Modified flaw-size toughness technique

A modified indentation technique for measuring fracture toughness without the use of residual stress parameters was previously described by Cook and Lawn. To determine the accuracy of this technique for testing glass-ceramics, barium aluminosilicate (BAS) glass-ceramics with a variety of microstructures were examined. Comparisons were made with the fracture toughness values calculated using a novel technique. The sizes of subcritical and critical flaws were compared. The modified flaw-size technique provided a closer estimate of fracture toughness than the modified indentation technique for all of the glass-ceramics examined. The semi-elliptical shape of indenter-induced flaws is responsible for the difference between test methods. These results explain a discrepancy between the theory and regression model of Cook and Lawn.