Crack Depth Profile Research Articles

An Efficient Phenomenological Inversion Method for Reconstruction of Crack Depth Profile in a Metal From ACFM Probe Output Signals

The paper proposes an efficient phenomenological inversion method to determine the depth profile of a surface-breaking crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The proposed method utilizes a conjugate gradient algorithm to minimize an objective function, representing the difference between probe predicted and actual signals in an iterative manner. The objective function is derived explicitly in terms of crack depth variables by considering a polynomial function for the field distribution in the depth direction and applying appropriate Greens functions. This approach enhances the accuracy and computational efficiency of the inversion process, regardless of the choice of initial crack depth profile or presence of noise in the measurement system. The validity and efficiency of the proposed method are demonstrated by comparing the reconstructed depth profiles of several simulated and machine-made cracks with their actual data, and those obtained using the conventional phenomenological approach based on an efficient stochastic optimization scheme along with a fast pseudoanalytic ACFM probe output simulator.

Output Signal Prediction of an Eddy-Current Probe When Scanning Arbitrary-Shape Surface Cracks in Metals

This article proposes an efficient modeling technique to predict the output signal of an eddy-current (EC) probe when scanning arbitrary-shape surface cracks in metals. The probe is excited by an arbitrary-shape wire carrying an ac current of arbitrary frequency with no restriction on the crack depth profile or magnetic permeability of the metal. In the proposed technique, the equivalence principle is adopted to replace the crack with the equivalent electric and magnetic current densities, and the governing volume integral equations (VIEs) are derived using appropriate Green’s functions. The VIEs are solved by the method of moments (MoM) for the unknown electric and magnetic fields inside the crack. This is done by expanding the electric and magnetic fields in terms of basis functions compatible with the problem. To support the validity of the proposed modeling technique, we compare the predicted output signals of various EC probes with their experimental counterparts and those available in the literature.

A Fuzzy Error Characterization Approach for Crack Depth Profile Estimation in Metallic Structures Through ACFM Data

The estimation of fatigue crack depth profile is one of the most important issues in nondestructive evaluation techniques. In this regard, providing a method which is able to determine the error rate of the depth profile estimation will be significant. Although several methods have been proposed to estimate the depth profile with reasonable accuracy, these mentioned methods are unable to determine the error rate of estimation process. In this paper, to eliminate mentioned drawbacks, a first and second fuzzy approximation methodology is proposed to estimate the surface crack depth profile of an arbitrary crack in metallic structures based on alternating current field measurement signals. In the proposed method, it is feasible to achieve a definite error rate to determine the surface crack depth profile under evaluation. Next, to achieve the most reasonable accuracy and the least computational complexity simultaneously, a solution will be proposed for the method presented in this paper. Finally, the results of several common methods are compared with the method presented in this paper in analytical and experimental forms.

Stress corrosion crack initiation of alloy 600 in PWR primary water

Stress corrosion crack (SCC) initiation of three mill-annealed alloy 600 heats in simulated pressurized water reactor primary water has been investigated using constant load tests equipped with in-situ direct current potential drop (DCPD) measurement capabilities. SCC initiation times were greatly reduced by a small amount of cold work. Shallow intergranular attack and/or cracks were found on most high-energy grain boundaries intersecting the surface with only a small fraction evolving into larger cracks and intergranular SCC growth. Crack depth profiles were measured and related to DCPD-detected initiation response. Processes controlling the SCC initiation in mill-annealed alloy 600 are discussed.

Life Prediction and Stress Intensity Factor (SIF) analysis for the base straight component and weld straight component (Pipes) used in the nuclear power plant

The present investigation, the fatigue crack growth behaviour of surface cracked piping, pressurized base straightcomponent(PBSC) andpressurized base straight weld component(PBWC)are performed on the basis of the linear elastic fracture mechanics (LEFM) principles. The stress intensity factors available in the open literature are evaluated to predict the crack growth behaviour of the component using the specimen tested material data. It is concluded that the available open literature stress intensity factor (SIF) solution like ASM and Bergman for external surface cracked straight pipe case having the semi-elliptical crack profile predicts the fatigue life of the component when having constant crack depth profile.

