Characterization of foreign object damage (FOD) and early fatigue crack growth in laser shock peened Ti-6AL-4V aerofoil specimens

Abstract

Foreign object damage (FOD) has been identified as one of the primary life limiting factors for fan and compressor blades, with the leading edge of aerofoils particularly susceptible to such damage. In this study, a typical aerofoil specimen of Ti-6Al-4V alloy was used. The leading edge of the specimen is representative of a generic aerofoil geometry. The specimens were treated by laser shock peening (LSP) to generate protective residual stresses in the leading edge region prior to impact. FOD was simulated by firing a cubical projectile at 250 m/s head-on at the leading edge using a laboratory gas gun. The specimens were then subjected to fatigue testing under high cycle (HCF), low cycle (LCF) and combined LCF and HCF loading conditions. Scanning electron microscopy (SEM) was used to characterise the damage features due to FOD. Crack initiation and early crack growth due to FOD and subsequent fatigue loading were examined in detail. An analysis with a backscatter electron (BSE) detector was also carried out to investigate the microstructual deformation due to FOD, LSP, as well as the early fatigue crack growth mechanism in a complex residual stress field.