STRESS REDISTRIBUTION IN CERAMIC METAL MULTILAYERS CONTAINING CRACKS/

Multilayer systems comprising either Al or Cu bonded to sapphire have been used to evaluate stress redistribution effects around cracks, cau sed by dip in the metal. Single precracks have been introduced into th e outermost sapphire layer and strain distributions measured around th ose cracks upon loading. Both moire interferometry and (Cr3+) fluoresc ence piezospectroscopy have been used for that purpose. Finite element calculations of the plastic zone size, L(s), the crack opening displa cement, delta, and the strains have been made, with the yield strength s of the metals, sigma(o), regarded as unknowns. Comparison of L(s) or delta with the measurements establishes sigma(o). Once the relevant y ield strengths have been determined, the calculated stresses and strai ns (as redistributed by slip in the metal layer) are found to be in re asonable correspondence with the values measured by either moire inter ferometry or piezospectroscopy. Some small discrepancies remain, which have yet to be explained.