The number of methods which allows for non destructive testings of opening cracks is always increasing but, for industrial applications, the visual testings are still the most widely used especially during the processing steps and the maintenance operations. Among these visual techniques, penetrant testing is one of the most propular methods because its cost is very low and its versatility is large. Nevertheless, sometimes, this simple technique cannot be used. A few typical cases should be mentionned: unsecured human interventions, forbidden contact with the surface specimen, possibility of false alarms when the surface roughness is important, necessity of an automatised control.
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