Failure analysis of scratch damaged shot peened simulated components at high temperature

The damaging effect of scratches in both as-machined and shot peened surfaces on fatigue behaviour was investigated for an advanced nickel base superalloy. Experimental fatigue tests and elastic–plastic FE analysis were carried out on specimens representative of a critical region of a gas turbine aero engine compressor disc operating under extreme conditions of stress and temperature. In some specimens scratches were created with a cutting tool in order to create a damaging effect in the shot peened surface. The scratches had 50 and 100 μm depth, lying within the layer of compressive residual stress created by shot peening. From fatigue tests it was observed that shot peening has a strong benefit in early crack propagation, this effect being less pronounced when a scratch is created. However many of the specimens tested did not fail after a large number of cycles. A detailed study of the fracture surface, for scratch damaged shot peened surfaces, was conducted in order to better understand the mechanism of crack growth and the reasons why some specimens did not fail after a large number of loading cycles. Observations revealed distinct changes of grow mechanism close to the scratch root. It was found that multi-initiation sites occurred along the root of the scratch which appeared to rapidly coalesce to form a long shallow crack running the length of the scratch, even for those specimens that did not fail. The results of the FE study are used to complement the observations and to explain some of situations when the crack arrested.