Characterization of Crack Propagation along Bond Coat/Topcoat Interface of a TBC System

The crack propagation along the bond coat / topcoat interface of a TBC system has been studied in a previous work under a certain mode ratio (K2/K1). In order to discuss a parameter that describes the crack propagation, the growth behavior of a crack has to be studied under various K2/K1 ratios. In this work, BEM was used to determine the modifications that change K2/K1 ratios of the specimen employed in the previous study. Six characteristics of the specimen were modified: location of the applied load; height; length, position and direction of the notch, and its separation from the bond coat / topcoat interface. Among these modifications, the change of location of load point can effectively change K2/K1 ratio to values greater and smaller than those of the previous study. Crack propagation tests with three different positions of load point were performed using specimens with topcoat porosity of 5%, 15% and 20%. The results indicated that, independently of K2/K1 ratio, the parameter Ki dominated the crack growth behavior for an interface crack under mixed mode.