COMBINED MODE-I AND MODE-II FRACTURE OF MONOLITHIC CERAMICS

The mode I, mode II, and combined mode I-mode II fracture behaviors of a coarse-grained silicon nitride, a fine-grained silicon nitride, and an alumina were investigated. These ceramics were fractured from two types of fracture initiating flaws: small surface flaws and large sing le edge precracks. The small surface flaws were introduced by Knoop in dentation in flexural samples at various angles to the tensile stress direction and fractured in four-point bending. The samples with large precracks were fractured in the asymmetric four-point-bend geometry. T he mixed-mode fracture toughness values obtained from the two flaw con figurations were in good agreement with each other. All three ceramics displayed very similar mixed-mode fracture behavior, although their m icrostructures were not similar. Comparison of experimental data to mi xed-mode fracture theories revealed that the minimum strain energy den sity theory best described the mixed-mode fracture behavior of all thr ee ceramics.