長嶋 利夫

Development of a low-invasive microneedle is currently desired in the medical field to mitigate the patients’ stress and pain. We have paid attention to mosquitoes that puncture the skin without giving humans no feelings of pain. We have observed mosquitoes and found that when their proboscis punctures human skin, they make the following three behaviors: apply tension to human skin; rotate their proboscis; vibrate their proboscis. In our previous studies, we developed a bundled set of three microneedle imitating the mosquito’s proboscis and experimentally proved the usefulness of their alternate vibrations, which is one of the mosquito’s puncturing behaviors. However, the setting of three needles with proper clearances from each other was difficult, making their driving system too complex to practically use it. Therefore, we have developed a simplified microneedle by reducing the number of needles from three to two or one. This paper has focused on the effects of the rotations of a single needle. Using our developed microneedle with a diameter of 90 µm and the thinnest commercial microneedle with a diameter of 180 µm, we evaluated the effect of reciprocating rotation, one of the mosquitoes’ puncturing behaviors, by puncture experiments using artificial skin and nonlinear finite element method (FEM) analysis. As a result, it was found that the reciprocating rotation suppresses the puncture resistance force and the skin deflection.

<title>Abstract</title> To predict fracture behavior for ductile materials, some ductile fracture simulation methods different from classical approaches have been investigated based on appropriate models of ductile fracture. For the future use of the methods to overcome restrictions of classical approaches, the applicability to the actual components is of concern. In this study, two benchmark problems on the fracture tests supposing actual components were provided to investigate prediction ability of simulation methods containing parameter decisions. One was the circumferentially through-wall and surface cracked pipes subjected to monotonic bending, and the other was the circumferentially through-wall cracked pipes subjected to cyclic bending. Participants predicted the ductile crack propagation behavior by their own approaches, including FEM employed GTN yielding function with void ratio criterion, are FEM employed GTN yielding function, FEM with fracture strain or energy criterion modified by stress triaxiality, XFEM with J or ?J criterion, FEM with stress triaxiality and plastic strain based ductile crack propagation using FEM, and elastic-plastic peridynamics. Both the deformation and the crack propagation behaviors for monotonic bending were well reproduced, while few participants reproduced those for cyclic bending. To reproduce pipe deformation and fracture behaviors, most of groups needed parameters which were determined to reproduce pipe deformation and fracture behaviors in benchmark problems themselves and it is still difficult to reproduce them by using parameters only from basic materials tests.

This study seeks to establish a high-fidelity mesoscale simulation methodology that can predict the progressive damage and resultant failure of carbon fiber reinforced plastic laminates (CFRPs). In the proposed scheme, the plastic behavior (i.e., pre-peak nonlinear hardening in the local stress-strain response) is characterized through the pressure-dependent elasto-plastic constitutive law. The evolution of matrix cracking and delamination, which result in post-peak softening in the local stress-strain response, is modeled through cohesive zone models (CZM). The CZM for delamination is introduced through an interface element, but the CZM for matrix cracking is introduced through an extended finite element method (XFEM). Additionally, longitudinal failure, which is dominated by fiber breakage and typically depends on the specimen size, is modeled by the Weibull criterion. The validity of the proposed methodology was tested against an off-axis compression (OAC) test of unidirectional (UD) laminates and an open-hole tensile (OHT) test of quasi-isotropic (QI) laminates. Finally, sensitivity studies were performed to investigate the effect of plasticity and thermal residual stress against the prediction accuracy in the OHT simulation. (C) 2016 Elsevier Ltd. All rights reserved.

