Fatigue crack growth in laser shock peened Ti-6AL-4V aerofoils subjected to foreign object damage

摘要

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 generic aerofoil specimen of Ti-6A1-4V alloy was used. The specimens were treated by laser shock peening (LSP) to produce compressive residual stresses in the leading edge region prior to impact. FOD was simulated by firing a cubical projectile at the leading edge using a laboratory gas gun at 200 m/s, head-on; and at 250 m/s, at an angle of 45°. The specimens were then subjected to 4-point bend fatigue testing under high cycle (HCF), low cycle (LCF) and combined LCF and HCF loading conditions. A real-time computer-controlled direct durrent potential (DCPD) system and optical crack detecting techniques were used for crack monitoring. Fatigue crack growth data under low cycle fatigue (LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions were correlated using a stress intensity factor range. The influence of impact angles and loading conditions on fatigue crack growth behaviour was assessed, and the results were compared with those from untreated FODed specimens. The fatigue crack growth behaviour in laser shock peened specimens indicate that residual stresses due to LSP and FOD play a key role in influencing the fatigue crack growth, and results in a significant reduction in fatigue crack growth rates compared to those of FODed only specimens of the same alloy.