Fatigue crack growth in laser shock peened aerofoils subjected to foreign object damage

Foreign Object Damage (FOD) is one of the main life limiting factors for aeroengine fan blades. The FOD impacts during takeoff and landing cause severe damage to aerofoils, resulting in reduced air safety and life time with an estimated annual cost of $4 billion for the aeroengine industry. Advanced surface treatments, such as Laser Shock Peening (LSP) have significantly improved the fatigue strength and crack growth resistance of critical components under FOD. However, it is not yet possible to predict the protective residual stresses and utilise their full potential for enhancing fatigue resistance and damage tolerance capacity in service. This research programme aims to utilise some of the established methods for fatigue tolerance assessment of critical components, based on fracture mechanics principles, to address the effects of complex residual stresses due to LSP and FOD on fatigue crack growth in aerofoils under simulated service loading conditions. The experimental study involved fatigue testing of LSPed and FODed specimens with a geometry representative of fan blades made from Ti-6Al-4V alloy. A four point bend fatigue test setup was designed and calibrated. A real-time computer-controlled crack growth monitoring system and optical crack monitoring techniques were developed. Scanning Electron Microscopy (SEM) and Back-Scatter Electron (BSE) were used to conduct metallographic and fractographic studies, including crack initiation, early fatigue crack growth and FOD damage characterisation. The fracture mechanics analyses used the weight function method and the finite element method to obtain a modified stress intensity factor considering residual stresses due to LSP and FOD. Fatigue crack growth data under low cycle fatigue(LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions were correlated using a standard and the modified stress intensity factors. 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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Evolution of a laser shock peened residual stress field locally with foreign object damage and subsequent fatigue crack growth

Foreign object damage (FOD) can seriously shorten the fatigue lives of components. On the other hand, laser shock peening improves fatigue life by introducing deep compressive residual stress into components. In this paper we examine how the non-uniform steep residual stress profile arising from FOD of laser peened aerofoil leading edges varies as a function of fatigue crack growth under high cycle fatigue and mixed high and low cycle fatigue conditions. The ballistic FOD impacts were introduced by impacting a cube edge head-on (at an angle of 0°) to the leading edge. The residual stress distributions have been mapped by synchrotron X-ray diffraction prior to cracking and subsequent to short (∼1 mm) and long (up to 6 mm) crack growth. The results suggest that the local residual stress field is highly stable even to the growth of relatively long cracks

Assessment of specific contribution of residual stress generated near surface anomalies in the high temperature fatigue life of a René 65 superalloy

International audienceThis study evaluates the influence of residual stresses induced by the fabrication of surface anomalies on the fatigue crack growth in a nickel based superalloy. To separate the notch effect of the geometry from the residual stress field induced by fabrication of the surface flaws, two V‐type anomalies are considered: scratches and dents with equivalent morphology and size. A specially designed heat treatment has been used to reduce the magnitude of residual stresses around these anomalies in order to highlight their effects on the different stages of the crack propagation, under low cycle fatigue conditions at 400 °C. The crack initiation life is short for both anomalies but in the presence of compressive residual stresses, a decrease of the fatigue crack growth rate has been observed during the first stages of the crack propagation. Furthermore, the results showed that without residual stresses, scratches and dents exhibit the same behaviour. Thus, the residual stress field below surface anomalies is the main parameter controlling the fatigue life from surface anomalies