Influence of crack length on crack depth measurement by an alternating current potential drop technique

Abstract

An alternating current potential drop (ACPD) technique is used for sizing depth of surface cracks in metallic components. Crack depth estimations are prone to large deviations when ACPD measurements are made on very shallow and finite length cracks, especially in low conducting materials such as austenitic stainless steel (SS). Detailed studies have been carried out to investigate the influence of crack length and aspect ratio (length to depth) on depth estimation by performing measurements on electric discharge machined notches with the aspect ratio in the range of 1 to 40 in SS plates. In notches with finite length, an additional path for current to flow through the surface along the length is available causing the notch depths to be underestimated. The experimentally observed deviation in notch depth estimates is explained from a simple mathematical approach using the equivalent resistive circuit model based on the additional path available for the current to flow. A scheme is proposed to accurately measure the depth of cracks with finite lengths in SS components.

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

“…b) Schematic description of bottom crack detection using ACPD. Fig.1.a) shows that when a surface crack with depth d exists, redistribution current flows along the crack profile [13]. Moreover, the crack causes additional current path and a larger potential drop [14].…”

Potential difference methods for measuring crack growth: A review

“…The DCPD method has been widely accepted due to its easy application and hence relative cheap cost [41,131,134,137]. Because of the nature of DC, DCPD is independent of magnetic permeability of conductive specimens hence the capacitance problem (the capacitance of conductive specimen which is usually ignored actually exerts an effect on measurement of PD) concerned in ACPD is eliminated [13,52,121,131,137]. The wide use of DCPD offers another advantage, in that previous results and guidance are available for certain specimen geometries [41].…”

“…The wide use of DCPD offers another advantage, in that previous results and guidance are available for certain specimen geometries [41]. However, in DCPD tests the whole specimen is tested in terms of the current path, and so the specimen geometry itself (rather than just some flaw geometry) influences the calibration [13]. Moreover, a relatively high current level is required in order to achieve measurable potential outputs [41,76,137,138].…”

“…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

“…ACPD offers good resolution, interference rejection, and accuracy with low excitation currents. 6 Typical applications include the measurement of surface crack depth in metal plates and cylinders, subsurface crack and pit depth in thick and thin metal plates, and conductive material properties such as conductivity and permeability. ACPD is often used to measure surface cracks in welded parts of metallic structures.…”