Nagashima Toshio

<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.

The quasi-three-dimensional XFEM is applied to damage propagation analyses of CFRP (Carbon Fiber Reinforced Plastics) laminate. Six-node triangular interface element and six-node pentahedral continuum element enriched with Heaviside function are employed to model delamination and matrix crack, respectively. Bi-linear type cohesive zone model is introduced between delamination and/or matrix crack and then implicit static or explicit dynamic method is utilized to solve system equations considering materially nonlinearity. Code verification was performed through DCB, ENF and TCT test specimen analyses and damage propagation analyses of NHT (No Hole Tension) and OHT (Open Hole Tension) test specimens were validated by comparison with experiment results. In addition, computation conditions for explicit dynamic analysis including mass scaling and energy balance, Zigzag CZM, and threshold parameter of crack shape definition were examined. It was shown that implicit static method using Zig-zag CZM is the most efficient for damage propagation analysis using the proposed method.

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 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.

Novel cohesive element which can deal with non-self-similar crack growth under mixed-mode II and III failure condition is proposed. The local coordinate system based on the crack front direction is determined in terms of relative displacement of the cohesive elements. Traction-relative displacement relation of the element is determined as a function of mode components calculated on the local coordinate system. This formulation allows us to simulate delamination growth when the non-self-similar crack growth occurs and the fracture toughness depends on the mode ratio. The numerical examples revealed that the crack growth strongly depends on its history when the mode II and III fracture toughness is different and the present formulation is valid even for the simulation of such history dependent crack growth.

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 authors propose a node-by-node meshless method (NBNM), which discretizes the weak-formed governing equations of continuum mechanics using only distributed nodal data. This method uses the following three core methodologies: (i) interpolation using the moving least squares method (MLSM) (ii) estimation of stiffness by nodal integration with stabilization terms, and (iii) a node-by-node iterative solver. This paper discusses the effect of the stabilization term introduced to the NBNM and examines the convergence of the NBNM approximation. An error indicator for NBNM analysis is proposed, and the development of a prototype CAE system based on this method is outlined. Moreover, results of two-dimensional plane stress analysis with adaptation are shown. In conclusion, this paper will show that the NBNM is capable of utilizing CAD data easily and performing effective adaptation analyses, and therefore may be used as a convenient CAE tool. (C) 2000 Elsevier Science S.A. All rights reserved.

The meshless method is expected to become an effective procedure for realizing a CAD/CAE seamless system for analyses ranging from modelling to computation, because time-consuming mesh generation processes are not required. In the present study, a new meshless approach, referred to as the Node-By-Node Meshless method is proposed, in which only nodal data is utilized to discretize the governing equations, which are derived using either the principle of virtual work or the Galerkin method. In this method, three key methodologies are utilized: (i) nodal integration using stabilization terms, (ii) interpolation by the Moving Least-Squares Method, and (iii) a node-by-node iterative solver. This paper presents the formulation of the proposed method along with numerical results obtained for two-dimensional elastostatic and eigenvalue problems. Copyright (C) 1999 John Wiley & Sons, Ltd.

As a fundamental research problem relating to the amplitude of Pogo oscillation of liquid propellant launch vehicles, an experimental study was conducted in which a cylindrical shell vertically hung and partially filled with liquid was longitudinally excited at the bottom in the authors' previous paper. The vibration of the shell wall was induced at the several specific ranges of exciting frequency by the mechanism of parametric excitation. To analyze this problem, a set of nonlinear equations of vibration is derived with the aid of the finite element method. Axial mode shapes of the axisymmetrical and circumferentially n-wave modes are calculated from the linear part of the set of nonlinear equations. Using these two modes, the set of nonlinear equations is reduced to the coupled equations of two degrees of freedom. The equations to calculate a stationary vibration, and its stability are derived. The range of the parametric excitation is obtained in the plane of exciting force amplitude and frequency.