Papers by Author: Lin Zhi Wu

Abstract: To a laminate model of 3D four-directional braided composites with periodic transverse cracks, a new normal stress function in weave direction is present. In this function, variations of stress with x and y are considered. By using the principle of minimum complementary energy, stress components in the representative unit are obtained. Tensile modulus of 3D four-directional braided composites with periodic transverse cracks is calculated by utilizing Hashin’s average theory and the principle of minimum complementary energy. To demonstrate the solution, some examples are analyzed.

Abstract: Uni-tension test and torsion test with specimens of A3 steel are completed, and deformation rate, the spin of Euler configuration to Lagrange configuration and generalized Jaumann rate of Kirchhoff stress in torsion problem are given. Large strain constitutive relationship with generalized Jaumann stress rate is studied based on torsion test. The simple shear problem is discussed by using the constitutive model given in this paper. The result indicates that the hardening coefficient of constitutive model with generalized Jaumann rate may be given by the same expression as that of constitutive model with Jaumann objective rate when determining by using torsion tests and the shear stress response curve in simple shear problem obtained by using the constitutive model in this paper is similar to the shear stress-strain curve given in solid circular shaft torsion tests.

Abstract: This paper addresses the numerical simulation of mixed-mode crack propagation in Functionally Graded Materials (FGMs) by means of eXtended Finite Element Method (XFEM), endowed with elastic and toughness properties which gradually vary in space. The method allows to follow crack paths independently of the finite element mesh, this feature is especially important for FGMs, since the gradation of the mechanical properties may lead to complex propagation paths also in simple symmetric tests. Each step of crack growth simulation consists of the calculation of the mixed-mode stress intensity factor by means of a non-equilibrium formulation of the interaction integral method, determination of the crack growth direction based on a specific fracture criterion. A specific fracture criterion is tailored for FGMs based on the assumption of local homogenization of asymptotic crack-tip fields in FGMs. The present approach uses a user-defined crack increment at the beginning of the simulation. Crack trajectories obtained by the present numerical simulation agree well with available experimental results for FGMs. The computational scheme developed here serve as a guideline for fracture experiments on FGM specimens (e.g. initiation toughness and R-curve behavior).

Abstract: In present work, the critical buckling load of metallic foam composite sandwich panels is calculated by experimental, finite element methods (FEM) and theoretical analysis. The experimentally investigated is based on an edgewise compression test program to examine buckling failure and compressive properties. The metallic foam sandwich panels under edgewise compression tend to collapse in overall buckling mode. The most important factor that determines the overall buckling load of a sandwich panel under edgewise compression is the shear properties the metallic foam core. The sandwich beam theory and the FE model are developed for prediction of the buckling load of metallic foam sandwich structure. In despite of some differences existed among experimental data, FE and theoretical results, considering the existence of initial defects in sandwich structures which can’t be calculated in FE model and theory anlysis, the differences are in the reasonable range. The FE program developed in this paper can effectively be used to simulation of edgewise compression response for metallic foam composite sandwich structures. Theoretical and FE model results are in agreement with experimental result.

Abstract: Since composite sandwich structures are susceptible to low-velocity impact damage, a thorough characterization of the loading and damage process during impact is important. In the present paper, the low-velocity impact response of carbon fiber composites lattice structures are investigated by experimental and numerical methods. Impact tests on composite plates are performed using an instrumented drop-weight machine (Instron 9250HV) and a new damage mode is observed. A three-dimensional finite element model is built by ABAQUS/Explicit and user subroutine (VUMAT) to predict the peak loading and simulate the complicated damage problem. It can be found that numerical predictions coincide well with experimental results.

Abstract: In this paper, coherent gradient sensing (CGS) and digital speckle correlation method (DSCM) are introduced to study the K-dominance of static crack tip in functionally gradient materials (FGMs) with a crack oriented along the direction of the elastic gradient. And the numerical simulation is analyzed through finite element method (FEM). Firstly, the CGS and DSCM equations at the mode-I static crack tip of FGMs are developed, which can be used to calculate the stress intensity factors of FGMs. Secondly, three kinds of FGMs specimens with different variation of the modulus are prepared to observe the influences of the property variation on the K-dominance. Then three-point-bending experiments are carried out. The interference fringe pictures of CGS and the speckle patterns for DSCM on the specimens are shot through the camera. Thirdly, based on the results of the experiments, the stress intensity factors of three kinds of FGMs specimens are calculated by CGS and DSCM. Meanwhile, the stress intensity factors are obtained by FEM. Finally, comparing the results from CGS, DSCM and FEM, the K-dominance of mode-I static crack tip in FGMs is discussed in detail. It is found that the K-dominance of FGMs and homogenous material is almost same when the gradient index in FGMs is relatively small.

Abstract: Functionally graded materials (FGMs) with continuous varying properties have absorbed great attention for the purpose of eliminating the mismatch of material properties which may result in cracking. In this paper, three-dimensional finite element method (3D FEM) based on nonhomogeneous elements is used to study the fracture behaviors of a 3D FGM plate. Since real material properties at Gaussian integration points are adopted during forming the element stiffness matrix, the nonhomogeneous material properties can be applied in each element. Moreover, 20-node singular elements are used around the crack front to deal with the singularity of stress fields at the crack front. By this way, the stress intensity factors (SIFs) can be calculated with high efficiency and accuracy. Therefore, compared with the general FEM using homogeneouos elements, the calculating efficiency and accuracy can be increased. Finally, parameter analysis is conducted. It is found that the material nonhomogeneity constant and the crack parameter have significant influences on the SIFs.

Abstract: The crack problem for a functionally graded orthotropic coating-substrate structure under an in-plane load is studied. The orthotropic coating is assumed to contain a crack perpendicular to the interface. Integral transform method is used to obtain singular integral equation. Stress intensity factors (SIFs) are evaluated. The influences of orthotropic material constants and the geometry parameters on SIFs are analyzed.

Abstract: In this paper, the non-local theory of elasticity was applied to obtain the dynamic behavior of a Griffith crack in functionally graded piezoelectric materials under the harmonic anti-plane shear stress waves. The problem can be solved with the help of a pair of dual integral equations. Unlike the classical elasticity solutions, it is found that no stress and electric displacement singularities are present at the crack tips, thus allows us to use the maximum stress as a fracture criterion.