Abstract
A multi-frequency alternating current field signature method (MACFSM) that combines multi-frequency excitation and zero phase is proposed to detect subsurface defects in metal pipes. In a numerical analysis, multi-frequency exciting currents are applied to a metal pipe (wall thickness T = 10mm) which contains different depth defects, and then zero phase is extracted from phase spectrum to assess defect depth. The investigation shows that in comparison with derivative extremum, the monotonic relationship between zero phase and defect depth can enable accurate determinations of defect depths. A general solution for calculating defect depth using zero phase is given in MACFSM. Pulse excitation field signature method is proposed for future research of quantitative nondestructive evaluation (QNDE) in the end of this paper.
Highlights
The potential drop (PD)[1] technique enables non-destructive evaluation for an increasing range of applications in the analysis of defective materials
Two features derivative extremum and zero phase are extracted from voltage derivative and phase spectra, respectively
The numerical analysis reveals that the derivative extremum in multi-frequency alternating current field signature method (MACFSM) is incapable to accurately determine the defect depths, especially shallow defects (
Summary
The potential drop (PD)[1] technique enables non-destructive evaluation for an increasing range of applications in the analysis of defective materials. It includes many derivatives such as the direct current PD (DCPD),[2] the alternating current PD (ACPD),[3] the direct current field signature method (DCFSM),[4,5,6] and the four-point PD. The voltage between the two electrodes can be expressed by: V. where I0 is the injected current amplitude, R1 and R2 are the outer and inner radii of the pipe, respectively, ∆ is the probe spacing, σ is the conductivity, and f is the frequency of exciting current. Internal defects at shallow depths cannot be accurately calculated, as described below in a numerical analysis
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