Micro-scale Effect On Dynamic Void Growth And Yield Behavior Of Porous Material Under Impact Loading

The processes of void nucleation, growth and coalescence are the principal mechanism in material ductile fracture. Once voids have been nucleated, they grow and then coalesce through plastic deformation and eventually render the ductile materials to fracture. During these three continuous phases, the void growth lasts for the longest time and has close relation to the safety life of the materials and structures, thus it is of great worth to be studied.In the present paper, a spherical cell model containing a spherical void is investigated. To describe the strain rate and size effects of the matrix material on the void growth and the macroscopic yield of porous material, the rate-dependent strain gradient plasticity constitutive theory by Fleck and Hutchinson is adopted. Following Gurson, the relation between microscopic fields and macroscopic responses under high-rate impact loading is developed. On these bases, we investigate in detail the void growth and the dynamic yield behavior of porous material under the high rate impact loading with a special focus on the coupling influences of scale effect, inertial effect and rate-dependent effect. The numerical results indicate that:(1) The macroscopic yield surface expands outward when the scale effect is considered, and the equivalent stress becomes higher even for the same hydro-stress. The scale effect decreases the rate of void growth under the same triaxial stress. For the typical spherical symmetrical loading case, the scale effect increases the threshold stress of the void growth.(2) The inertial effect also makes yield surface expand outward, and for the void with larger radius, this phenomenon becomes more notable. There exists a critical stress triaxiality, the inertial effect accelerates void grow when the stress triaxiality less than this critical value, otherwise, the inertial effect decreases the rate of void growth. Besides, the inertial effect increases with increasing impact loading acceleration.(3) The inertial effect and size effect seem to be uncoupled. When the void size is larger, the inertial effect is dominant and the size effect is negligible, and vice versa. Under higher stress triaxiality, both the inertial effect and the size effect suppress markedly the void growth.(4) The material rate sensitivity also makes yield surface expand outward and decreases the rate of void growth; moreover, it makes the scale effect more remarkable. For the spherical symmetrical loading case, the threshold stress of void growth increases with the rate sensitive coefficient increasing.