The Fracture Behavior And Discrete Dislocation Dynamic Mechanism Of Crystalline Materials

The physical mechanism for damage and fracture behavior of material is one of the key problems of solid mechanics.The macroscopic damage and fracture behavior of materials is derived from mesoscale deformation and damage evolution.At mesoscopic scale,the deformation and damage behavior of materials are closely related to the interaction of discrete dislocation with defects such as grain boundary(GB),void and microcrack in the material.It is important to capture the interactions between dislocations with other defects such as grain boundaries,voids and microcracks,which has theoretical significance for revealing the physical mechanism of damage and fracture behavior in material.Therefore,it is necessary to develop algorithms for capturing the interaction of discrete dislocations with grain boundaries,voids and microcracks at mesoscopic scale.Based on the algorithms,the processes of interaction between discrete dislocations with grain boundaries,voids and microcracks can be performed quantitatively.The study of these problems belongs to the intersection of solid mechanics and material physics,which has important academic significance and potential engineering application value.The discrete dislocation dynamics(DDD)can tracks the dynamic evolution of discrete dislocations and simulate problems in larger spatial scale and longer time scale,so it is becoming a popular and effective tool to depict the plastic deformation and its physical mechanism at submicron scales.By the employment and extension of the DDD framework,several relevant works are performed in this thesis to study the damage and fracture behavior for crystalline materials at mesoscopic scale as follows:(1)The interaction between a type-I blunt crack and a near-tip void were modelled by DDD simulations to reveal the void growth mechanisms and their size effect.Results indicate that: even for the type-I opening crack,the horizontal slip bands connecting the crack tip and the void surface are easily formed,especially when the ligament between the crack-tip and void surface is not too long.There exists a critical void radius,under which the near-tip void tends to change into a flat shape.(2)The Discrete Dislocation Dynamic(DDD)framework was further developed with the employment of a crack-tip dislocation emission scheme based on Rice-Thomson model and a energy based GB-dislocation penetration model.With the employment of the extended DDD framework,detailed simulations are performed to study the influence of dislocation nucleation mechanisms,penetrable GB and the grain size on the dislocation-induced shielding effect at the crack-tip in poly-crystallines.Related results indicate that: the DECT(dislocation emission from the crack-tip)mechanism plays more important role compared with the DNFR(dislocation nucleation from the Frank-Read source)one for the shielding effect of dislocation on the crack-tip.The shielding effect of dislocation is dominated by the grain size as well as the energy barrier for dislocation transmission across GB.With the increase of grain size,the shielding effect of dislocations on the crack-tip increases significantly,showing strong grain size effect.On the other hand,with the increase of the GB energy barrier for dislocation transmission,the shielding effect of dislocations on the crack-tip first increases and then reaches its peak and finally decreases,showing an interesting mountain-shaped dependence on the GB energy barrier.This mountain-shaped dependence is gradually flattened with the increases of grain size.(3)A hybrid discrete dislocation dynamic approach based on the extended finite element method has been developed to model the evoluation of multiple dislocations.Compared with other DDD algorithms,the present XFEM-based DDD scheme can not only deal with the non-uniform gradient mesh,but also solve the boundary value problem containing complex surfaces/interfaces and discrete dislocations induced displacement discontinuity in a unified framework directly with satisfactory accuracy and efficiency.(4)The DDD framework based on the extended finite element method is further developed by introducing a crack tip dislocation emission model based on the Rice Thomson theory and the cohesive surface model.Then the extended DDD framework has been applied to study the competition between the crack tip dislocation emission and cohesive crack decohesion of single crystal materials.Related results indicate that: the dislocations emitted from the crack tip can generate a local stress field at crack tip,the local stress field can not only shield the crack from the applied load but also inhibit the emission of subsequent dislocations from the crack-tip.The competition between the two different shielding effects under the applied load leads to the repeated dislocation emission and crack propagation process within the crack-tip zone.The mechanism of ductile brittle transition at the crack tip and its discrete dislocation dynamics details can be captured successfully by the the present extended algorithm,which shows good agreement with the pervious MD simulations.