Dynamic fracture initiation criterion in ceramics under combined mode impact loading

陶瓷工业的最新进展使我们能够使用高性能陶瓷制造机械部件，如涡轮机。然而，高性能陶瓷的使用需要在动态断裂力学方面进行进一步的研究。在一般情况下，当冲击载荷作用于结构时，应力波会斜向冲击结构部件中的各种裂纹状缺陷。应力波在裂纹尖端周围产生混合应力状态。因此，必须明确组合载荷下的动态起裂准则，以评估与冲击断裂有关的结构完整性。为了准确地评估动态断裂准则，我们开发了一种利用中心预裂盘试件进行混合模式冲击断裂试验的新方法。通过改变中心预裂纹的夹角，可以在I型和II型的任意组合下进行冲击断裂试验。模式I和模式II的动态断裂韧性值由断裂起裂时的临界动态应力强度因子确定。动态应力强度因子是通过将阶跃响应函数与断裂试验中测得的冲击载荷历史叠加来计算的。对预裂盘试件进行了有限元分析，得到了阶跃函数。使用裂纹尖端附近的应变片来检测断裂的开始。在目前的研究中，对氧化铝陶瓷和一些脆性材料在组合模态载荷下进行了动静态断裂试验。动态结果与直径压缩断裂试验中获得的准静态数据进行了比较，并拟合了本研究提出的一个简单的经验准则。

Recent progress in ceramics industries has enabled us to use high performance ceramics for manufacturing components of machinery such as turbines. However, the use of high performance ceramics has required further investigation in dynamic fracture mechanics.In an ordinary case that an impact load acts on a structure, stress waves obliquely impinge to various cracklike flaws in the structural components. The stress waves produce mixed-made stress states around the crack-tips. Thus the dynamic fracture initiation criterion under combined mode loading has to be made clear for assessments of structural integrity in relation to the impact fracture.For the accurate evaluation of the dynamic fracture criterion, we have developed a new test method for mixed-mode impact fracture using the centerprecracked disk specimen. The impact fracture tests canbe performed under arbitrary combinations of mode I and II by varying the inclibation angle of the centerprecrack. The dynamic fracture toughness values of mode I and II are determiced form critical dynamic stress intensity factors at fracture initiation. The dynamic stress intensity factors are calculated by superposing the step-response functions with respect to the impact load history measured in the fracture test. The stepresponse fuctions have been obtained from finite element analyzes for the precracked disk specimen. The onset of fracture initiation is detected using the strain gage near the crack-tip.In the resent research, dynamic and static fracture tests are performed for alumina ceramics and some brittle materials under combined mode loadings. The dynamic results are compared with the quasi-static data obtained in diametralcompression fracture tests, and fitted to a simple empirical eriterion proposed in this study.