Crack-shape effects for indentation fracture toughness measurements

TLDR: In this article, the authors compared various methods to measure fracture toughness using indentation precracks using soda-lime glass as a test material and found that stress intensity factors based on classical half-penny crack shapes overestimate toughness values and produce an apparent R-curve effect.

Abstract: Various methods to measure fracture toughness using indentation 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[sub c] and the residual-stress factor [chi] 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.