We present herein a comparative study of non-destructive testing performed by an induction coil, with a sensitivty of 24muV at 70Hz, a flux-gate and a HTc-SQUID-based magnetometer with a field noise density of 20pT/Hz and 0.3pT/Hz respectively. Single-layered and multi-layered aluminium-alloy structures with artificial defects commonly encountered inthe aircraft industry, i.e. holes, slots and cracks, have been investigated. Experimetnal data are discussed in erms of signal-to-noise ratio in the frequency range 10-1000 Hz. The effect of stand-off variation and probe tilting on the output signals is also investigated.
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