Shot-peening intensities vs. eddy current signals as seen in iterative treatment-measurement experiment

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Nakagawa, Norio
Frishman, Anatoli
Shen, Y.
Lo, Chester
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Center for Nondestructive Evaluation

We report on progress in a swept high frequency eddy current (SHFEC) technique for characterization of surface residual stress on shot‐peened superalloy surfaces. Our aim here is to demonstrate the sensitivity of our measurement for practical shot peening intensities, i.e. at 4 ∼ 6 A. First, we present our improved probe and instrumentation being sufficiently sensitive to resolve the surface conditions at these low Almen intensities, where our earlier measurements encountered noise problems. The previous coil was also larger (18 mm in diameter) than desirable. Our new probe integrates smaller coils (12 mm in diameter, forming an AC bridge) and on‐board electronics on a common printed circuit board, mutually connected at the shortest possible distance. The operational‐amplifier‐based electronics acts as impedance buffers, and maintains the cabling impedance at the characteristic 50 Ω between the probe board and the instruments. We have thus reduced the instrumentation noises. Second, we present the result of an iterative treatment‐measurement experiment, performed on a 2”‐by‐3” Inconel 718 block specimen, initially polished to a mirror finish. After an initial baseline SHFEC measurement, we performed shot peening, an Almen strip deflection measurement, and a SHFEC measurement as one iteration cycle, and repeated the cycles multiple times at predetermined intervals. We will show the resulting SHFEC signals (i.e. lift‐off normalized vertical‐component signals) plotted against the Almen intensities. We then draw several conclusions from the experimental data, including a) the SHFEC signals increase monotonically in correlation with the Almen intensity increase, and b) the SHFEC signals exhibit sufficient deviations to resolve 4 ∼ 6 A intensities, while c) the SHFEC signals indicate saturation of the Inconel 718 response against peening, but the saturation occurs later in the iteration than indicated by the A‐series Almen strip.


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This article appeared in AIP Conference Proceedings, 1096 (2009): 1356–1362 and may be found at:

Tue Jan 01 00:00:00 UTC 2008