In this study, some benchmark problems on fracture tests for circumferentially through-wall/surface cracked pipes were provided. The participants predicted the ductile crack propagation behavior by their own approaches, including nucleation, growth, and coalescence of voids simulated by Gurson model, ductile crack propagation using stress modified fracture strain (SMFS) model, J-integral based ductile crack propagation using XFEM, CTOA based ductile crack propagation using FEM, stress triaxiality and plastic strain (STPS) based ductile crack propagation using FEM, and ductile crack propagation using peridynamics. Among them, GTN, CTOA and STPS models were not applied to surface crack problems. Discrepancies between the experimental maximum loads and calculated maximum loads were within 10% in most cases and 25% in the maximum case. Element size dependency of analysis parameters were considered in SMFS and GTN models while those were determined from independent material tests. Gurson model can predict slanting crack propagation directions. XFEM which did not need analysis fitting parameters cannot analyze beyond the peaks of load-LPD curves. Crack propagation directions were given and fixed in both CTOA and STPS models. Parameters in Gurson model and peridynamics were optimized to reproduce load-LPD curve in one of the benchmark problems.

The level set extended finite element method (XFEM) is applied to two-dimensional and quasi-three-dimensional crack propagation analyses using cohesive zone models (CZMs). The proposed method uses no asymptotic basis functions near the crack tip and uses only the Heaviside function. The crack geometry is approximated by two signed distance functions (SDFs). Elements that include a crack are then classified into several partitioned patterns according to nodal SDF values, and enriched nodes are determined. A CZM is introduced to the crack line or the surface including a discontinuous displacement field modeled by XFEM. In order to solve the discretized governing equations, the implicit method and the explicit dynamic method are used. The proposed method is applied to the crack propagation analysis of a three-point bending beam and fracture analyses of carbon fiber reinforced plastic (CFRP) laminates considering the interaction between the matrix cracks and delamination. (C) 2016 Civil-Comp Ltd and Elsevier Ltd. All rights reserved.

The aim of this study is to establish a high-fidelity mesoscale numerical simulation tool which can predict the progressive damages and resultant failure of carbon fiber reinforced plastics (CFRPs) laminates. In the proposed tool, the plastic behavior (i.e. pre-peak nonlinearity in the local stress-strain response) is characterized through the pressure-dependent elasto-plastic constitutive law. Moreover, the evolution of matrix crack and delamination, which result in post-peak softening in the local stress-strain response, is modelled through cohesive zone model (CZM). While the CZM for delamination is introduced through the interface element, the CZM for matrix crack is introduced through the extended finite element method (XFEM). Additionally, fiber failure which typically depends on the specimen size is modelled by Weibull criterion. Finally, the validity of proposed methodology was tested against the off-axis compression (OAC) test of unidirectional laminates and the open-hole tensile (OHT) test of quasi-isotropic laminates.

拡張有限要素法による二次元亀裂進展解析について、従来の漸近解基底を拡充した方法と、拡充しない方法の差異について比較した結果を比較考察した．

The extended finite element method (XFEM) using a crack tip element (TIP element), which is enriched through only the Heaviside function, is applied to crack and its propagation analysis in two-dimensional elastic problems. In the proposed method, two-kind of signed distance functions are utilized in order to express crack geometry implicitly and finite elements, which has interaction with crack, are appropriately partitioned according to the level set values and then integrated numerically for derivation of stiffness matrix. The results by XFEM using TIP elements were compared with those by the conventional XFEM using both the asymptotic bases and the Heaviside function. It was shown that the TIP element provides appropriate stress intensity factors and crack propagation path.

Purpose - The purpose of this paper is to investigate the effects of the finite element models on the response of a free surface or a floating roof, which is important for safety assessment of oil storage tanks. Design/methodology/approach - Structural analyses of shell structures using the three-dimensional finite element method (FEM), potential flow analyses by FEM, and fluid-structure interaction analyses by strong coupling of the structural and fluid analyses were performed. In-house software was utilized for computations shown in this paper except the solver for non-symmetric sparse matrix. Findings - A model with a rigid tank and an elastic roof was confirmed to be able to perform the seismic response analysis most effectively from the viewpoint of computational cost with no reduction in accuracy. Research limitations/implications - The stress distribution on the floating roof will be evaluated to assess the safety of oil storage tanks subjected to seismic waves in the future research. Originality/value - This paper shows the dynamic responses of a liquid storage tank subjected to seismic motion using four different analysis models and the results were compared. It was concluded that a model with a rigid tank and an elastic roof can perform the seismic response analysis most effectively from the viewpoint of both accuracy and computational cost.

