On The Fracture Mechanism Of Transparent Ceramics Under Shock Wave Compression

Understanding the shock fracture mechanism of transparent ceramics is important to the enhancement of shock resistibility of bulletproof window,missile fairing,and anti-fall mobile phone screen.Brittle materials such as ceramics exhibit unique compression damage and failure under shock wave compression,but the analysis and inversion of experimental results cannot directly reveal the internal process of failure and the mesoscopic evolution mechanism inside the material.The purpose of this paper is to simulate the shock failure process of polycrystalline transparent ceramics,and to reveal the physical mechanism of impact damage,which would provide physical understanding for preparing new transparent ceramics with excellent shock resistibility.A "lattice spring model" that effectively describes the impact of large deformation and a large number of cracks on brittle materials is used as a calculation method.An impact compression failure model of polycrystalline transparent ceramics containing grains,grain boundaries and different crystal directions was established.The "Vorinoi tessellations"technology is ultilized to construct the grain structure and the random grain orientation for different grain is given;the finite element-discrete element quantitative parameter mapping method proposed by Gusev is used to calculate model spring stiffness coefficient;the fracture energy criterion based on Griffith energy balance principle is used to describe the fracture in medium,and set different fracture strength for grains and the grain boundary.The study of mesoscopic evolution indicates that the shock wave front propagating in the heterogeneous transparent ceramics shows inhomogeneity.In the Hugoniot state,most of the grains have become the plastic deformation,but a small amount of grains are still in the elastic deformation stage due to its orientation.Cracks nucleate at grain boundaries and when the crack propagates to the grain boundary,in one case,the cracks propagate along the grain boundary and no longer enters the inner part of the grain;in another case,the cracks enter the adjacent grain,but the direction of propagation is deviated from the original direction,and the crystallographic orientation would effect the path of cracks.The calculation results of macro shock response and the association between meso damage and macroscopic response show that,when the impact pressure is below HEL,intergranular fracture occurs and it does not significantly affect the overall elastic response,and it is a single elastic shock wave.In contrast,dramatic changes in the mode of fracture behaviour dominated by transgranular fracture were found at stresses greater than HEL,causing the nominal macroscopic plastic response and the shock wave broadening into a double-wave structure consisting of elastic wave and deformation wave.The HEL decreases with the increase of materials thickness,and the elastic precursor will decay as the shock wave evolves.The relationship between the amplitude of Hugoniot elastic limit value and the propagation distance is an exponential function,although the decay rate is different under different impact pressure.