Products featuring very irregular surfaces, such as-cast slabs or billets, have remained an issue for non-destructive detection of cracks with eddy-current testing. However multiplexed arrays probes seem a promising solution to overcome the productivity/resolution challenge; they also allow easy imaging. A specific multiplexing pattern based on transmit receive elements, which are almost insensitive to lift-off variations on normal metal allows detecting such surface-breaking flaws with little perturbations from the surface. To meet the challenge of the lift-off/resolution compromise, a special geometrical arrangement was imagined, which has to be respected. Finally multiplexing features inspection in attainable widths typically 100 mm with a 5 mm cross resolution. We describe here the work comparing the figures of merit (resolution, lift-off) of several multiplexing patterns, with first investigations on a high surface quality finished sample with EDM notches, then on notches and apparent cracks on rough as-cast surfaces: 5 mm long cracks seem to be detectable with a 3 mm nominal stand-off. Finally we discuss the possibilities which this new technique opens for applications in the steel industry, especially for as-cast surfaces and hot-rolled products.
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