The extended finite element method is applied to stress analyses of composite laminates modeled by shell elements. In the proposed method, a thin-walled structure containing an interface is modeled by shell elements, and the nodes on the interface are enriched in order to model the delamination. The X-FEM code for thin-walled structures based on the proposed method is developed and is applied to buckling analyses of Carbon Fiber-Reinforced Plastic laminate with delaminations. The proposed X-FEM for shell elements was shown to provide appropriate results, which agree well with those obtained by the X-FEM for solid elements and conventional FEM analyses. (C) 2010 Elsevier Ltd. All rights reserved.

Conventional finite element method is continually used for the flaw evaluation of pipe structures to investigate the fitness-for-service for power plant components, however, it is generally time consuming to make a model of specific crack configuration. The consideration of a propagating surface crack is further accentuated since the crack propagation behavior along the crack front is implicitly affected by the distribution of the crack driving force along the crack front. The authors developed a system to conduct crack propagation analysis by use of the three-dimensional elastic-plastic extended finite element method. It was applied to simulate ductile crack propagation of circumferentially surface cracks in pipe structures and could realize the simultaneous calculation of the J-integral and the consequent ductile crack propagation. Both the crack extension and the possible change of crack shape were evaluated by the developed system.

A structural analysis software referred to as V-X3D, which is based on the extended finite element method (X-FEM) using the VCAD framework provided by RIKEN, was developed and applied to perform a crack propagation analysis. In the method utilized for the developed software, crack geometry is expressed with triangular patches explicitly for finite element models and elastostatic analyses are performed by X-FEM. In crack propagation analyses, crack geometry is updated for adding triangular patches after each loading cycles using Paris' law, where crack growth rate is related with range of stress intensity factor (SIF) around a crack tip. SIF is evaluated by energy release rate, which is calculated by the domain integral method. As numerical examples crack propagation analyses for a plate with a circular crack under tension loading and a round steel bar with a straight notch under four points bending loading were performed by V-X3D. The validity was examined through comparisons with numerical results by another method and experiment results.

Recently, attempts have been underway to simulate rolling contact fatigue (RCF) crack growth in the railhead, including also the effect of wear on maintaining the integrity of the rail and saving cost. At this juncture, it is essential to confirm whether the past analyses are adequate and what extent of differences exists when the different mechanisms or numerical procedures are applied to the same conditions in the RCF problem. Therefore, boundary-element analyses of stress intensity factors (SIFs) at the inclined surface crack tip under RCF conditions have been performed. Comparisons were made between SIFs calculated by the present analyses and those done by the numerical procedures of other researchers in the RCF problem. From this study, it was recognized that a special program should be developed to analyse the SIFs when the fluid pressure is taken into account. It was also found out that, for the analyses of SIFs, the iteration procedure with convergence calculation to specify the extent and location of locked, slipped, and separated regions on the crack faces should be used.

The extended finite element method (X-FEM), in conjunction with the level set method, is applied to sloshing analysis of a rigid container filled with liquid. The governing equations for liquid with a free surface based on the potential flow theory are discretized using the framework of level set X-FEM. Once the space domain of a container is modeled by tetrahedral elements, sloshing analysis for arbitrary liquid levels and configurations can be performed without remeshing. Natural frequencies of free surface sloshing motion in rigid containers of various shapes were computed by the proposed method and the results were compared with those obtained by theoretical solutions and experiments. The proposed method was demonstrated to perform sloshing analysis efficiently for rigid containers with various liquid levels and configurations. Copyright (C) 2008 John Wiley & Sons, Ltd.

