Study On The Mechanism Of Failure Wave In Brittle Materials Under Dynamic Impact Loading

Brittle materials such as glass and ceramic have been widely applied in national defense and civil industry, because of their good properties of high strength, high sound speed and low density. Brittle materials have so excellent performance especially in the field of armor protection field for enhancing effectively the living ability of military objectives such as tanks and battleships. Therefore, research on the dynamic failure mechanism of such materials has importantly scientific significance as well as widespread application background. Failure wave is a special failure mode only in glass. However, it is still disputed issue that whether such a failure wave can be generated in ceramic materials. For this purpose, in this dissertation, both the failure wave in glass and the plastic–like response in ceramic were investigated systemically, which include experimental study, numerical simulation and theoretical analysis. The main work is as follows:(1) The planar impact experiments were carried out. By using the VISAR(Velocity Interferometer System for Any Reflector) and the DISAR(Displacement Interferometer System for Any Reflector) systems, the particle velocity time–history curves on the back surfaces of the alumina ceramics specimens were measured under different experimental conditions. It is found, in alumina ceramics, that there is no recompression signal characterizing the failure wave phenomenon, but a plastic–like response behavior under impact loadings can be observed.(2) An elastic micro–crack damage model is proposed and inserted into the LS–DYNA application software to simulate the dynamic response of both the glass and the ceramic materials. The computational results are in agreement with the experimental data, which show that such a damage model is available in characterizing the dynamic response behaviors of the two kinds of material. Moreover, it is also shown that the two kinds of material have similar dynamic damage mechanism, and the difference presented in the experimental results is just due to the different microstructures and loading conditions. In addition, in the elastic micro–crack damage model, a new damage variable is defined by using the two statistic variables ? crack size and crack number density. The evolution of this damage variable can be used as a criterion for the generation of failure wave.(3) The formation mechanism of unbalance growth of the original micro–cracks for a failure wave is proposed for the dynamic failure of brittle materials, based on the mechanism of “in situ activation and growth” of initial micro–cracks and the micro–crack growth feature of brittle materials under the dynamic loading, which includes the two main aspects: one is that the original micro–crack means that existed one in the material, namely, ignoring the nucleation effect; the other one is that the micro–cracks grow near their original locations without moving to another places, and moreover, along the shock wave propagating direction, the micro–cracks at different locations grow at the different velocities after the shock wave, which means the unbalance growth.