Computational modelling of crack initiation in a single crystal superalloy under fatigue-oxidation conditions

Crack initiation in a single crystal nickel base superalloy was studied under fatigue-oxidation conditions, using a crystallographic constitutive theory in conjunction with a mass diffusion model. Finite element (FE) analyses were carried out on a notched compact tension (CT) specimen with a void close to the notch surface. The number of cycles to crack initiation was predicted using a strain ratchetting based failure criterion. Microcracks were predicted to initiate from the void due to the more severe ratchetting at the void surface. The applied load level and ratio, as well as the void location, strongly affect the number of cycles to crack initiation from the void. A high temperature oxidising environment is predicted to reduce the number of cycles to crack initiation by enhancing the ratchetting in the vicinity of the void, a consequence of microstructural degradation of the material near the notch due to oxidation linked diffusion processes at the notch root.