Evaluation of Fracture Toughness Measurements Using Chevron-Notched Silicon and Tungsten Microcantilevers

Measuring the fracture behavior of small specimens of semibrittle materials such as tungsten is often difficult due to the lack of crack stability and a high ratio of the crack tip plastic zone size to specimen dimensions. To overcome this, microcantilever bending tests were used with a stable chevron notch geometry coupled with elastic–plastic fracture mechanical (EPFM) analysis. The chevron notch geometry was first validated by measurement of the \( (111) \) cleavage toughness in single-crystal Si, then fracture resistance curves (R-curves) were calculated via EPFM analysis of fracture data obtained from a semibrittle W-1%Ta alloy. The accuracy of the fracture resistance curves measured from W-1%Ta was evaluated by means of ASTM standard macroscopic fracture tests. The conditional fracture toughness (KQc) prior to crack instability was found to be five times larger than the macroscopic fracture toughness (KIc), due to the combination of plastic tearing of ductile ligaments and the extensive microplasticity ahead of the crack tip. These results suggest that use of chevron-notched microcantilevers is suitable for evaluating the fracture toughness of brittle silicon but overestimates the fracture toughness value for semibrittle tungsten.