KSEF R&D Excellence: Stress and Texture in Surface-Enhanced Titanium Alloys: Their Nondestructive Inspection and Effects on High-Cycle Fatigue Properties

Surface conditioning such as shot peening, laser shock peening, and low plasticity burnishing 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 U.S. 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 induce stress relaxation to occur. This proposed project, which will be focused on Ti-6AI-4V (a titanium alloy commonly used for turbine engine fan blades) and the emerging surface-enhancement technique of low plasticity burnishing, 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) 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 and texture on the high-cycle fatigue performance of Ti-6AI-4V samples. Our objectives are as follows: . To investigate the possible change in subsurface texture induced by low plasticity burnishing as well as the effect of this change on fatigue behaviour and on ultrasonic evaluation of stress in Ti-6AI-4V samples. . To study how different stress profiles affect the fatigue properties of Ti-6AI-4V samples, and to determine the preferred subsurface condition for optimal fatigue performance. The proposed research will be conducted in close consultation and collaboration with the Behavior and Life Prediction Branch and the Nondestructive Evaluation Branch of the Materials and Manufacturing Directorate, Air Force Research Laboratory.