Fatigue crack sizing in steel bridge decks using ultrasonic guided waves

A baseline-free quantitative sizing methodology utilizing ultrasonic guided waves for fatigue cracks under welded stiffeners in steel bridge decks has been developed. An inverse wavefield extrapolation-based formulation for obtaining the crack reflectivity and depth profile has been presented in the kernel of which, the presence of welded orthogonal stiffeners has been accounted for. Having conducted experiments on a test bridge deck subject to fatigue loading, it has been demonstrated that the crack profile can be estimated from the reflection coefficients obtained. In comparison with the reference measurements, the maximum crack depth estimation error turned out to be about 20%.

A Fuzzy Learning Approach for Identification of Arbitrary Crack Profiles Using ACFM Technique

This paper presents a new inversion approach to estimate the depth profile of a fatigue crack from the output signal of an alternating current field measurements (ACFM) probe. We first show that the mapping between the ACFM signals and corresponding crack depth profiles can be expressed by an affine transformation, then we utilize a generalized version of the fuzzy alignment algorithm (FAA) to find this mapping more efficiently. The proposed method is then extended using the Dave's objective function previously proposed for the fuzzy clustering. To evaluate the accuracy and robustness of the proposed inversion technique we reconstruct cracks of arbitrary shapes from both simulated and measured ACFM signals. The results show that the proposed approach is more robust and accurate than existing methods and gives reasonable results even in the absence of sufficient training data.

Developments in efficiently modelling eddy current testing of narrow cracks

Further developments in a Boundary Element Method model for eddy current inspection of ideal or narrow cracks are presented. First, the required Green’s function is rapidly computed in the spatial domain using the concept of discrete complex images. The approach is based on the analytical evaluation of the Sommerfeld type Integrals through the use of the Generalized Pencil of Function method. Computation times are recorded in order to show the remarkable reduction in the moment matrix fill time as well as in the total computational time. Second, the model is extended to arbitrary crack depth profiles. Theoretical results are compared to published benchmark measurements of the coil impedance change due to a semi-elliptic and an epicyclic crack and excellent agreement is achieved.

Neural network approach for determination of fatigue crack depth profile in a metal, using alternating current field measurement data

A neural-network-based technique is described to determine the depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the measurements along the crack opening. The network uses the multilayer perceptron structure for which the training database is established by systematically producing semi-elliptical multi-hump cracks with narrow openings and random lengths and depth profiles. A fast pseudo-analytic ACFM probe output simulator is also used to produce network input data around each crack for a specified inducer. To demonstrate the accuracy of the proposed inversion technique, the simulated results of cracks with both common and complex geometries are studied. The comparison of the actual and reconstructed depth profiles substantiates the technique introduced here. To further validate the technique, the experimental results associated with several fatigue cracks of complex geometries are presented.

Using a wavelet network for reconstruction of fatigue crack depth profile from AC field measurement signals

This paper presents a wavelet-network-based technique for reconstructing the crack depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the ACFM probe output signals along the crack opening. The database for training the network is established by developing a random crack-depth generator, using a fast pseudo-analytic ACFM probe output simulator. The validity of the proposed technique is demonstrated by comparing the actual and reconstructed depth profiles of several simulated and machine-made cracks with no predetermined geometries.

Method for Measuring Short‐Crack R‐Curves without Calibration Parameters: Case Studies on Alumina and Alumina/Aluminum Composites

A method to measure short‐crack R‐curves is introduced which does not require an independent determination of calibration factors and which utilizes an equilibrium crack shape condition to compute the shape factor of a surface crack, R‐curves of an alumina with two different grain sizes and of an aluminum/alumina composite were measured in the crack length regime of below 100 μm up to almost 1000 μm. Calculated and experimentally obtained crack depth profiles were in good agreement with deviations being between −22% and +26% for the crack ellipticity and confirmed the reliability of the method.