Abstract
Fracture toughness testing of two transparent materials, fused silica and PMMA (polymethyl methacrylate), has been carried out according to a method suggested by ISRM (International Society for Rock Mechnics) in order to know the crack front evolution and also to examine the unloading compliance method for determining the crack length, and a K-resistance curve has been evaluated by the extended ISRM suggested method for these materials.The main results obtained in this study are as follows:(1) The crack front in brittle fused silica preceeds at the ends when the crack length is small, and it approaches to be a straigth line as the crack grows. On the contrary, the crack front in the ductile PMMA always preceeds at the center. Thus, the shape of the crack front depends on the material characteristics.(2) The unloading compliance method according to the ISRM suggested method can evaluate the crack length almost accurately for fused silica while it underestimates the crack length for PMMA, since the time-dependent crack growth occurs in the early stage of unloading. The gradient of a straigth portion in a load-displacement curve during unloading should be used to evaluate the crack length accurately for the materials such as PMMA.(3) A K-resistance curve can be determined almost accurately for brittle fused silica by the extended ISRM suggested method.
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