Controlled Crack Shapes for Indentation Fracture of Soda–Lime Glass

Radial cracks for indented soda–lime glass aged in distilled water were highly elliptical because of truncation by lateral cracks. Indentation in silicone oil minimized radial/lateral crack interaction but still produced cracks having nominally constant ellipticity during bend testing. Analysis of applied stress/indentation crack length data using stress intensity factors based on half-penny crack shape resulted in apparent R -curve behavior and/or overestimation of the fracture toughness. Incorporation of elliptical shape factors eliminated the R -curve behavior and reduced measured toughness to near the accepted value for soda–lime glass.