Dynamic fracture initiation criterion in ceramics under combined mode impact loading

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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.