MICROMECHANICS OF SHEAR LIGAMENT TOUGHENING

A new toughening mechanism is proposed based on the fracture of shear ligaments which are formed in the process zone as the result of mismatched crack planes of deflected cracks. The anticipated toughness enhancement due to these shear ligaments has been evaluated by a micro-mechanical approach that considers the plastic dissipation associated with fracturing of the shear ligaments. Additional toughness increases due to frictional dissipation at fracture surface asperities that experience rubbing have also been considered. The amount of toughening, as represented by a toughening ratio, has been shown to depend on the ligament length, a ligament toughness parameter representing the work to fracture, the area fraction of the ligament, and the area fraction of frictional contacts, if rubbing of the fracture surfaces is present. The proposed model is used to explain roughness-induced toughness in general, and that in alpha + beta Ti-alloys and two-phase TiAl-alloys in particular.