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Title: Validation of a Residual Stress Measurement Method by Swept High-Frequency Eddy Currents

Abstract

This paper reports on a swept high-frequency eddy current (SHFEC) measurement method developed for electromagnetic nondestructive characterization of residual stresses in shot peened aerospace materials. In this approach, we regard shot-peened surfaces as modified surface layers of varying conductivity, and determine the conductivity deviation profile by inversion of the SHFEC data. The SHFEC measurement system consists of a pair of closely matched printed-circuit-board coils driven by laboratory instrument under software control. This provides improved sensitivity and high frequency performance compared to conventional coils, so that swept frequency EC measurements up to 50 MHz can be made to achieve the smallest skin depth of 80 {mu}m for nickel-based superalloys. We devised a conductivity profile inversion procedure based on the laterally uniform multi-layer theory of Cheng, Dodd and Deeds. The main contribution of this paper is the methodology validation. Namely, the forward and inverse models were validated against measurements on artificial layer specimens consisting of metal films with different conductivities placed on a metallic substrate. The inversion determined the film conductivities which were found to agree with those measured using the direct current potential drop (DCPD) method.

Authors:
; ; ;  [1]
  1. Center for NDE, Iowa State University, Ames, Iowa 50011 (United States)
Publication Date:
OSTI Identifier:
21054934
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 894; Journal Issue: 1; Conference: Conference on review of progress in quantitative nondestructive evaluation, Portland, OR (United States), 30 Jul - 4 Aug 2006; Other Information: DOI: 10.1063/1.2718104; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; DIRECT CURRENT; EDDY CURRENT TESTING; EDDY CURRENTS; FILMS; HEAT RESISTING ALLOYS; LAYERS; MHZ RANGE; NICKEL BASE ALLOYS; RESIDUAL STRESSES; SUBSTRATES; SURFACES

