Takehiro Fujimoto

Concepts of Separated J-Integrals, Separated Energy Release Rates, and the Component Separation Method of the J-Integral for Interfacial Fracture Mechanics

First, this paper presents the concepts of separated J-integrals and separated energy release rates. The path-independent separated J-integrals have the physical significance of energy flows into an interfacial crack tip from adjacent individual material sides or, equivalently, separated energy release rates. Thus, the J-integral and the energy release rate can be evaluated by the sum of the path-independent separated J-integrals. Second, the relations between the separated J-integrals and the stress intensity factors are derived. Third, the component separation method of the J-integral is extended for interfacial crack problems to allow accurate evaluation of the stress intensity factors. Finally, pertinent numerical analyses are carried out to demonstrate the usefulness of the separated J-integrals and the component separation method.

Finite element analyses of near-tip deformation in inhomogeneous elastic-plastic fracture specimens

Abstract Two types of path-independent expressions were derived for the nonlinear fracture parameter T ∗ integral in inhomogeneous multilayer materials. Finite element analyses were carried out for inhomogeneous elastic-plastic fracture specimens consisting of A533B steel and HT80 steel: these two materials have considerably different yield stresses, although their elastic properties are exactly the same. The T ∗ -integral for inhomogeneous materials demonstrated excellent path independence even in the stages of large deformations around the crack tip and material interfaces. Numerically generated moire fringe patterns are in good agreement with experimentally recorded patterns. The shapes of plastic zones appearing in the specimens reveal large inhomogeneity effects. The applicability of a hybrid moire-finite element method is demonstrated briefly.

Modeling and estimation of deformation behavior of particle-reinforced metal–matrix composite

Abstract In order to estimate the characteristic feature of the deformation behavior of materials with a length scale, the strain gradient plasticity theories, corresponding variational principle and a finite element method are given. Then the finite element method is applied to the estimation of the mechanical characteristics of the particle reinforced metal–matrix composites modeled under plane strain conditions. The effects of the volume fraction, size and distribution pattern of the reinforcement particles on the macroscopic mechanical property of the composite are discussed. It has been clarified that the deformation resistance of the composite is substantially increased with decreasing particle size under a constant volume fraction of the reinforcement material. The main cause of the increase of the deformation resistance in the plastic range is the high strain gradient appearing in the matrix material, which increases with the reduction of the distance between particles.

Moiré interferometry measurements of neartip deformation in inhomogeneous elasticplastic fracture specimens

Abstract Moire interferometry was used for direct measurements of the crack tip behavior in the homogeneous compact tension (CT) specimens of A533B and HT80 steels, and in electron-beam welded inhomogeneous CT specimens of the two materials. These two materials have considerably different yield stresses, although their elastic properties are the same. Five types of homogeneous and inhomogeneous specimens were used in the experiment. Moire fringe patterns of the five specimen types were directly compared for elastic, elastic-plastic and unloaded stages. Although the global deformations in terms of the load versus COD relations exhibit little inhomogeneity effects, large inhomogeneity effects were observed in the moire fringe patterns in the elastic-plastic and unloaded stages. The near-tip deformations were also compared with the corresponding HRR singular fields. In a horizontal weld specimen, due to strong hardening in the weld region, the slope of the displacement field does not shift the characteristic HRR field for the A533B or the HT80 steel for increasing load.

Verification of the Combination Rules of Multiple Flaws in ASME B&PV Code Section XI: A Case Study of Two Adjacent Surface Planar Flaws

In nuclear pressure vessels, multiple surface cracks are often found by regular inspection. In order to evaluate the integrity of the vessels, ASME BP however, its accuracy has not been clarified yet. For the analyses of interacting multiple semi-elliptical surface cracks, in 1983 Nishioka and Atluri developed the Vijayakumar, Nishioka, and Atluri (VNA) solution-finite element alternating method which is highly accurate and cost effective. Using this highly accurate VNA-finite element alternating method, the case of extremely closely located two interacting coplanar cracks was analyzed. From the numerical results, it is found that the B&PV Code Section XI provides a conservative flaw combination rule. Therefore, the B&PV Code Section XI is precisely verified by modern and accurate computational technologies.

