TOUGHENING OF LAYERED CERAMIC COMPOSITES WITH RESIDUAL SURFACE COMPRESSION

Effects of macroscopic residual stresses on fracture toughness of mult ilayered ceramic laminates mere studied analytically and experimentall y, Stress intensities for edge cracks in three-layer, single-edge-notc h-bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a fu nction of the crack length by the method of weight function. The selec ted weight function and the method of calculation were validated by ca lculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress-intensi ty equation recommended in ASTM Standard E-399. The stress-intensity c alculations for the three-layer laminates with the macroscopic residua l stresses mere used to define an apparent fracture toughness, The the oretical predictions of the apparent fracture toughness were verified by experiments on three-layer SENB specimens of polycrystalline alumin a with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer, A residual compression of similar to 400 MPa developed in the outer l ayers by the constrained transformation of the unstabilized zirconia f rom the tetragonal to the monoclinic phase enhanced the apparent fract ure toughness to values of 30 MPa . m(1/2) in a system where the intri nsic fracture toughness was only 5 to 7 MPa . m(1/2).