A Reversing Direct Current Potential Drop System for Detecting and Sizing Fatigue Cracks along Weld Toes

Steel welded T-joints with 102 mm thick base plates were fatigue tested in three-point bending. A reversing direct current potential drop (DCPD) system was used to detect the initiation of multiple surface cracks along the transverse weld toes of these joints and to monitor the size and shape of fatigue cracks that developed from these cracks. Potential drop readings were obtained by fixed probes straddling the weld toes and normalized with respect to potential drop readings from a remote reference probe. The normalized potential drop readings from each probe were related to the local crack depth by two-dimensional (2-D) calibrations derived by boundary element analyses and the foil analogue method. The fixed-probe arrangement was able to detect 0.4 mm to 1.0 mm deep surface cracks along the transverse weld toes of the T-joints. The 2-D calibrations were able to predict the crack depth at the deepest points of these cracks to within ±10%.