Fracture mechanics analysis of crack shapes due to cyclic loading in baria-silicate glass

The objective of this study was to determine the geometric characteristics associated with the critical crack caused by cyclic loading. In an attempt to simulate an intraoral loading condition, the Hertzian cyclic loading of baria-silicate glass was performed using a type 302 stainless steel indenter under an aqueous environment using clinically relevant parameters, i.e., a low loading frequency (∼3 Hz) and a low load level (≤200 N). The indenter diameter (4.76 mm) approximated the cuspal radii of molar and premolar teeth. Ten bar specimens each were subjected to loading cycles of 0, 10³, 10⁴, and 10⁵ cycles. A four-point bending test was used to quantify the severity of the strength reduction caused by the repeated loading test. There was a decrease in fracture stress after 10³ cycles that was associated with cone crack formation. No significant additional reduction was found after 10⁵ cycles for specimens tested both in air and in deionized water. Stress-corrosion fatigue accelerated the surface crack propagation rate in baria-silicate glass specimens. Four different crack geometries were identified along with failure mechanisms. Various fracture mechanics approaches were tested against observed crack geometries. The previously unobserved triangular crack geometry was found.