Search

The alternating current field measurement (ACFM) is an excellent nondestructive testing method for the inspection of cracks in the steel structure. Generally, the research of the ACFM focus on the inspection of one crack with specific direction. However, the crack may extend to an irregular crack with multi-direction due to the complex stress and the special structural shape. In this paper, the irregular crack was inspected visually without cleaning the coating using the ACFM technique. The distorted electromagnetic field was analyzed around the irregular crack by the finite element method (FEM). The double gradient fusion algorithm (DGFA) was presented to reconstruct the surface morphology of the irregular crack. The ACFM system was developed to inspect the irregular crack under the coating. The results show that the current disturbance effect and the magnetic leakage effect both present at different parts of the irregular crack. The surface morphology of the irregular crack can be inspected visually by the DGFA using the ACFM technique. The visual inspection results give more visible morphology information for the structural health monitoring of the steel structure. [ABSTRACT FROM AUTHOR]

As an important connection method between drill pipes, the thread is prone to fatigue cracks and stress corrosion cracks due to long-term exposure to complex forces and corrosive environments. The thread crack detection system of drill pipe based on altering current field measurement (ACFM) is proposed. The crack detection model of the drill pipe thread using ACFM is developed by the finite element software COMSOL. The distribution of the induced magnetic field above the thread is extracted. The relationships between the characteristic signal and the depth and angle of the crack are analyzed. The drill pipe thread detection probes and the detection system are established, and different types of cracks on two different specifications of threads are detected. The experimental results show that the detection system can achieve full coverage detection of the thread crest and root, and cracks at different angle can be identified effectively on the thread. [ABSTRACT FROM AUTHOR]

Alternating Current Field Measurement (ACFM) technique has been widely used in various fields such as oil & gas, aerospace areas. However, during the detection process, the probability of detection (POD) is influenced by multi-factors such as lift-off height, scanning angle and detection speed. In this paper, a bicharacteristic probability of detection (BPOD) model is developed for quantifying the crack detection reliability using integrated Bx and Bz signals in the ACFM technique. A multidimensional BPOD model is established to study the influence of multiple factors on the detection probability of cracks using ACFM technique. A Bayesian network-based method is proposed to reverse the hierarchical impact weights of influence factors on the BPOD of the ACFM technique. The results show that the BPOD model considering both Bx and Bz response signals can evaluate smaller defects than the conventional POD model. The multiple influencing factors can substantially decrease the BPOD of cracks. The hierarchical impact weights on BPOD for cracks are ranked as follows: lift-off height, detection speed, personnel involved in the detection process and scanning angle. This approach can retrospectively find the potential contributors to the missed detections based on the BPOD. [ABSTRACT FROM AUTHOR]

Corrosion is a common damage form in various underwater structures. However, the corrosions are usually covered by the coatings and attachments in the underwater environment. In this paper, the 3D visual reconstruction methods for evaluating the corrosion defect using the alternating current field measurement (ACFM) technique are presented. The FEM model is set up with corrosion and pitting defects to analyze the disturbed current field and the distorted magnetic field. A gradient imaging algorithm based on the Bz signal is proposed to reconstruct the surface morphology of the corrosion and the pitting defect. The inner contour of the corrosion defect is reconstructed using the Bx signal sensitive characteristics of probe lift-off. The calibration method is proposed to measure the depth of the pitting defect. The testing system is built, and the experiments are carried out to verify the 3D visual reconstruction methods. The maximum error of the surface area reconstruction is 8.13%. The maximum error of the corrosion inner reconstruction is 10%. The maximum error of the pitting depth is 3.25%. • A 3D visual reconstruction method is presented for the reconstruction of the corrosion under the coating. • A gradient imaging algorithm of the Bz signal is proposed to reconstruct surface morphology of the corrosion. • The inner contour of the corrosion is reconstructed using the Bx signal sensitive characteristics of the probe lift-off. • The calibration method is proposed to measure the depth of the pitting defect. [ABSTRACT FROM AUTHOR]

• A theoretical model is proposed to explore current disturbance mechanism for subsurface cracks at different frequencies. • The variation pattern of Bx signal with frequency is discovered and employed for the identification of subsurface cracks. • A novel transition frequency is proposed and utilized for assessing the burial depth of subsurface cracks. Subsurface cracks are more challenging to detect and evaluate compared to surface cracks. This paper introduces a novel transition frequency to identify and evaluate subsurface cracks using alternating current field measurement (ACFM) technique. A theoretical model is proposed to explore current disturbance mechanism for subsurface cracks under different frequencies. A 3D simulation model is established to validate the proposed theoretical model and investigate the effects of crack length, shape and specimen thickness on transition frequency. The ACFM testing system is used to detect subsurface cracks with different burial depths and explore the effect of lift-off on transition frequency. The results demonstrate that, with increasing frequency, the shape of Bx signal for subsurface cracks transitions from a trough to a peak, which can be utilized for identifying subsurface cracks. The burial depth can be accurately assessed by transition frequencies. The transition frequency is hardly affected by crack length, shape and lift-off. [ABSTRACT FROM AUTHOR]

Alternating current field measurement (ACFM) technique is an emerging non-destructive testing method for detecting cracks in conductive material. However, the existence of lift-off effect brings great challenges to the identification and accurate quantification of cracks using ACFM technique. In this paper, crack identification and quantization methods are studied under the condition of lift-off effect using ACFM technique. A novel array image feature method based on a longitudinal array probe is proposed to identify the crack with lift-off variations. The influence of constant lift-off height on characteristic signal is studied. The Peak-Trough distance of the Bz signal is not affected by the lift-off height, which can be used to measure the length of cracks. A newfound attenuation coefficient is proposed to develop a lift-off compensation method for evaluation of the depth of crack at any constant lift-off height. The crack identification and quantification methods are verified by crack testing experiments in an aluminum workpiece. The results show that the crack can be identified effectively under the condition of lift-off variations using the array image feature method. The length and depth of cracks with different constant lift-off heights can be measured by the Bz Peak-Trough distance and the lift-off compensation method respectively. The measured errors of the crack depth and length are limited within 10% [ABSTRACT FROM AUTHOR]