EXTRINSIC FACTORS IN THE MECHANICS OF BRIDGED CRACKS

Calculations for single-edge notch specimens under uniform remote tension are used to demonstrate the profound influence of the extrinsic factors of specimen shape and load distribution on the propagation of bridged cracks when the bridging zone length is comparable to any of the dimensions of the crack or the specimen. The influence of the extrinsic factors is so strong that there is a grave risk of seriously nonconservative predictions of strength and reliability if standard engineering methods are used for materials exhibiting such bridged cracks. However, this difficulty can be resolved by regarding the relationship between the bridging tractions p and the crack opening displacement u as a fundamental material property. Extensive calculations are summarized for one class of relations p(u) that rise to a peak corresponding to ligament failure and then fall gradually to zero at large u. Resistance curves of great variety are displayed. The central role of the "bridging length scale," the initial crack extension over which the bridging zone matures, is demonstrated. The roles of various features of p(u) in determining the transition from noncatastrophic (or ductile) failure to catastrophic (or brittle) failure are examined.