Crack depth measurement by means of the AC Potential Drop method.

Alternating Current Potential Drop (AC-PD) is an electrical-magnetic NDT method where an electric current is passed between contacts placed on the area under test. The presence of a crack between the contacts increases the electrical resistance and thus a change in potential (voltage) when compared to a defect free part of the component. This effect is used to size cracks found during NDT examination.

The inspection process

In short; the alternating currents flow close to the surface of the component due to its skin effect. The depth of penetration depends on the utilized frequency. When a defect extends below the depth of penetration, the current flows down along the crack face and passes under.

The difference in potential measured on the surface by a known distance and placed across the crack will be greater than the potential difference between the same distance and placed on good material, due to the greater current path.

The depth of the crack can be calculated by comparing the ratios of the potential drop between a defect free sample and the area under test.

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