.

© 2020 Elsevier Ltd The fracture toughness of an adhesively bonded carbon fiber reinforced plastic joint under mixed mode (I and II) conditions was determined by means of the Fernlund–Spelt type double cantilever beam (DCB) test, where a rubber-modified epoxy adhesive was used. The R curves were determined by calculating the energy release rates, GI and GII, based on the beam theory, along with a finite element analysis. The total energy release rate GT (= GI + GII) strongly depended on the I–II mixed mode. This means that it remarkably increased with increasing mode-II component; furthermore, it varied with crack growth from GTC (at the onset of cracking) to GTS (at the steady crack-propagation region). The difference between GTC and GTS increased with the increase in the mode II component. Such R-curve characteristics were discussed based on observation of the fracture surfaces with a scanning electron microscope and the corresponding crack propagation-path behavior.

© 2020 Trans Tech Publications Ltd, Switzerland. This study presents an experimental study on failure modes and resistances of polycarbonate (PC)/Acrylonitrile Butadiene Styrene (ABS) blends and their ABS constituents under a drop weight impact test (DWIT). Failure modes and impact resistances such as impact strength and impact toughness of such blends are generally influenced by molecular weight of the PC, rubber content and size of rubber particle in ABS system. A preliminary study on ABS materials using a DWIT showed that size of rubber particle not only determining their failure modes but also influencing their resistance characteristics. However, in a previous study performed using the similar DWIT on PC/ABS blends with a 10 wt% rubber content, it was revealed that size of rubber particle did not significantly influence their resistances. Their failure modes were even macroscopically very difficult to be distinguished. This study, hence, is aimed to further explore role of the size of rubber particle on failure mode and impact resistance characteristics of the PC/ABS blends and their ABS constituents with a higher rubber content. The impact test results have revealed that with a 20 wt% rubber content, size of rubber particle only influenced the resistances of the PC/ABS blends. It did not significantly contribute to affect failure mode of the PC/ABS blends. Whilst, it significantly influenced failure modes and resistances of the ABS. The DWIT results also re-confirmed that blending a brittle ABS into PC led to produce a tougher PC/ABS blend.

Hydrogen power sources are expected to be the main power source for a sustainable society in the future. Therefore, the development of mechanical parts or systems to adapt to the usage of hydrogen is a key issue. Ionic polymer metal composite (IPMC) actuators are composed of an electrolyte polymer membrane coated with electrodes. Owing to their light weight, flexibility, and ease of miniaturization, IPMC actuators are promising candidates for use as micro-actuators in small robots, microvalves, or artificial muscles. In this study, a hydrogen-assisted IPMC actuator is developed, and its deformation mechanism is investigated. The hydrogen absorption produced by water splitting and the hydrated cation movement in the electrolyte are the driving forces involved in this actuator. The effects of applied voltage, thickness of palladium electrodes, and length of the actuators are studied. Moreover, the contributions of bending moments by a hydrated cation movement and those by hydrogen absorption to the bending deformation are separately clarified. The results indicate that the deformation mechanism of the hydrogen-assisted IPMC actuator depends on the applied voltage, palladium-electrode thickness, and actuator length. The contribution of the hydrogen assistance decreases with the applied voltage, while that of the hydrated cation movement increases. In addition, the specimen with a shorter length exhibits a larger hydrogen-induced moment ratio. These results suggest that the hydrogen assistance to the bending deformation is more effective in IPMC actuators with a shorter and thinner palladium electrode.

This present study has been re-established to investigate failure mode and resistance characteristics of the PC/ABS blends and their ABS constituents under impact for a range of rubber contents. This present study has still been experimentally performed under an instrumented-drop weight impact test (DWIT) at a room temperature. It has been finally revealed that with a particular size of rubber particle, content of rubber significantly influenced impact failure modes and impact resistances of the PC/ABS blends and their ABS constituents as well. The test results showed that impact strength of the blends was improved about 23.22% and 155.33% due to increase in content of rubber up to 15 wt% and 20 wt%, respectively. There was also found that an increase in impact toughness of the blends for 57.48% and 239.23% was due to increase in content of rubber up to 15 wt% and 20 wt%, respectively. Whilst, impact strength of the ABS was improved about 392.98% and 190.12% due to increase in content of rubber up to 15 wt% and 20 wt%, respectively. An increase in impact toughness of the ABS for 308.20% and 172.56% was due to increase in content of rubber up to 15 wt% and 20 wt%, respectively.