An approach to reduce lift-off noise in pulsed eddy current nondestructive technology

• An approach to reduce lift-off noise in the PEC technique is proposed and verified.
• The linear curve (peak value of difference signal–lift-off) is investigated.
• For the specific defect, the slope of linear curve is same for all lift-offs.
• The slope of the linear curve regularly changes with the defect depth/width.
• The slope is presentsed to charaterize defect width/depth in the proposed approach.

The pulsed eddy current (PEC) technique, as an emerging technique of the eddy current technique, has been used in engineering, such as aircrafts, oil/gas pipelines, nuclear steam pipes and high-speed rails, due to its richer information in time domain and frequency domain. However, the lift-off noise, introduced by varying coating thicknesses, irregular sample surface or movement of transducers, has a serious influence on the accuracy of the detection for the defects in these key structures. It greatly limits the application of PEC in quantitative nondestructive testing. In order to reduce the effect of the lift-off, the lift-off effect is analyzed theoretically and experimentally; based on the investigation of the relationship between the peak value of the difference signal and the lift-off, an approach to reduce the lift-off noise for detection the defect depth or width is proposed. In this approach, the defect depth and width are determined by the slope of the linear curve of the peak value of the difference signal and the lift-off. The proposed approach is verified by experiment and the results indicate that it can highly reduce the lift-off noise in the PEC technique. Therefore, it can be applied in characterization of the surface defects in sample with non-ferrous material.