Crack In Ferromagnetic Materials Research Articles

An inversion scheme for sizing crack from signals of the motion-induced eddy current testing method based on a new formula of signal gradient of ferromagnetic materials

In this paper, an inversion scheme for the profile reconstruction of a fatigue crack in rails from the motion-induced eddy current testing (MIECT) signals is proposed based on the conjugate gradient (CG) optimization method and the fast MIECT forward simulator developed by authors. To calculate the gradient of MIECT signals with respect to the crack parameters, a simplified analytical gradient formula is deduced, which enables a high efficiency crack reconstruction from MIECT signals for a crack in ferromagnetic material. The validity of the inversion schemes is demonstrated at first by reconstructing cracks with numerical simulated signals for ferromagnetic materials with an oblique elliptic crack model. The good reconstruction results reveal that the new signal grandient formula is efficient for reconstructing crack in ferromagnetic material with the CG algorithm. Finally, sizing of artificial cracks are carried out with use of measured MIECT signals due to cracks in plates of both non-magnetic and ferromagnetic materials. The reconstructed crack parameters are in good agreement with the true values, which experimentally proved the validity and efficiency of the proposed inversion scheme and the new signal gradient formula for MIECT inversion.

Quantitative evaluation of surface crack in ferromagnetic materials based on Bayesian network in eddy current testing

Quantitative evaluation of surface cracks using detection signal is of great significance for accurate prediction of cracks in eddy current testing. It is very difficult to evaluate both the width and depth of small cracks. A quantitative evaluation method based on Bayesian network is proposed for estimating the width and depth of surface cracks in the ferromagnetic materials. First, the simulation model of eddy current testing (ECT) is established and verified by the experimental results. Then, the variation of induced voltage with crack size is studied. Four feature points of real and imaginary part of induced voltage of the receiver coil are selected to characterize the crack size. Finally, a Bayesian network is applied to evaluate the crack size based on numerical simulation results. The evaluation results show that Bayesian network can accurately estimate the width and depth of small cracks.

Quantitative Inversion of Stress and Crack in Ferromagnetic Materials Based on Metal Magnetic Memory Method

Ferromagnetic materials are widely used in engineering structures. Damages caused during manufacture and the use of ferromagnetic materials may seriously undermine the safety of the engineering structure, and even lead to serious industrial accidents. For the early damage testing problem of ferromagnetic materials, it is imperative to propose and adopt new nondestructive testing methods. The metal magnetic memory (MMM) method, which is also known as the micromagnetic testing method, can achieve the early detection of damages in ferromagnetic materials such as steels. However, due to the lack of the quantitative research, the MMM method is only applicable for damage localization. In this paper, the MMM method is utilized to quantitatively evaluate the stress and crack in ferromagnetic materials. The inverse model including the objective function and optimization parameters is proposed. The reconstruction approach is established based on the conjugate gradient inversion method and the exact line search algorithm. In the theoretical analysis, the effectiveness of the MMM method in quantitative determination regarding the position and size of defects is verified with the experimental signal of a hole defect. Following this, the theoretical analyses of localization and sizing of stress and crack are conducted by using the simulation results as the measured signals. It can be found from the results that the MMM method can be applicable for quantitative evaluation of surface rectangular cracks, surface stress concentration zone, convex-type defect, and so on. In addition, the influences of the signal selection, sampling rate, lift-off value, stress level, and noise amplitude on the quantitative analysis of early stress concentration are discussed in details.

Multi-source effect in magnetizing-based eddy current testing sensor for surface crack in ferromagnetic materials

The mechanism of widely applied magnetizing-based eddy current testing (MB-ECT) sensor for ferromagnetic materials is simply emphasized by “the magnetization eliminates the non-uniformity of the permeability and increases the penetration depth”, resulting in rough application for non-destructive testing (NDT). Hence, the neglected permeability distortion around the surface crack is pointed out and analyzed at different magnetization states. In this paper, the multi-source effect of MB-ECT method is analyzed using the equivalent source method. The disturbed magnetic field source is actually composed of the primary magnetic leakage field, secondary disturbed magnetic field caused by the crack itself and the permeability distortion around the crack. Furthermore, the influence of the magnetizing current and the probe lift-off are investigated by finite element analysis (FEA). The simulation result shows “concave” feature as a consequence of the permeability distortion. A series of experiments are designed to analyze the dominant signal components under different conditions. The magnetizing current selection in MB-ECT method corresponds to a unique shape while plotting the signal amplitude as a function of the magnetizing current and lift-off of the probe. This study reveals the multi-source effect in the mechanism of MB-ECT sensors and provides the basic theory or analysis principles for the precision crack evaluation.

