CRACKING OF LAMINATES SUBJECTED TO BIAXIAL TENSILE STRESSES

During the processing of laminar ceramic, biaxial residual stresses ca n arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the lay er and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that f or a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. He re, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analy sis, laminates composed of alternating layers of zirconia and alumina/ zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the bi axial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and t heir extension into the adjacent layers. These observations were compa red to the theoretical predictions.