The effect of fatigue on residual peening stresses in aerospace components

The Rolls-Royce Wide Chord Fan Blade (WCFB) is an innovative aerospace component, manufactured from Ti-6A1-4V alloy. The blade roots experience fatigue loading during flight, and are subject to fretting fatigue where they contact the fan disc. Shot peening (SP) and laser shock peening (LSP) are mechanical surface treatments which have been applied to the blade root to improve fatigue strength and damage tolerance. Both techniques cause local plastic deformation of the material, resulting in a layer of compressive residual stress. The compressive stress from LSP may reach a depth of 2mm in titanium, compared with only 0.25mm for conventional SP. These compressive stresses are balanced by subsurface tensile stress. LSP is reported to cause less work hardening than shot peening. It is important to understand the stability of the residual stresses during service conditions so that accurate fatigue life assessments can be made. Fatigue testing has been carried out on blade root section test pieces and on notched three point bend samples. Residual strain and stress measurements have been made on both types of sample. Residual strain measurements have been made using monochromatic and pulsed time-of- flight neutron diffraction techniques. The large penetration lengths of these types of radiation allow accurate residual strain measurements to be made nonrdestructively, deep inside a component. Because no material removal is required, no stress relaxation occurs, allowing the measurement of balancing tensile stresses. LSP and SP samples have been studied, both in the as-received condition and after fatigue.