This paper reports on development of an electromagnetic method for residual stress characterization in shot-peened aerospace materials.  The residual stress improves crack-initiation resistance of aerospace components, thus prolonging their service life. Due to possible in-service stress relaxation, component life extension can only be realized if the residual stress profile, which typically extends to about 200 μm, can be characterized nondestructively. X-ray diffraction measurement is limited to about 20 μm thick surface layer. Detection of sub-surface stress requires removal of surface layer, rendering the technique destructive. In this paper, we present the work on an eddy current (EC) technique that combines sweep-frequency EC measurement and model-based inversion for assessing residual stress in nickel-based superalloys. We have developed a high-frequency EC measurement system and proprietary probes fabricated by the PCB technology that can operate up to 50 MHz with the smallest penetration depth of 80 mm. An experimental procedure was devised to extract from EC data the horizontal component reflecting lift-off effects, and the vertical component which is lift-off free and is related to conductivity change. A numerical simulation based on the Born approximation shows that surface roughness affects the horizontal component somewhat, but has virtually no effects on the vertical component.  Measurements on Inconel 718 specimens have shown a clear difference before and after shot-peening in the vertical component.  The EC signals were then inverted via a multi-layer model to yield conductivity depth profile, from which residual stress and cold work profiles can be determined when a material-based profile model is assumed. The forward and inverse models themselves were validated with simulated layer specimens.
This work was performed at the Center for NDE at Iowa State University with funding from the Air Force Research Laboratory through S&K Technologies, Inc. on delivery order number 5007-IOWA-001 of the prime contract F09650-00-D-0018.