Effect Of Defects In Materials On The Behavors Of Crack Growth

Failure of most of materials is induced by crack growth.Crack growth is closely linked with the safety of structures,so the problem of crack growth has been an important research field since last century.The behaviors of crack growth are different for different kinds of materials.Due to the influence of microstructures,the behaviors of crack growth are also different in the same material.Thus,crack growth problem is complex,and some mechanisms are still unclear.In order to prevent the failure of materials effectively or prepare materials having high resistance of crack growth,it is essential to understand the mechanisms of crack growth.For the problem of the effect of defects on crack growth,some phenomena and mechanisms need be studied.Based on this,main works in this paper are as follows:(1)For brittle materials,crack-tip plasticity is not considered.Based on the distributed dislocation method,the solution of a two-dimensional elastic plane containing multiple straight and kinked cracks is presented,and the stress intensity factors at crack tips and the stress field are obtained.Futher,the effects of micro-crack length,micro-crack orientation,micro-crack angle on the behaviors of macro-crack growth are analyzed systematically.(2)For ductile materials,the effect of micro-cracks on the plasticity of macro-crack tip is studied by two ways.On the one hand,crack-tip plasticity is replaced equivalently by the continuously distributed dislocations.A theoretical model is established to represent crack-tip plasticity.This model can be applied to predict crack opening displacement and the size of plastic zone.Based on the model,the effect of micro-cracks on the plasticity of macro-crack tip is analyzed.On the other hand,based on the elastic stress field and von Mises yield criterion,the boundary of plastic zone can be determined,and then the effect of micro-cracks on the plastic zone ahead of the macro-crack is analyzed.(3)For crystalline metallic materials,considering the grain size effect on plastic deformation,it is studied from three aspects,that is:i)In coarse-grained metals,the plastic deformation is mainly contributed by the generation and motion of lattice dislocations.Crack-tip dislocation emission and pileup are considered;the effects of dislocation pileup,crack length and grain size on crack growth behaviors are analyzed.In addition,dislocation pileup can cause the stress concentration at the tip of dislocation pileup.Thus,the problem of inclusion cracking due to lattice dislocation pileup is investigated.ii)In nanocrystalline metals,the generation and movement of lattice dislocations are suppressed greatly,and the motion of grain boundary dislocations becomes more active.Therefore,grain boundary sliding induced by local high shear stress near a crack tip is considered,and nano-crack nucleation at triple junctions of grain boundary is analyzed.The theoretical solution of this problem is obtained by the distributed dislocation method,and then the structures of triple junctions of grain boundary on nano-crack nucleation are analyzed comprehensively.iii)In gradient nano-grained metals,a theoretical model is established to analyze crack-tip dislocation emission and movement,and the theoretical solution of this model is obtained.The evolution of dislocations in grains and the effect of grain size gradient on dislocation distribution are analyzed.The results show the grain size gradient effect can restrain crack growth by increasing crack blunting and the shielding effect of dislocations.