Characterization of residual stresses in ferrous components by magnetic anisotropy measurements using a hall effect sensor array probe

A new surface sensor probe comprising an angular array of Hall effect sensors has been developed for characterization of residual stresses in ferrous materials by means of stress‐induced magnetic anisotropy measurements. The sensor probe applies a radially spreading ac magnetic field to a test sample, and detects stray fields in different directions simultaneously to determine the principal stress axes. In situ measurements were conducted on a annealed steel plate under four‐point bending stresses to evaluate the probe performance. The ratio of stray field signals measured along and perpendicular to the stress axis varies linearly with the surface stress, indicating the possibility of characterizing residual stresses in ferrous components using the sensor array probe

Nondestructive evaluation of residual stresses in case hardened steels by magnetic anisotropy measurements

This paper reports on a recent study aimed at developing the stress-induced magnetic anisotropy (SMA) technique for characterizing residual stresses in case hardened steel components. The results of SMA measurements performed on flat induction hardened steel discs with different case depths confirm the feasibility of detecting principal stress axes by measuring the angular variation of magnetic permeability. The permeability signals along the principal axes were found to vary monotonically with the residual stresses measured by XRD, but the signals are in general smaller for samples with a larger case depth. The magnetomechanical properties of the martensitic case and ferritic/pearlitic core of the induction hardened sample were studied by measuring magnetostriction curves from strip samples that were cut from the case and core regions, respectively. The case strip shows a significantly lower magnetostriction than the core strip, indicating a weaker stress dependence of magnetic properties for the martensitic case than for the ferritic/pearlitic bulk of the case hardened samples

Evaluation of eddy current and magnetic techniques for inspecting rebars in bridge barrier rails

This paper reports on a feasibility study of using eddy current (EC) and magnetic flux leakage (MFL) methods to detect corrosion damage in rebars that anchor concrete barrier rails to the road deck of bridge structures. EC and MFL measurements were carried out on standalone rebars with and without artificial defects of 25% and 50% material loss, using a commercial EC-based rebar locator and a MFL system that was developed using giant magnetoresistance sensors to detect leakage fluxes from the defects. Both techniques can readily detect the defects at a distance of 2.5″ (63.5 mm). The amplitudes of the EC and MFL signals vary monotonically with the amount of material loss, indicating the potential of using the techniques to quantify material loss of standalone rebars

Unconventionally high and low frequency eddy current methods for material surface characterizations

This paper reports on a type of multiple frequency eddy current NDE methodologies. A specific focus was placed on a multiple low frequency methodology that is suitable for case depth characterization of case‐hardened steel samples. A nominally uniform excitation field approach has been developed, and demonstrated to work for discriminating the case depths ranging between 1 mm and 6 mm, while meeting additional requirements of one‐sided access and a finite stand‐off distance

Detection of anamolous machining damages in Inconel 718 and TI 6–4 by eddy current techniques

This paper reports on an eddy current (EC) study aimed at detecting anomalous machining damages in Inconel 718 and Ti 6–4 samples, including (i) surface discontinuities such as re‐depositing of chips onto the machined surface, and (ii) microstructural damages manifested as a white surface layer and a subsurface layer of distorted grains, typically tens of microns thick. A series of pristine and machine‐damaged coupons were studied by EC scans using a differential probe operated at 2 MHz to detect discontinuous surface anomalies, and by swept high frequency EC (SHFEC) measurements from 0.5 MHz to 65.5 MHz using proprietary detection coils to detect surface microstructural damages. In general, the EC c‐scan data from machine‐damaged surfaces show spatial variations with larger standard deviations than those from the undamaged surfaces. In some cases, the c‐scan images exhibit characteristic bipolar indications in good spatial correlation with surface anomalies revealed by optical microscopy and laser profilometry. Results of the SHFEC measurements indicate a reduced near‐surface conductivity of the damaged surfaces compared to the undamaged surfaces

Investigation of the effects of notch width on eddy current response and comparison of signals from notches and cracks

This paper reports on work conducted to investigate the effect that electrical discharge machining (EDM) notch width has on the eddy current (EC) signal as a function of coil drive frequency. The notch results are also compared to EC signals from laboratory‐grown fatigue cracks. This study builds upon previous work with titanium, Inconel and aluminum materials where the signal amplitude was shown to decrease, as expected, as the notch width decreases. The trend was captured well by numerical results and this allowed estimates to be made about the signals from idealized “zero‐width” notches. The results indicated that the signal reduction factor from a 0.127 mm (0.005 inch) wide, rectangular notch to a theoretical zero‐width semi‐elliptical notch of the same size ranged from 25 to 42% for low conductivity materials when data was collected at 2 MHz. For aluminum, the difference between signals from 0.127 mm wide notches and estimated signals for zero‐width notches was approximately 50%. However, 2 MHz is an uncommonly high frequency for inspecting aluminum alloys so additional work was necessary to investigate the notch width effect at lower frequencies. This study sought to determine how the notch‐width effect changed as a function of frequency for high conductivity materials such as aluminum

Analysis of Barkhausen effect signals in surface-modified magnetic materials using a hysteretic-stochastic model

The effects of microstructural variations with depth on Barkhausen effect (BE) signals in surface-modified ferrous materials have been studied through measurements and simulations based on a hysteretic-stochastic model. The BE signals measured from an unhardened sample show a peak near zero field. In contrast, the BE signals in surface-hardened samples with different case depths exhibit low-amplitude pulses near zero field and a peak at a high reverse field, which are attributed to irreversible magnetization processes in the soft core and the hardened surface layer, respectively. Theoretical analysis showed that the model parameters k and ξ, which describe the domain-wall pinning strength and the range of interaction of a domain wall with pinning sites, respectively, are related to each other as ξ=a ρ−1/2=b/k via the pinning site density ρ, where a and b are constants. The relationship was used to simulate BE signals of the surface-hardened samples as a sum of signals generated at different depths by taking into account signal attenuation due to eddy current shielding. The simulated results were found to exhibit the general features observed in the experimental results

Evaluation of surface-modified materials by model assisted analyses of Barkhausen effect signals

Barkhausen effect (BE) measurements offer a useful technique for evaluation of surface conditions of magnetic materials. Theoretical analysis of BE signals has nevertheless remained a challenge due to a lack of adequate model description of the phenomenon. This paper presents model‐assisted analysis of BE signals with an aim of evaluating surface‐modified materials whose magnetic properties vary continuously with depth. The approach is based on an extended BE model which describes the signal power spectrum. In the extended model, the local fluctuation of pinning field and correlation length parameters, which characterize domain wall motion and hence the generation of BE signals, are related to the domain wall pinning strength which vary with depth as described in terms of a parametrized function. In this study, the BE signals measured over a broad frequency range from carburized steels and oxidized Ni97Al3 were analyzed using multiple frequency passbands to extract signals generated at different depths. Depth profiles of the pinning field fluctuation and correlation length were determined by obtaining the best fit of the modeled BE signal power to the experimental data. The inverted profiles show changes at depths which agree with case depths of the carburized steel samples, and with the oxidization depths of the Ni97Al3 samples

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

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