HCF Performance and FOD Tolerance Improvement in Ti-6Al-4V Vane with LPB Treatment

Abstract

Mechanical surface treatments that introduce a layer of residual surface compression improve high cycle fatigue (HCF) performance. If the depth of compression extends through the thickness of blade or vane edges, foreign object damage (FOD) tolerance can be dramatically improved. The effect of low plasticity burnishing (LPB) on the HCF performance and FOD tolerance of a first stage Ti6Al-4V turbine engine vane have been investigated in both tension-tension (R=0.1) and fully revered bending (R=-1). Actual vanes from fielded engines and blade-edge feature samples were fatigue tested with FOD simulated by EDM notches. The fatigue strength for LPB processed blades increased over 4-fold for both vanes and vane-edge feature specimens with FOD 0.020 in. deep, and was undiminished by 0.030 in. deep FOD. Assuming a Kt = 3 HCF performance criteria, LPB provided tolerance of FOD up to 0.10 in. deep. The beneficial through-thickness compression was retained even for compressive loading in fully reversed bending. The fatigue and FOD tolerance improvement are shown by linear elastic fracture mechanics modeling to be due to the deep stable compressive layer produced by LPB.