Novel Method for Sizing Metallic Bottom Crack Depth Using Multi-frequency Alternating Current Potential Drop Technique

Abstract

Potential drop techniques are of two types: the direct current potential drop (DCPD) technique and alternating current potential drop (ACPD) technique, and both of them are used in nondestructive testing. ACPD, as a kind of valid method in sizing metal cracks, has been applied to evaluate metal structures. However, our review of most available approaches revealed that some improvements can be done in measuring depth of metal bottom crack by means of ACPD, such as accuracy and sensitivity of shallow crack. This paper studied a novel method which utilized the slope of voltage ratio-frequency curve to solve bottom crack depth by using a simple mathematic equation based on finite element analysis. It is found that voltage ratio varies linearly with frequency in the range of 5-15 Hz; this range is slightly higher than the equivalent frequency and lower than semi-permeable frequency. Simulation and experiment show that the novel method can measure the bottom crack depth accurately.

Potential difference methods for measuring crack growth: A review

“…2, which refers to a confined current region beneath the specimen surface caused by interaction of original AC and induced eddy current. Compared with the whole body affected in DCPD, in ACPD the current field concentrates in a narrower layer near the material surface, thus a lower excitation current is sufficient to supply easily measurable potentials [10,13,29,40,41,74,76,84,96,124,125,127,131,137,139], thereby minimising the risk of localised heating [13,96,139]. Hence, ACPD is able to achieve higher sensitivity than DCPD, especially for small crack lengths near the surface of materials [13,34,40,41,132,141] and the effect of specimen geometries is reduced [13].…”

“…As a result, if a component, is predicted to be serviceable for a life, it is not inconceivable that a 50% safety factor will be applied to this, in order to compensate for the uncertainty. The capability of accurate, in-situ, non-destructive determination of crack geometry is therefore an important challenge faced by engineers today and is the subject of significant ongoing research activity [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

An SVM approach with alternating current potential drop technique to classify pits and cracks on the bottom of a metal plate

“…10 Recently, the multi-frequency alternating current potential drop technique which is an extension of ACPD has been used to test bottom cracks in metal plates. 11 ACPD has been applied to test defects in both top and bottom surfaces.…”

Inner defect depth detection using a multifrequency alternating current potential drop technique

“…© 2016 Author (s Alternating current potential drop (ACPD) is a nondestructive technique used to detect the depth of defects, especially surface defects, by applying a high-frequency current into a measured metal structure. [1][2][3] Figure 1 shows a metal plate with a defect; I 1 and I 2 are the current injection points. The potential drop V 1 is measured by a pair of probes that form a gap of constant length ∆; another potential drop V 2 , which can be viewed as a reference voltage, is measured when moving this pair of probes far away from the defect.…”

“…As mentioned above, when applying a high-frequency excitation current, most current flows in a thin layer close to the outer surface. Equation (1) shows that this current layer can gradually penetrate into the inner wall as the frequency decreases, so the potential drop also decreases as the frequency decreases.…”