The extended finite element method (X-FEM), which can model the domain without explicitly meshing the crack surface, can be used to perform stress analyses for solving fracture mechanics problems efficiently. In the present study, the principle of superposition is used to solve crack problems in conjunction with the X-FEM. In the proposed method, the surface load distributed on the crack surface, which is modeled implicitly by the interpolation functions with enrichment terms, is introduced to X-FEM analysis. Moreover, the energy release rate at the crack front is evaluated by the domain integral method with boundary integral terms for the surface load. The proposed method is verified through numerical analyses of two- and three-dimensional crack problems in linear fracture mechanics.

The extended finite element method (X-FEM), which can model the domain without explicitly meshing the crack surface, can be used to perform stress analyses for efficiently solving fracture mechanics problems. In the present study, the constraint condition enforcement for X-FEM analysis considering symmetry is presented. Since the interpolation functions utilized in X-FEM analysis include the enrichment basis functions, the freedoms of the node on the symmetric plane should be constrained properly in the X-FEM model with symmetric conditions. Moreover, evaluation of the energy release rate by the domain integral method should be performed considering the symmetry conditions. In the present paper, the constraint conditions for three-dimensional X-FEM analysis considering symmetric conditions are summarized, and numerical examples using symmetric X-FEM models are shown. The proposed procedure can be used to perform efficient X-FEM analyses of practical fracture problems.

The extended finite element method (X-FEM) can model arbitrary cracks independently of the finite element mesh. Therefore, X-FEM can be used to efficiently perform stress analyses in the field of fracture mechanics. This paper describes the application of X-FEM to stress analyses and crack propagation analysis. As numerical example, the problem of a three-dimensional body with a planar crack is solved and the distribution of energy release rate is evaluated. The obtained results are verified by comparing with those obtained using conventional finite element analysis. Moreover, the crack propagation analysis is performed by X-FEM in conjunction with the fatigue crack propagation law, which gives the relation between the energy release rate and the crack extension length.

The extended finite element method (X-FEM) is applied to the stress analysis of composite laminates having interlaminar planar delamination. In X-FEM analysis, the geometry of such delaminations can be modeled independent of the finite elements. The domain form of the contour integral can be used to compute the energy release rate in conjunction with X-FEM. As numerical examples, three-dimensional analyses for DCB and ENF test specimens were performed by X-FEM with various enrichment nodes, and the obtained results were examined. In addition, a model of the no-friction-contact condition by X-FEM was proposed and applied to ENF test analysis. Moreover, eigenvalue buckling analyses of a CFRP plate with delamination were performed by X-FEM as a practical example related to Compression After Impact (CAI) problems of composite materials. The numerical results show that X-FEM is an effective method for analyzing stress in composite laminates with delamination.

The X-FEM in conjunction with the level set method can be used to simplify the modeling of continua containing several cracks and hence perform effective stress analyses related to fracture mechanics. In this study, numerical procedures for elastic-plastic X-FEM analysis including selections of the near-tip enrichment functions are examined. As numerical examples, two-dimensional crack problems of elastic-plastic materials under plane strain condition are solved to evaluate the crack opening displacement and the J-integrals around the crack tip. Obtained results are compared with those obtained using conventional finite element analysis.

In this study, numerical procedures for thee-dimensional elastic-plastic X-FEM analysis including selections of the near-tip enrichment functions are examined. The utilized enrichment functions approximate the asymptotic displacement field near the crack tip obtained by the HRR solution. As a numerical example, a surface crack problem of elastic-plastic materials is solved to evaluate the crack opening displacement and the J-integrals along the crack front, which is evaluated using the domain integral method. Obtained results are compared with those obtained using conventional finite element analysis.

This series of articles summarize the current progress of meshfree/particle method. In this issue, Extended/Fictitious finite element method is focused and then the extended finite element method (X-FEM) using interpolation functions based on the partition of unity is introduced. The theoretical formulation is outlined and the several numerical examples for two-dimensional elasticity are illustrated.