DEPSCOR: Residual Stress, Micra and Macro-Texture in Surface-Enhanced Titanium Alloys: Their Nondestructive Inspection and Effects on Hihg-Cycle Fatigue Properties

. ~urf~ce conditioning such as shot peening, laser shock peening, and low plasticity burnIshmg Imparts on the metal parts so treated a thin layer of subsurface compressive residual stress, which greatly enhances the high-cycle fatigue performance of the parts. Both the Air Force and manufacturers of aircraft engines are very interested in incorporating the beneficial effect of residual stress into component-life predictions. To this end, (currently unavailable) nondestructive methods must be developed by which the profile of the subsurface stress layer can be accurately and reliably measured or inferred, both before and after the component is exposed to conditions common in the engine environment, e.g., cyclic thermal and mechanical loading, which may force stress relaxation to occur. This proposed project comprises two parts. The main objective of the first part is: . To develop a nondestructive ultrasound technique to extract information on the profile of the inhomogeneous residual stresses induced by low plasticity burnishing (LPB) and by shot peening on Ti-6AI-4V samples. In our efforts we will continue and extend the very recent and promising exploratory study by Man, Koo (GE Aircraft Engines), and Shepard (AFRL/MLLMN), where they successfully inferred some characteristics of the residual stress profile in a LPB-treated Ti-6AI-4V sample from the dispersion of Rayleigh waves in that sample. The second part of the proposal concerns the effects of subsurface compressive residual stress, micro- and macro-texture on the high-cycle fatigue performance of titanium alloys. Our objectives are as follows: . To study the effects of micro- and macro-texture on the high-cycle fatigue behavior of alpha or near alpha Ti alloys with a view to building up a model that quantifies the process of short fatigue crack propagation in these alloys. . To investigate the possible change in subsurface texture induced by low plasticity burnishing and shot peening as well as the effect of this change on fatigue behaviour and on ultrasonic evaluation of stress in Ti-6AI-4V and other Ti alloys. . To study how different stress profiles affect the fatigue properties of the Ti alloys, and to determine the preferred subsurface condition for optimal fatigue performance.