The effects of scratch damage on the fatigue performance of a nickle-based superalloy used for aerospace applications

Abstract

Surface damage can be introduced into rotor disc components during assembly, maintenance and overhaul operations for aircraft engines. This is known as handling damage and is often in the form of surface scratches, which are known to reduce total component fatigue life. This work provides a holistic understanding of the effect of artificial scratch damage on the fatigue performance of a nickel-based superalloy used for compressor and turbine disc applications.

Following extensive analysis of results from previous test programmes completed at Rolls Royce plc., a series of low cycle fatigue tests were performed on Alloy 720Li specimens. A test matrix was designed to isolate and identify every factor that may contribute to the fatigue properties of scratch damaged components. The geometry of a scratch, which causes a local stress concentration and increases the ‘effective Kt’ at the scratch root, is the most significant factor in controlling fatigue performance. Scratches cause a decrease in crack initiation life, thereby reducing total component fatigue life. Compressive residual stresses induced by shot peening and the scratching process itself are beneficial to component life.

Varying levels of scratch damage were fully characterised by scanning electron microscopy, microhardness testing and electron backscatter diffraction to show the severe microstructural modification and increased hardness caused by the scratching process. Computational modelling was carried out to determine geometrical effects of scratches on local stress and strain fields. The ‘effective Kt’ values under scratches were calculated, which helps to predict fatigue performance. This type of comprehensive investigation is important for damage tolerance lifing approaches and helps to minimise premature component retirement.