Finite Element Research On Thermo-elasticbehaviors And Fracture Characteristics Of Graded Composites

Graded materials are new kind of nonhomogeneous composites whose constitute volume fractions, micromechanics structure and effective properties are all varied continuously in gradation directions. Because of the excellent thermal-mechanical property and designable structure, graded composites have received increasingly applications in aerospace, mechanical engineering and biological medicine industry fields. Studies on the thermal stress reduction and fracture characteristics are the fundation of structural optimization design for graded materials, which have become the hot and difficult areas of the current research. In the paper, graded finite element is adopted to study thermal-elastic behaviours, stress intensity factors and crack propagation paths in graded composites. The main contents are summarized as follows:(1) Based on classic micromechanics methods, micromechanics governing equations and effective thermal-mechanical properties of graded composites are derived. Interactive influences among constitute particles are fully introduced in the current fomula, which makes it more suitable for effective property predictions in the whole gradation range of graded composites. And corporating with conventional micromechanics models, the spatial distributions of effective thermoelastic and thermal conductivity properties of graded materials are investigated in detail.(2) Considering the property inhomogeneity in graded composites, stiffness matrices of two-dimensional graded thermal conduction element and graded plane stress(plane strain) element are calculated, and a continuously graded finite element model is established. The calculation capacities of graded element in different serving conditions are then demonstrated. Under thermal cycling loadings, the transient temperature and thermal stress fields of one-directional and two-directional graded composites are investigated, and effects of nonhomogeneous parameters on the thermal-mechaincal behaviours are discussed detailedly. Finally, optimization design schemes for graded structures are given according to the minimum temperature field and minimum thermal stress field criterions, respecively.(3) Stiffness matrice of two-dimensional graded extended finite elements are calculated by embedding property gradations into conventional homogeneous elements. A continuously graded finite element model is then established for cracked graded structure and displacement and stress fields are calculated. Finally, the interactive energy integral incoorporating property inhomogeneity is employed to extract mixed-mode stress intensity factors. The advantages of graded extended finite elements for calculating stress intensity factors in graded materials are validated. Effects of nohomogeneous parameters on stress intensity factors are discussed in detail. Furthermore, stress intensity factors in some typical graded structures are calculated, and fracture characteristics of one-directional and two-directional graded composites are compared.(4) Quasi-static crack propagation paths in graded structures are simulated by graded finite element method. In the model, the maximum hoop stress and maximum hoop stress ratio criterions are used to calculate crack growth directions, and the rule of mixture is adopted to predict the effective fracture toughness. Crack paths modelled by these two criterions are compared to demonstrate the effect of fracture toughness gradation on crack propagations in graded materials. Effects of nonhomogeneous parameters on crack propagation paths are investigated in detail. Furthermore, crack propagations in some typical grade structures are studied, and some advice for optimization design of graded composites are given from the fracture mechanics scope.