Effect of laser peening and shot peening on fatigue striations during FCGR study of Ti6Al4V

The fatigue life of steam turbine blades is enhanced by subjecting its fir tree blade root section to mechanical treatments like shot peening and laser peening to introduce compressive stresses in the surface layers. In the case of higher power rated steam turbines, light alloys such as Ti6Al4V are now being adopted in the last stage blades since their exit diameters are very large and hence it is not technically possible to use steels for these machines due to the enormous stresses that would be experienced at the blade roots. This paper studies the effect of shot peening and laser peening on the fatigue crack growth of Ti6Al4V alloy at a stress ratio R of 0.1 and the changes observed in the fracture microstructure during the fatigue crack growth (FCGR). In specific, study of the striations developed during the FCGR on the fracture surface at regular intervals of crack movement have been recorded; an attempt has been made to correlate the striation spacing with crack front growth. It is observed that the fatigue life in the initial stages of FCGR testing is affected by the shot peening effect whereas it is more affected by laser peening effect at higher values of delta K (ΔK). The effect of closure is seen in both the shot peened and laser peened samples to different degrees.