| Surface enhancement treatment by shot peening has been widely used in industrial applications, especially for aircraft engine components. Typical peening processes use small shots of a few hundred micrometer in diameter blasted on component surfaces, resulting in residual stress near the surface in the depth range of a few hundred micrometers nominally. Compressive surface residual stress is useful for improving crack initiation resistance that prolongs service life of the part. To implement this highly desirable maintenance strategy, an in-service nondestructive method is needed to monitor the residual stress state of parts periodically, so that appropriate maintenance actions can be taken when residual-stress protection is lost, by either replacing or re-treating the part. X-ray and neutron diffraction methods are the only two standard methods considered the most reliable. However, conventional XRD methods can achieve relatively low penetration depth (<10 mum for most metals), and hence destructive layer removals are needed for measuring residual stress profiles which typically range from 200 mum to 2000 mum in depth for shot-peened materials of practical interest. Neutron diffraction method has also a practical limitation in terms of its cost and resulting radioactivity. In this dissertation, we developed a swept high frequency eddy current (SHFEC) measurement methodology for conductivity characterization of shot peened nickel based alloys. A model-based, eddy current inversion method is presented and applied to the SHFEC data obtained from a series of shot peened nickel based alloys to determine the depth profiles of actual conductivity up to 400 mum below the samples' surfaces. By developing a modified piezo-resistivity theory that includes the effect of texture on stress-induced conductivity changes, the residual stress profile of a shot peened IN718 sample is obtained from eddy current data. The obtained residual stress depth profile agrees with that measured by the standard layer removal XRD method. Texture profile of the shot peened IN718 sample is demonstrated by an XRD partial pole figure and orientation image microscopy (OIM). A new procedure of analyzing conventional theta--2theta XRD data is also developed in this dissertation for determining residual stresses in shot peened surfaces assuming an isotropic plane stress state. Collectively, this work lays foundation to the eddy current technique to assess residual stress in shot peened nickel based alloys that have extensive applications in aircraft engines. |