Citation Formats

Lee, C., Shen, Y., Lo, C. C. H., and Nakagawa, N.. Validation of a Residual Stress Measurement Method by Swept High-Frequency Eddy Currents. United States: N. p., 2007. Web. doi:10.1063/1.2718104.
Lee, C., Shen, Y., Lo, C. C. H., & Nakagawa, N.. Validation of a Residual Stress Measurement Method by Swept High-Frequency Eddy Currents. United States. doi:10.1063/1.2718104.
Lee, C., Shen, Y., Lo, C. C. H., and Nakagawa, N.. Wed . "Validation of a Residual Stress Measurement Method by Swept High-Frequency Eddy Currents". United States. doi:10.1063/1.2718104.
@article{osti_21054934,
title = {Validation of a Residual Stress Measurement Method by Swept High-Frequency Eddy Currents},
author = {Lee, C. and Shen, Y. and Lo, C. C. H. and Nakagawa, N.},
abstractNote = {This paper reports on a swept high-frequency eddy current (SHFEC) measurement method developed for electromagnetic nondestructive characterization of residual stresses in shot peened aerospace materials. In this approach, we regard shot-peened surfaces as modified surface layers of varying conductivity, and determine the conductivity deviation profile by inversion of the SHFEC data. The SHFEC measurement system consists of a pair of closely matched printed-circuit-board coils driven by laboratory instrument under software control. This provides improved sensitivity and high frequency performance compared to conventional coils, so that swept frequency EC measurements up to 50 MHz can be made to achieve the smallest skin depth of 80 {mu}m for nickel-based superalloys. We devised a conductivity profile inversion procedure based on the laterally uniform multi-layer theory of Cheng, Dodd and Deeds. The main contribution of this paper is the methodology validation. Namely, the forward and inverse models were validated against measurements on artificial layer specimens consisting of metal films with different conductivities placed on a metallic substrate. The inversion determined the film conductivities which were found to agree with those measured using the direct current potential drop (DCPD) method.},
doi = {10.1063/1.2718104},
journal = {AIP Conference Proceedings},
number = 1,
volume = 894,
place = {United States},
year = {Wed Mar 21 00:00:00 EDT 2007},
month = {Wed Mar 21 00:00:00 EDT 2007}
}
  • This paper reports on an on-going project toward developing electromagnetic residual-stress characterization method. Specifically, we present recent work on an eddy current technique and model-based inversion method, specific to nickel-based superalloys. In our approach, we treat residual stress measurements essentially as a 'layer-substrate' problem (a shot-peened layer on an alloy substrate), to which the swept-frequency EC technique is applicable. To this end, a high-sensitivity eddy current system with operating frequency up to 50MHz has been recently developed and validated. We present a description of our instrumentation that includes proprietary probes fabricated by the PCB technology, and laboratory-grade instruments under softwaremore » control. The paper also describes the experimental procedure for performance studies, supported by preliminary data that distinguish clearly between surfaces before and after shot-peening for nickel-based alloys. In addition, we present forward and inverse model validations which will determine their reliability in the subsequent use in model-based residual stress profile inversion.« less
  • Recent research indicated that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stress in surface-treated components. This technique is based on the so-called piezoresistive effect, i.e., the stress-dependence of electric conductivity. Previous experimental studies were conducted on excessively peened (Almen 10-16A peening intensity levels) nickel-base superalloy specimens that exhibited harmful cold work in excess of 30% plastic strain. The main reason for choosing peening intensities above recommended normal levels was that the eddy current penetration depth could not be decreased below 0.2 mm without conducting accurate measurements above 10 MHz, which is beyond the operationalmore » range of most commercially available eddy current instruments. In this paper we report the development of a new high-frequency eddy current conductivity measuring system that offers an extended inspection frequency range up to 80 MHz with a single probe coil. In addition, the new system offers better reproducibility, accuracy, and measurement speed than the previously used conventional system.« less
  • Because of their frequency-dependent penetration depth, eddy current measurements are capable of mapping the near-surface depth profile of the electric conductivity. This technique can be used to nondestructively characterize the subsurface residual stress distribution in certain types of shot-peened metals, e.g., in nickel-base superalloys. To predict the depth-dependent, but frequency-independent, intrinsic electric conductivity from the frequency-dependent apparent eddy current conductivity (AECC), a highly convergent iterative inversion procedure is presented. The proposed technique exploits three specific features of the subsurface electric conductivity variation caused by near-surface residual stresses in shot-peened metals. First, compressive residual stresses are limited to a shallow surfacemore » region of depth much less than typical probe coil diameters. Second, the change in electric conductivity due to residual stresses is always very small, typically less than 1%. Third, the electric conductivity profile is fairly smooth and continuous. The accuracy of the proposed iterative inversion procedure is one order of magnitude better than that of the previously developed simpler method (J. Appl. Phys. 96, 1257 2004)« less
  • Traditional methods for inverting swept frequency or pulsed eddy current signals to get material information involve iterating with a forward model until the response from the model under the same excitation condition is as close to the measured signal as possible. Although the feasibility of the model based inversion has been demonstrated, the complexity of such procedures and the computational resources that this technique requires has hampered its widespread acceptance in industry. Recent approaches include using the look up tables for features extracted from the signals. The performance of look up table approach depends on the choice of the featuresmore » extracted. The authors propose an innovative approach of using a neural network (NN) to solve this inversion problem. Although the use of NN for inverting uniform field eddy current data has been demonstrated, this is the first effort to investigate the feasibility of NN inversion of swept frequency and pulsed eddy current data for thickness measurements of metallic coatings of metal substrates. The authors previously reported initial results from this research. The current paper focuses on the PC based instrumentation and software developed for the swept frequency material profiler. Results of the NN based classification are summarized, and potential applications discussed.« less
  • Typical aero engine alloys, such as IN718, can be surface-treated by shot peening to induce near-surface compressive strains. To calculate the remaining operation time for those critical aero engine components, a quantitative nondestructive determination of near-surface strain gradients has to be developed. We have demonstrated in the past, that it is possible to obtain a characteristic depth profile (surface and sub-surface) of the electrical conductivity of shot peened specimen by using high-frequency eddy current techniques. The measured conductivity profile is resulting from residual stresses, cold work, surface roughness, and the microstructure of the material. The objective is to measure residualmore » stresses (separately from other material properties) in such components after a defined life time. It can be assumed, that surface roughness and microstructure remain unchanged in IN718 materials over their lifetime, but cold work and residual stresses can change independently. Consequently, there is a need to clearly separate the information from both material properties of received eddy current conductivity signals in order to obtain specific information related to residual stresses. This paper presents results acquired from different experiments, conducted to separate both effects by using the eddy current technique on shot peened IN718 materials. We present different physical approaches and illustrate the experiments to solve them. In addition, we will demonstrate that there is a need to use additional techniques, for example ultrasonic time-of-flight measurements, to separate the effects of residual stresses from compound (mixed) signals obtained on cold work samples.« less