On the General Solutions for Mixed-Mode Penny-Ohaped Crack and Their Applications

The general solutions for a penny-shaped crack in an infinite solid, subjected to arbitrary tractions on the crack surfaces were derived. The applicability was demonstrated deriving the closed-form solutions for a penny-shaped crack subjected to the lower-order loading such as constant tension, shear, bending, and torsion. Furthermore, we consider a circular crack subjected to cubic-order normal stresses. It is shown that the stress intensity factor distribution derived from the general solution exactly agree with the analytical solutions derived by Shah-Kobayashi.

Numerical Simulation of Impact Transonic Interfacial Fracture

In transonic interfacial crack propagating fracture problem, the generation-phase simulations were done using the moving finite element method based on Delaunay automatic mesh generation. And the contact function based on the penalty method was newly developed to consider the crack face contact near the propagating interfacial crack tip. It was succeeded to visualize in 3-dimensions the Mach shock wave emanated from the propagating crack tip. And it was tried for the transonically propagating crack problem that solving the energy flows through the contact zone or along the Mach shock wave line emitted from the crack tip. The energy flow patterns into the crack tip were also visualized. Furthermore, from the values of the separated dynamic J integrals, it was found that the dynamic J integral is non-zero even for transonic fracture region and the most of the energy release rate is provided from the more compliant material epoxy.

Experimental and Numerical Study on the Dynamic Fracture Characteristics of Gray Cast Iron FC200

Recently, some improvements made to machine performance have caused accidents as a result of impact fracture. These fractures were caused by unexpected dynamic loads. To suppress the damage in these accidents, it is necessary to clarify the dynamic fracture mechanism, many reports have been published on dynamic fracture phenomena [1, 2, 3]. Cast iron is used to repair some structural and mechanical parts following fracture accidents. The brittle behavior of cast iron is not desirable for preventing dynamic fracture. It is necessary to clarify the dynamic fracture mechanism of cast iron for the safety design and maintenance of structures. The dynamic behavior of deformation and fracture depends on the size of a structure. In some cases, an experimental approach using specimens at industrial scale is difficult. As a first step, dynamic fracture without a huge mass effect should be discussed. In this study, a normal sized three point bending specimen consisting of cast iron was used in dynamic experiment. An ultra-high speed camera was used to observe crack propagation. Some fractures were caused under eccentric loading, non-straight cracks propagated in this condition. According to the experimental results, the path and velocity of crack propagation were estimated. Fracture criteria were discussed from the results of numerical simulation. To simulate the behavior of crack propagation a moving finite element method based on Delaunay automatic triangulation was used. The prediction of fracture paths based on the fracture mechanics theory was demonstrated in these numerical simulations. The predicted fracture path agreed with the experimental fracture path.

Development of three-dimensional moving finite element method based on Delaunay automatic tetrahedronization

Up to now, it has been impossible to simulate dynamic crack propagation with distortion and heaving. In this study, a three-dimensional moving finite element method based on automatic tetrahedronization (MFEM BOAT) is developed. This method makes it possible to simulate a complicated dynamic fracture. The moving fine elements subdivision is used to evaluate an exact solution for a singular field. The equivalent domain method is applied to the solution to calculate the dynamic fracture parameters along a dynamically propagating crack front.

Experimental and numerical study for crack propagation in aluminum alloy A2024-T351

In some catastrophic structural accidents, impact loading causes metal failure and fracture. High impact loads cause fast crack propagation, and distinct plastic deformations can be observed around the crack propagation path. The fracture behavior depends on the material ductility, loading velocity, and various other factors. Therefore, the mechanism of dynamic ductile fracture is intricately complex. This study focused on the dependence of dynamic fracture on loading velocity. Experiments were conducted for not only pure mode I fractures but also mixed mode fractures under eccentric impact loading. Fracture experiments under quasi-static loading were also conducted for comparison. The moving finite element analysis of these crack propagation phenomena was performed on the basis of the experimental observations. The numerical solutions were used for domain integration to estimate the fracture mechanics parameter. The crack propagation path was observed to depend on the loading velocity under eccentric loading. Under eccentric impact loading, the cracks did not propagate toward the loading point. The relationship among crack propagation velocity, roughness of the crack surface, and crack propagation path was also determined from these experiments. The histories of the fracture mechanics parameter during fast crack were calculated from the moving finite element analysis results. The fracture energy resistance was evaluated from the fracture mechanics parameter and was found to increase under impact loading.