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来制造涡轮机等机械部件。然而，高性能陶瓷的使用需要进一步研究动态断裂力学。在通常情况下，冲击载荷作用于结构上，应力波斜向冲击结构部件中的各种裂纹状缺陷。应力波在裂纹尖端产生混合应力状态。为了准确评价复合型冲击断裂的动态起裂准则，提出了一种新的复合型冲击断裂试验方法--中心预裂纹圆盘试件法。通过改变中心预裂纹的倾斜角度，可以进行任意组合的I型和II型冲击断裂试验。由裂纹起裂时的临界动态应力强度因子确定了Ⅰ型和Ⅱ型的动态断裂韧度值。动态应力强度因子的计算是通过叠加阶跃响应函数相对于在断裂试验中测得的冲击载荷历史。对预裂纹圆盘试件进行有限元分析，得到了阶跃响应函数。在本研究中，对氧化铝陶瓷和一些脆性材料进行了复合加载下的动态和静态断裂试验。将动态结果与径向压缩断裂试验中获得的准静态数据进行了比较，并与本研究中提出的一个简单的经验公式进行了拟合。

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.