Determining the propagation angle for non-vertical surface-breaking cracks and its effect on crack sizing using an ACFM sensor

Abstract

Alternating current field measurement (ACFM) probes are used to detect and size cracks in a range of engineering components. Crack sizing for this, and other electromagnetic (EM) based NDT systems, relies on relating the signal obtained to the actual crack length. For cracks that do not propagate vertically, such as rolling contact fatigue cracks in rails, predicting the crack depth, which determines the rail depth to be removed by grinding, requires an assumed propagation angle into the material as no method to determine crack vertical angle from the EM signals has been reported. This paper discusses the relationship between ACFM signals and propagation angles for surface-breaking cracks using a COMSOL model. The Bx signal accurately predicts the crack pocket length when the vertical angle is 30-90° but underestimates pocket length for shallower angles, e.g. a 50% underestimate is seen for a 3.2 mm pocket length crack propagating at a vertical angle of 10°. A new measure, the Bz trough-peak ratio, is proposed to determine the crack vertical angle. These are verified by experimental measurements using a commercial ACFM pencil probe for cracks with a range of vertical angles between 10° and 90°.

Characterisation of clustered cracks using an ACFM sensor and application of an artificial neural network

“….5 mm is used in this model. Shen, et al [52] showed that, for RCF cracks propagating into a rail at a range of angles from 30-90 , the ACFM signal response to changing crack vertical angle is insignificant. Hence, the °s ignal can be directly used to determine the crack pocket length.…”

Identification of Tiny Surface Cracks in a Rugged Weld by Signal Gradient Algorithm Using the ACFM Technique

“…As the first step, how to find a tiny crack in a weld is critical work in the ACFM field. The probability of detection (POD) using the ACFM technique decreases drastically for tiny cracks whose lengths are less than 10 mm [16]. Smith et al applied the ACFM technique to inspect the welds in stainless steel nuclear storage tanks [17].…”

“…Yuan et al presented the two-step interpolation algorithm for the measurement of long and short cracks in the pipe string using the uniform alternating current field [19]. Leng et al proposed the combined metal magnetic memory (MMM) and the ACFM The probability of detection (POD) using the ACFM technique decreases drastically for tiny cracks whose lengths are less than 10 mm [16]. Smith et al applied the ACFM technique to inspect the welds in stainless steel nuclear storage tanks [17].…”

Prediction of RCF clustered cracks dimensions using an ACFM sensor and influence of crack length and vertical angle

“…Vertical depth varies depending on the crack vertical angle and crack pocket length, as shown in Figure 1c. It has recently been reported that the Bz trough-peak ratio, derived from a single measurement line across the centre of the crack at 45°, can be used to obtain the crack vertical angle, which can then be used with the determined crack pocket length to give the vertical depth [14]. This provides a guide for railway grinding to remove the RCF cracks.…”

“…Responses of ACFM Bx signals to various surface lengths, inner spacings and crack numbers for crack clusters have been reported but only cracks with vertical angles of > 30° were considered [16]. Previous studies [14,22] have shown that shallow vertical angles (< 30°) give different Bx signals compared to those obtained for cracks with vertical angles >30° for single and clustered cracks, leading to under estimation of crack pocket lengths. It is important to be able to characterise crack clusters, as RCF cracks usually present in the form of clusters and with vertical angles less than 30°.…”