Abstract: Three kinds of fracture specimens are tested under different constraint conditions. One is 3PB(Three Point Bending) specimen, another is CCT(Center Cracked Tension) specimen, and the third one is called CCB(Center Cracked Bending) specimen. By the SEM (Scanning Electron Microscope) observation, it is shown that the roughness of fracture surface is different from each other largely. They are the effect of constraint condition. The dimple fracture process is simulated by the finite element method using Gurson's constitutive equation, and the crack tip stress fields are obtained. The distributions of stress triaxiality components are qualitatively agree with the experimental results. The J-R curves obtained also qualitatively agree with those of experiments, and the fracture surface roughness is well simulated.
Abstract: In this paper, a simple but highly accurate analytical method to evaluate stress intensity factors based on an element overlay technique is presented. When a crack exists in a structure, we need to analyze its global deformation as well as local deformation field around the tip of the crack. This naturally leads us to a consideration on some kind of multi-scale analysis strategies. Thus, we adopt the element overlay technique (S-FEM) which is a kind of multi-scale methodologies.
In this paper, we present i) numerical implementation of element overlay technique (S-FEM), ii) method to calculate stress intensity factors (virtual crack closure-integral method, VCCM), iii)accuracies of evaluated stress intensity factors and iv) some discussions.
Abstract: The implementation of inter-element model to simulate crack propagation by using finite element analysis with adaptive mesh is presented. An adaptive finite element mesh is applied to analyze two-dimension elastoplastic fracture during crack propagation. Displacement control approach and updated Lagrangean strategy are used to solve the non-linearity in geometry, material and boundary for plane stress crack problem. In the finite element analysis, remeshing process is based on stress error norm coupled with h-version mesh refinement to find an optimal mesh. The
crack is modeled by splitting crack tip node and automatic remeshing calculated for each step of crack growth. Crack has been modeled to propagate through the inter-element in the mesh. The crack is free to propagates without predetermine path direction. Maximum principal normal stress criterion is used as the direction criteria. Several examples are presented to show the results of the implementation.
Abstract: This paper compiles solutions of plastic η factors for standard and non-standard fracture toughness testing specimens, via detailed three-dimensional (3-D) finite element (FE) analyses. Fracture toughness testing specimens include a middle cracked tension (M(T)) specimen, SE(B), single-edge cracked bar in
tension (SE(T)) and C(T) specimen. The ligament-to-thickness ratio of the specimen is systematically varied. It is found that the use of the CMOD overall provides more robust experimental estimation than that of the LLD, for all cases considered in the present work. Moreover, the estimation based on the load-
CMOD record is shown to be insensitive to the specimen thickness, and thus can be used for testing a specimen with any thickness.
Abstract: Based on detailed two-dimensional (2-D) and three-dimensional (3-D) finite element (FE)analyses, this paper attempts to quantify in-plane and out-of-plane constraint effects on elastic-plastic J and cracked tip stresses for biaxially loaded plate with a through-thickness crack and semi-elliptical surface crack. It is found that the reference stress based approach for uniaxial loading can be applied to estimate J under biaxial loading, provided that the limit load specific to biaxial loading is used, implying that quantification of the biaxiality effect on the limit load is important.
Investigation on the effect of biaxiality on the limit load suggests that for relatively thin plates with small cracks, in particular with semi-elliptical surface cracks, the effect of biaxiality on the limit load can be neglected, and thus elastic-plastic J for a biaxially loaded plate could be estimated, assuming that such plate is subject to uniaxial load. Regarding the effect of biaxiality on crack tip stress triaxiality, it is found that such effect is more pronounced for a thicker plate. For plates with semi-elliptical surface cracks, the crack aspect ratio is found to be more important than the relative crack depth, and the effect of biaxiality on crack tip stress triaxiality is found to be more
pronounced near the surface points along the crack front.
Abstract: Based on the cellular automata of the plane truss structure, a 2D cellular automata model is presented to simulate the fracture of rock at meso-level. Cellular automata are made up of cell, states, lattice, neighbor and rule. Rock is divided into lattice in which each lattice point presents a cell. Each cell is assumed to connect with several cells, which are called as its neighbors, in virtue of truss elements. The truss elements can adopt some different simple local laws, i.e. constitutive law, which may be elastic or elastic-plastic and the simple fracture rule. It also can adopt different mechanical properties, which present their heterogeneity and anisotropy. This model can make full use of the advantages of cellular automata such as its intrinsic parallelism, localization and so on. In the meantime, as a powerful tool to analyze the nonlinear, complex system, cellular automata can be used to study the nonlinear, complex fracture process. The model is used to simulate the direct tensile of the rock plates, the complete fracture process and the stress-strain curves are attained which are accordance with the experiment.
Abstract: A numerical study on the effects of the distribution of rubber particles size on the fracture toughness of rubber-modified polymer alloys was computed. FEM analyses were conducted on the deformation field near the crack tip under mode I for small scale yielding condition. Near the crack tip is modelled as composite of matrix materials and rubber particles. On the other hand, outer region is modelled as homogeneous material whose constitutive equation has been obtained by analysing unit cell model of matrix and rubber particle. Perfect bonding or partial debonding of the interface is assumed in the computation. Matrix and rubber particles are treated as Mises and Mooney-Rivlin materials, respectively. It is shown that energy flux into fracture process zone; Ĵ -integral is smaller for bimodal type than monomodal one. This behavior largely occurred on the partial debonding case. These results imply that the screening effects occurred in the bimodal type larger than monomodal
Abstract: Body-centered cubic unit cell models and three-dimensional finite element method are used to study the inelastic deformation of rubber particle modified polymers. Calculations are carried out for three loading conditions, i.e. uniaxial loading, plane strain deformation loading and the so-called 'equivalent shear' loading. Distributions of the localized shear deformation are presented to understand the microscopic deformation mechanisms of the polymers. Effects of particle size, particle volume fraction and loading conditions on the micro- and macroscopic deformation
behavior of rubber particle modified polymers are discussed.
Abstract: In this paper, a diatomaceous soft rock is studied. Triaxial tests had been conducted on this soft rock. From the test results, it is found that the stress-strain curve of this soft rock has a notable strain-softening tendency. In order to study its time-dependent stress-strain behavior, a constitutive model that can describe not only the strain-hardening behavior, but also the strain-softening behavior must be constructed. Based on Perzyna’s fundamental assumptions of the elastic visco-palstic theory, a visco-plastic flow rule, and Yin and Graham’s 3-D elastic visco-palstic constitutive model (3-D EVP model), the constitutive formulation under a triaxial stress state is obtained in this paper. The derived formulation can be used to simulate the time-dependent stress-strain behavior of both consolidated undrained and consolidated drained triaxial tests of soils and rocks. In this paper, the constitutive formulation is used to simulate the time-dependent stress-strain behavior of consolidated undrained triaxial tests of the soft rock studied in this paper. The simulated results are compared with the triaxial test results. The comparison of the results shows that model predictions agree well with measured results. This demonstrates that the EVP model can be used to describe the time-dependent stress-strain behavior of the soft rock studied in this paper.
Abstract: A strategy of coupling length scales from atomistic to continuum is investigated on the basis of quasicontinuum model for interface fracture problems. In the model, an atomistic region of the interest is discretized by finite elements and the positions of atoms are prescribed by means of nodal displacements of the elements with shape function for the reduction of the degrees of freedom. Total energy of the system consists of interatomic potentials, and minimized through variational method employed in conventional finite element formulation. In this study, we deal with the fracture behavior of Cu-Fe interface crack in two dimensional problems, and investigate the adequate discretization manner around the crack tip in use of quasicontinuum method.