Dynamic Piezoresistivity Calibration for Eddy Current Nondestructive Residual Stress Measurements

This paper reports on a microstructural study of a nickel-base superalloy, Inconel 718, with a focus on quantifying precipitate density and their effects on conductivity variations. The study is motivated by eddy current (EC) characterization of residual stresses, where observed EC signals are attempted to correlate with stress profiles of shot peened superalloy surfaces. It has been observed that the correlation is less universal than anticipated, and in fact strongly influenced by the material hardness, or the aging conditions. For example, the soft sample surface exhibits significantly stronger EC signals than the fully hardened sample when both are shot peened at the same Almen intensity. Thus, the objective of the present study is to examine this complex material response against aging and shot peening treatments at the microstructure scale, by the use of techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We will describe preparations of a series of Inconel 718 samples that was aged and shot peened at various conditions, and present microstructural data obtained from SEM and TEM images such as precipitate densities, correlated with bulk properties such as the hardness and conductivity. ABSTRACT. This paper reports on a microstructural study of a nickel-base superalloy, Inconel 718, with a focus on quantifying precipitate density and their effects on conductivity variations. The study is motivated by eddy current (EC) characterization of residual stresses, where observed EC signals are attempted to correlate with stress profiles of shot peened superalloy surfaces. It has been observed that the correlation is less universal than anticipated, and in fact strongly influenced by the material hardness, or the aging conditions. For example, the soft sample surface exhibits significantly stronger EC signals than the fully hardened sample when both are shot peened at the same Almen intensity. Thus, the objective of the present study is to examine this complex material response against aging and shot peening treatments at the microstructure scale, by the use of techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We will describe preparations of a series of Inconel 718 samples that was aged and shot peened at various conditions, and present microstructural data obtained from SEM and TEM images such as precipitate densities, correlated with bulk properties such as the hardness and conductivity.

Section: Introductionmentioning

confidence: 99%

Quantification of precipitates and their effects on the response of nickel-base superalloy to shot peening

Chandrasekar¹,

Lo²,

Frishman³

et al. 2012

“…It is based on the well-known piezo-resistive effect, which is the linear relationship between conductivity and elastic stress. The success of this method on certain Nickel-base superalloys such as IN100 and Waspaloy is attributed to the rare feature that both the parallel and normal piezo-resistive coefficients are negative [4]. The main limitation for Eddy current spectroscopy is the lack of selectivity as surface treatment not only induces subsurface residual stress but also cold work that is difficult to separate.…”

Section: Introductionmentioning

confidence: 99%

Stress dependence of Hall coefficient in a nickel-base superalloy

Shao

Zheng

Castagne³

2018

Abstract. The paper presents the development of a non-destructive technique for selective residual stress measurement based on Hall coefficient measurements. The Hall coefficient method is anticipated to be less affected by spurious interference from microstructural changes as compared to other non-destructive techniques [1]. A lock-in measurement method was used to measure the Hall voltage non-destructively on a 1 mm thick Nickel-base superalloy IN100 specimen with adequate precision (0.03% standard deviation) and consistency. Linear stress dependence of the Hall coefficient was observed. It is found that there is an approximately 2% change of the Hall coefficient from stress-free condition to 200MPa tensile stress. The promising experimental results indicate the possibility of exploiting this technique for nondestructive measurement of subsurface residual stresses.

“…Eddy current has been identified as a leading candidate for nondestructive characterization of residual stresses in engine components, particularly in nickel-base superalloys [2]. Correlations between EC signals and residual stresses in shot-peened alloys have been reported, and were attributed to the piezoresistivity effect, which refers to stress-induced changes in electrical conductivity [3,4]. Despite the significant potential of EC, several challenges have been identified against development of the measurement technique for residual stress characterization.…”

Section: Introductionmentioning

confidence: 99%

Effects of cold work on near-surface conductivity profiles in laser shock peened and shot peened nickel-base superalloy

Lesthaeghe¹,

Larson²,

Chandrasekar³

et al. 2013

This paper reports on a study of the effects of cold work induced by surface enhancement treatment on conductivity profiles in nickel-base superalloys, as part of the on-going efforts aimed at evaluating the feasibility of characterizing near-surface residual stress profiles in peened engine components using a swept frequency eddy current (SFEC) technique. The approach is based on the empirical piezoresistivity effect that correlates conductivity changes with residual stress, but recent studies have shown that conductivity changes induced by peening processes are also influenced by metallurgical factors such as cold work. In this study, conductivity deviation profiles were obtained by model-based inversion of SFEC signals from a set of aged Inconel 718 samples, which were either shot peened or laser shock peened to produce different residual stress and cold work profiles. The laser shock peened samples exhibit a larger increase in surface conductivity and deeper conductivity profiles, which are attributed to a smaller amount of surface cold work and deeper residual stress profiles created by laser shock peening than by shot peening. ABSTRACT. This paper reports on a study of the effects of cold work induced by surface enhancement treatment on conductivity profiles in nickel-base superalloys, as part of the on-going efforts aimed at evaluating the feasibility of characterizing near-surface residual stress profiles in peened engine components using a swept frequency eddy current (SFEC) technique. The approach is based on the empirical piezoresistivity effect that correlates conductivity changes with residual stress, but recent studies have shown that conductivity changes induced by peening processes are also influenced by metallurgical factors such as cold work. In this study, conductivity deviation profiles were obtained by model-based inversion of SFEC signals from a set of aged Inconel 718 samples, which were either shot peened or laser shock peened to produce different residual stress and cold work profiles. The laser shock peened samples exhibit a larger increase in surface conductivity and deeper conductivity profiles, which are attributed to a smaller amount of surface cold work and deeper residual stress profiles created by laser shock peening than by shot peening.