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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 250m/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.

The current study investigates the effect of foreign object damage (FOD) on the pre-existing compressive residual stress field associated with laser shock peening (LSP) and its evolution upon combined LCF/HCF cycling. FOD was introduced onto an aerofoil-shaped specimen that had been previously LSP treated through ballistic impacts at angles of 0° and 45° to the leading edge. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0° impact. Significant through thickness compression was introduced parallel to the leading edge as a result of the LSP process. The residual strain distribution was mapped around the FOD notch by synchrotron X-ray radiation. The results show predominantly compressive stresses ahead of the notch, being greater for the 0 compared to 45 impact. No significant stress relaxation was observed after a combined (1000 HCF cycles superimposed on 1 LCF cycle) cycle.

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-6Al-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. omenclature A = amplitude of oscillation a = cylinder diameter C p = pressure coefficient Cx = force coefficient in the x direction Cy = force coefficient in the y direction c = chord dt = time step Fx = X component of the resultant pressure force acting on the vehicle Fy = Y component of the resultant pressure force acting on the vehicle f, g = generic functions h = height i = time index during navigation j = waypoint index K = trailing-edge (TE) nondimensional angular deflection rate