EFFECT OF THERMOELASTIC ANISOTROPY ON THE R-CURVE BEHAVIOR OF MONOLITHIC ALUMINA

The postfracture tensile (PFT) test has previously been used to evalua te the wake stresses of both cubic and noncubic monolithic ceramics at room temperature. The present study applies this procedure to the eva luation of the characteristic stress-crack face separation relationshi ps for alumina at temperatures ranging from room temperature through 6 00 degrees C, which is slightly below the softening temperature for th e glassy grain boundary phase. For comparison with the PFT behaviors, R-curves are obtained throughout this range of temperatures using a sh ort double cantilever beam geometry. The average residual stresses ari sing from thermal expansion anisotropy diminish with increasing temper ature, causing two effects evidenced by a general downward shift of th e characteristic PFT stress-displacement curves. The resulting lower p eak stresses and smaller limiting crack opening displacements (COD) ma nifest through a decrease of the COD-grain size ratio criteria for bri dging. Also, the residual stresses contribute to a significant fractio n of large grains fracturing transgranularly. This fraction is modifie d somewhat by test temperatures of 800 degrees C and above; however, n o detectable change can be claimed for the present results, which are limited to 600 degrees C.