Impact of defect extent on swept frequency eddy current responses in non-destructive evaluation

Novel approach using swept frequency eddy current technique is employed in non-destructive inspection of conductive materials. Possibility of defect extent evaluation from response signals is experimentally studied. Two plate specimens made of austenitic steel SUS 316L containing artificial electro-discharge machined notches are inspected. The notches vary in dimensions, and they are inspected by two eddy current probes. In particular, a length and a depth dimensions of the notches are concerned here. A wide frequency range is used for the experiments, while the probes containing a transmitting and a receiving coils are located at fixed positions over the plates' surface during entire inspection. The voltage transfer frequency characteristics of the probes are recorded as the response signals. The frequency range is adjusted in such a way that the electrical resonance is clearly visible from the acquired characteristics. Influences of the defects' length and depth on the response signals are evaluated in order to investigate possibilities of cracks' evaluation. The presented results clearly show that the resonance frequency depends on a defect extent.