Influence of macroscopic shear loading on the growth of an interfacial crack initiated from a ceramic blister processed by laser shock

An original methodology was developed to analyze the interfacial toughness of a ceramic coated superalloy. Cobalt based superalloy cruciform specimens were coated by air plasma sprayed alumina. A circular crack of a few millimeters processed by laser shock was introduced at the interface in order to form a circular blister of the layer. Macroscopic shear loading was then applied to the pre-cracked system using a biaxial device. The experimental analysis has showed that initial circular blister and interfacial crack lost their circularity when macroscopic shear loading was applied. Image analysis has enabled to quantify the interfacial crack growth rate as a function of shear level. A Finite Element Analysis (FEA), using both Linear Elastic Fracture Mechanics (LEFM) and Cohesive Zone Model (CZM) of the ceramic/metal interface, has been achieved taking into consideration the 3D blister shape. Crack opening and closure associated to the blister shape under macroscopic shear loading have been evidenced to directly influence crack growth rate. The final shape of the interfacial crack was seen as a direct consequence of mode II, dominant failure mode in the analyzed case. Indeed, local analysis has shown that, for macroscopic shear loading, the crack front experiments a continuous variation of mode mixity as a function of the observed angular position.