Nagashima Toshio

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.

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.

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.

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.