Concealed cracks in rivet structuresThe MDK probe is a low-frequency probe in a multi-differential circuit. It is designed to recognize cracks extending outward from rivet bores without having to remove the rivet itself. The frequency range lies between 100 Hz and 100 kHz and its penetration depth in aluminum is 5 mm. Test pieces with defined cracks are required in order to adjust the settings. Changes in distance arising from bulges and deviations in lacquer coats are suppressed. The adjustable guide block enables the probe to be moved over the edges of the sheet. Cracks starting from 2 mm in length can be recognized in the 2nd and 3rd level of depth.