Mechanics and 1hermodynamics of Britile In1erfacial Failure in Bimaterial Systems

We discuss elastic-brittle fracture theory for cracks along interfaces between elastically dissimilar solids. Crack tip fields in such cases are characterized by one real and one complex stress intensity factor, where the latter couples two of the classically separate crack tip modes. Solutions for complex stress intensity factors for a variety of cases, including geometries of interest for toughness testing, are cataloged and the theory is applied to the problem of dislocation nucleation from a crack tip along a dissimilar material interface, such as a metal/ceramic interface. The variation of the-local phase angle, indicating stress field mode coupling, at the near atomic scale of dislocation nucleation is an important aspect of the problem. The dislocation problem arises in evaluating the competition between atomically brittle decohesion and plastic blunting at interfacial crack tips. Results confirm that both the properties of the interface and the direction of attempted cracking along it are important to the outcome of that competition; the latter arises because of the different stressing of slip systems associated with different directions of cracking. We also briefly review effects of solute adsorption at ceramic and metal/ceramic interfaces on their embrittlement, or in some cases on their ductilizing. Finally, we outline the thermodynamic interrelations between adsorption and alterations of the ideal work of interfacial separation, and formulate the. thermodynamic theory in a manner consistent with the possibility of strong dissimilarity in adsorption properties of the tWo surfaces which joined at the interface before fracture.