Improved Damage Tolerance Of Ti-6Al-4V Aero Engine Blades And Vanes Using Residual Compression By Design

The deep stable layer of compressive residual stress produced by low plasticity burnishing (LPB) has been demonstrated in laboratory testing to improve damage tolerance in engine alloys IN718, Ti-6Al-4V, Ti-6-2-4-6, and 17-4PH. This paper describes the fatigue and FOD tolerance benefits afforded by LPB treatment of a Ti-6Al-4V first stage fan blade and vane. FOD sensitive blades and vanes removed from fielded engines were LPB processed to protect the leading edge of the blade and the trailing edge of the vane. Both components were fatigue tested in cantilever bending mode at R>0 using specially designed test fixtures. FOD was simulated with machined notches for the blade and electrical discharge machined (EDM) notches for the vane. Residual stress and cold work distributions were measured using x-ray diffraction mapping techniques.
LPB produced a zone of nominally -100 ksi (-690 MPa) through-thickness compression in the leading edge of the blade and trailing edge of the vane. The HCF strength for LPB processed blades was 125 ksi (860 MPa) without FOD, and equal or greater than the as-received blades for FOD up to 0.050 in. (1.3 mm) deep - an order of magnitude improvement in damage tolerance. For both vanes and vane simulation specimens with 0.020 in. (0.5 mm) deep FOD, the HCF strength after LPB was over 4 times higher than the unprocessed counterparts. The HCF performance was largely unaffected by FOD up to 0.030 in. (0.7 mm) deep. If the traditional design criterion of Kt=3 is used, both the LPB processed blade and vane could be considered tolerant of even 0.10 in. (2.5 mm) deep FOD.
Linear elastic fracture mechanics analysis using AFGROW including the residual stress fields confirms the HCF and FOD performance and the minimal effect of stress ratio, R, in the presence of high residual compression. A novel Haigh diagram based method of predicting the improvement in damage tolerance is described and demonstrated for the Ti-6-4 blade and vane application using the fatigue data developed in the IHPTET HCF program and from the current work. The method allows estimation of the minimum compression required to achieve a desired damage tolerance or the optimum tolerance possible for a given application in terms of R-ratio, applied stress and Kt for the damage mechanism. The method provides a means of improving the damage tolerance of engine components by design, potentially reducing operating costs and improving fleet readiness by reducing inspection and maintenance requirements.