Improved detection of surface defects at sample edges using high-frequency eddy current amplitude and phase measurements

Abstract

ABSTRACT The detection of surface cracks at or close to sample edges is a challenging problem because the interaction of the eddy current with the sample edge can make it difficult to distinguish changes in the eddy current signal due to a defect. Samples with poor electrical conductivity such as titanium alloys used extensively in aerospace applications can be more difficult to inspect due to the low amplitude eddy currents induced in them and increased electromagnetic skin depths due to lower electrical conductivity. As fatigue surface cracks or manufacturing surface defects can often occur close to edges, the challenges of detecting small defects close to sample edges is an important research area to address. High-frequency eddy currents of over 10 MHz are used in a transmit-receive configuration using two solenoid type coils adjacent to each other. While conventional eddy current sensors are commonly designed for operating at frequencies into the low MHz region, the supporting electronics here will be positioned immediately behind the coils to improve electrical stability and reduce induced noise. The magnitude and phase of the voltage on transmit and receiver coils are measured, and finite element modelling is used to validate the experimental measurements and gain insight into the system behaviour. Small defects of down to 1 mm are easily detected, on the edge and at the corner of a titanium alloy sample with excellent signal-to-noise.