R-curve and strength behavior of a functionally graded material

The effects of loading conditions, specimen size and metal particle size on the crack growth resistance curve (R-curve) and residual strength of a ceramic-metal functionally graded material (FGM) are studied based on the crack-bridging concept. It is found that the FGM exhibits strong R-curve behavior when a crack grows from the ceramic-rich region toward the metal-rich region and the residual strength of the FGM with an edge crack at the ceramic side is notch-insensitive. The R-curve is more significant under central tension loading than under pure bending after the crack has grown for a certain amount. However, the residual strength of the FGM decreases with increasing initial crack length more slowly under pure bending than under tension. A larger metal particle usually results in more toughness increase and higher residual strength. Regarding the effect of the specimen size, in terms of the relative crack length a:b, the R-curve is weaker for a smaller specimen when the crack has not grown very deep, and the opposite occurs for deep grown cracks. Based on the real crack growth length, however, the smaller specimen exhibits stronger R-curve behavior. The R-curve also significantly depends on the initial crack size. © 1998 Elsevier Science S.A