Crack-Shape Effects for Indentation Fracture Toughness Measurements

Various methods to measure fracture toughness using in-dentation precracks were compared using soda-lime glass as a test material. In situ measurements of crack size as a function of applied stress allow both the toughness K_c and the residual-stress factor χ to be independently determined. Analysis of the data showed that stress intensity factors based on classical half-penny crack shapes overestimate toughness values and produce an apparent R -curve effect. This is due to a constraint on crack shape imposed by primary lateral cracks in soda—lime glass. Models based on elliptical cracks were developed to account for the crack-shape effects.