Pulsed eddy current thickness measurement using phase features immune to liftoff effect

• Published in: NDT & E international : independent nondestructive testing and evaluation Vol. 86; pp. 123 - 131
• Main Authors: Fan, Mengbao, Cao, Binghua, Sunny, Ali Imam, Li, Wei, Tian, Guiyun, Ye, Bo
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2017
• Subjects: Liftoff; Metallic plate; Nondestructive testing; Phase; Pulsed eddy current; Spectral response; Thickness measurement; Spectral response; Metallic plate; Phase; Nondestructive testing; Pulsed eddy current; Liftoff; Thickness measurement
• ISSN: 0963-8695; 1879-1174
• DOI: 10.1016/j.ndteint.2016.12.003

Conventionally, peak value and peak time are extracted from pulsed eddy current (PEC) response as features for thickness measurement. However, they suffer from liftoff variations. In this work, the phase of spectral PEC response from a Hall sensor are proposed to serve as robust features for thickness evaluation. The presented novel features are immune to liftoff effect, because phase signals remain nearly constant against liftoff variations. An analytical model was formulated, and simulations were performed to uncover the physics of the characteristics of the phase feature and establish the relationship between the phase feature and sample thickness. Experiments were carried out to validate the proposed phase feature. Eventually, the proposed phase feature was evaluated for accurate thickness measurement and some key factors were discussed. Main contribution: The phase of spectral PEC response from a Hall sensor based probe is proposed to serve as an original feature for thickness measurement, and the physics of the new feature immune to liftoff effect were revealed. [Display omitted] •The phase of spectral PEC response is proposed to serve as an original feature.•The feature is immune to liftoff effect, and the physics were revealed.•The behavior of the feature against liftoff variation was demonstrated.•Performances of the phase feature was evaluated for thickness measurement.•A strategy was presented to determine frequency and select superior feature.