Research On Mechanism Of Crack Propagation Of Sapphire Single Crystal Under Vibration Conditions

Sapphire single crystal has numerous excellent properties, such as highhardness, high strength, good abrasive resistance, provides excellent corrosionprotection, so it is increasingly widely used in the field of civil use and nationaldefense. In particular, sapphire single crystal gradually replaces the traditionalmaterials such as spinel and become a new kind of space optical window materialsbecause of its outstanding optical transmittance.However, sapphire optical window is so fragile that it inevitably hasmicro-crack during the process of preparation and assembling. In addition, sapphireoptical window usually suffer from vibration load when it works in space. In thiscondition, the original crack will grow quicker and decrease the properties ofsapphire， even total system lose its effectiveness. Therefore, it is of importantsignificance to study on mechanism of crack propagation of sapphire single crystalunder vibration conditions.The FE analysis software Abaqus is used to simulate the crack propagation ofsapphire single crystal under the horizontal vibration condition and theperpendicular vibration condition, while the simulation is based on the exponentialfunction model of cohesive theory. It indicated that the crack propagation show achanging feature by stages and the whole process accords with the crack growthfeature of brittleness materials, which presents rapid expansion after the suddencracking until failure. Through discussed the crack growth behavior under differentvibration condition finds out that the impact of vibration frequency and amplitudeon stress increase rate and concentration degree at the crack tip region, crackinitiation time, growth rate and final length has obvious regularity. Under thehorizontal vibration condition, the effect of vibration angle on crack growth isdiscussed by adjusting Ⅰ, Ⅱ load size, it indicated that with the vibration anglesmaller, the crack initiation time will increase.According to the vibration load applied in the simulation, the experiment ofsapphire single crystal under vibration is accomplished, which prefabricated crack iscreated by indentation method. The conclusion that crack propagation is in periodicand the influence of the frequency on crack propagation are experimentally verified,although the crack propagation time obtained by experiment is quite different fromsimulation, which is caused by the limited shaker output energy, ideal simplificationin simulation, etc.The Chen-Chu crack tip dislocation theory and Zhen-Hirt model are introducedto explain the conclusions of simulation and experiment. It indicated that the crack tip of sapphire single crystal will form plastic zone by dislocation emission andbecome passive by deflection, furcation and new crack initiation, etc. Then the crackwill grow in the form of micro-zone cleavage fracture. It also indicated that thefatigue crack growth curve of sapphire single crystal is made up of thenear-threshold region, the stable propagation region and the rapid propagationregion as well as other solid materials. However, because of the small fracturetoughness of sapphire, the intermediate stable crack propagation region is so narrowthat crack will go into the rapid propagation region and grow quickly after suddencracking.