We study the frequency-dependent inductance of a small air-cored coil of wire placed flat upon various ferromagnetic metal plates. The change in the complex inductance of the coil, measured with an HP 4194A impedance analyzer, is reported for frequencies between 1 kHz and 1 MHz. The metal plates consisted of commercially pure (99.7% and 99.9%) Ni, commercially pure (99.9%) Fe, and a suite of medium carbon steels. For the steel plates, inductance changes were consistent with a simple halfspace model that treats the metal as a continuum defined by a conductivity and a relative initial-permeability where these material parameters are isotropic, local, and uniform throughout the plate. The inductance changes for Ni and Fe could not be fit to the half-space model for any values of and , but were consistent with a model that assumes a thin (10 m) surface layer with a significantly reduced permeability-a dead layer. We tested the existence of the hypothetical dead layer in several ways. We found that the inductance increased when the surface was chemically etched (presumably eroding the dead layer) and decreased when the surface was mechanically polished (presumably increasing the dead layer). We also found that the inductance of the Fe and Ni samples decreased substantially over the course of days and months when exposed to air.
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.
Eddy current inspection is widely used in nondestructive evaluation to detect cracks in metal structures. The impedance plane measurement response collected using our motion controlled eddy current inspection system, are used in the analysis. A scalar reduction from the impedance plane response is used to minimize human-factor variation. We apply a noise interference model to the data from a large-scale experiment taking measurements on fastener rivet holes in multi-layer structures with fatigue cracks, and estimate the probability of detection (POD). The estimates of POD as a function of crack size will be valuable for future modelassisted POD study. KeywordsCenter for Nondestructive Evaluation, automated eddy current inspection, maximum likelihood, noise interference model, POD, rivet hole fatigue cracks Disciplines Statistics and Probability CommentsThis preprint was published as Mink Li, Norio Nakagawa, Brian F. Abstract: Eddy current inspection is widely used in nondestructive evaluation to detect cracks in metal structures. The impedance plane measurement response collected using our motion controlled eddy current inspection system, are used in the analysis. A scalar reduction from the impedance plane response is used to minimize human-factor variation.We apply a noise interference model to the data from a large-scale experiment taking measurements on fastener rivet holes in multi-layer structures with fatigue cracks, and estimate the probability of detection (POD). The estimates of POD as a function of crack size will be valuable for future model-assisted POD study.
A sometimes stated rule of thumb is that the eddy current signal from a fatigue crack can be 60% of the strength produced by a similarly sized, rectangular EDM notch. A study was conducted to explore the effect that the width of a discontinuity has on signal strength when inspecting low conductivity materials for small cracks with eddy current surface probes. EDM notches of different sizes and shapes were planted in Ti-6246 and IN-100 specimens. Each of the two materials received six 0.020 inch long by 0.010 inch deep notches and six 0.030 inch and 0.015 inch deep notches. Three of the notches of each size were rectangular shaped and three were semicircular shaped. One of the notches in each material size group was 0.005 inch wide, one was 0.003 inch wide and one was 0.001 inch wide. Each of the notches was scanned using absolute and differential pencil probes driven at several different frequencies. The experimental results were compared with numerically generated results, which allowed for a zero width notch to be considered. The results indicated that the signal reduction factor from a 0.005 inch wide, rectangular notch to a theoretical zero-width notch of the same size ranged from 25 to 42%.Development of eddy current testing system for inspection of combustion chambers of liquid rocket engines Rev. Sci. Instrum. 84, 014701 (2013) Eddy current effects in plain and hollow cylinders spinning inside homogeneous magnetic fields: Application to magnetic resonance J. Chem. Phys. 137, 154201 (2012) Defect characterisation based on heat diffusion using induction thermography testing Rev. Sci. Instrum. 83, 104702 (2012) Fully automated measurement setup for non-destructive characterization of thermoelectric materials near room temperature Rev. Sci. Instrum. 83, 074904 (2012) Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings
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