The nondestructive evaluation (NDE) for the crack inside a graphite slab was investigated by using the high-T(c) SQUID NDE system in unshielded environment. The crack-depth dependence of the crack signal was explored with the excitation frequency varied from 2 to 20 kHz. It was found that the phase of the crack signal is a linear function of the crack depth for various excitation frequencies. In addition, forlinear phase depth relation scaled in an empirical equation with the skin depth as the only variable. The empirical equation was useful in determining the proper excitation frequency for the eddy-current NDE of conductive samples.
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