An I-integral method for the crack-tip intensity factor in ferromagnetic materials with domain switching

An I-integral method is developed to calculate the crack-tip stress intensity factors (SIFs) in the ferromagnetic thin plate subjected to the external loads. The value of I-integral is not affected by the integral area size, the magnetization distributions or domain walls, which shows the applicability to the large-scale domain switching. Based on the stress and magnetic field obtained from phase-field simulations, the I-integral method is successfully employed to obtain the crack-tip intensity factors and decouples different modes of the crack-tip intensity factors in ferromagnetic thin plates with an impermeable edge crack under different mechanical and/or magnetic loadings. The influence of domain switching on the SIFs is investigated for different loadings. It is found that domain switching under a constant stress loading increases the crack-tip intensity factors, whereas domain switching decreases the crack-tip intensity factors under a constant strain loading. The results indicate that the domain switching-induced shielding or anti-shielding of a crack in ferromagnetic materials with an impermeable edge crack is dependent on the loading condition.

Phase field simulations on domain switching-induced toughening in ferromagnetic materials

Abstract A real-space phase field model is employed to investigate the domain switching-induced shielding or anti-shielding of a crack in ferromagnetic materials with single and multi-domain states. Phase field simulations demonstrate that magnetization switching takes place from the crack tip due to the highly concentrated stress in a ferromagnetic thin plate with a stationary crack. Based on the stress obtained from phase field simulations, the crack-tip stress intensity factors (SIFs) are calculated by linear extrapolation in the ferromagnetic thin plate subjected to different values of mechanical loads. The calculation results indicate that domain switching decreases the crack-tip stress intensity factors, resulting in domain-switching toughening in ferromagnetic materials. Furthermore, the domain switching in the multi-domain ferromagnetic plate induces a much larger decrease of stress intensity factor than that in the single-domain one, which suggests that engineering magnetic domains leads to the design of tougher ferromagnetic materials.

Fracture and deformation properties of Ni–Fe superalloy in cryogenic high magnetic field environments

The present paper includes experimental and analytical data on the fracture properties of a nickel–iron superalloy, a ferromagnetic austenite, at 4 K in magnetic fields of 0 and 6 T. The tensile, notch tensile and small punch tests are employed. A finite element analysis is also performed to convert the experimentally measured load–displacement data into useful engineering information. To interpret the results we review the available theory of the influence of magnetic field on the stress intensity factor for a crack in ferromagnetic materials.

Loading effect on ACPD of a crack in ferromagnetic material

The loading effect on alternating current potential drop (ACPD) for a ferromagnetic material containing a two-dimensional surface crack was investigated under opening mode loading without shear (mode I). The change in potential drop due to load was obtained with and without a magnetic field around the specimen. To remove the magnetic field from the circumference of the specimen, a new measuring system using the characteristic of coaxial transmission line was made. When the magnetic field does not exist around the specimen, the change in potential drop with load is governed by the change in electromagnetic properties near the crack tip. The results obtained by using the new measuring system are the basis for an application of the ACPD technique to the experimental determination of the stress intensity factor, since they are independent of the arrangement of the measuring probe lines and the current supply lines. The relationship between the change in potential drop and the change in load is linearized by demagnetization. The change in potential drop per unit change in the stress intensity factor is independent of the crack length.

Near crack in ferromagnetic materials for magnetic NDT

Near crack in ferromagnetic materials for magnetic NDT

Near crack in ferromagnetic materials for magnetic NDT: 37163 Qi, Z.; Lunyuan, Y. 11th World Conference on Nondestructive Testing, Las Vegas, Nevada (United States), 3–8 Nov. 1985. Vol. 1, pp. 216–225. Taylor Publishing Co., Dallas (1985)

Near crack in ferromagnetic materials for magnetic NDT: 37163 Qi, Z.; Lunyuan, Y. 11th World Conference on Nondestructive Testing, Las Vegas, Nevada (United States), 3–8 Nov. 1985. Vol. 1, pp. 216–225. Taylor Publishing Co., Dallas (1985)