Fatigue damage monitoring using the direct current potential drop method on notched steel specimens under multiaxial loadings

During fatigue testing, engineers often need to determine the number of cycles at which a crack reaches a defined small length, marking the onset of crack initiation. Various experimental non-destructive testing (NDT) techniques can be used to estimate the size of an initiated crack, among which the direct current potential drop (DCPD) method is widely utilised. This study investigates the multiaxial fatigue behaviour of specimens made of 42CrMo4 quenched and tempered steel. Approximately 150 fatigue tests were conducted on cylindrical specimens, featuring three different geometries: plain (hourglass-shaped), bluntly U-notched (circumferential U-notch, notch tip radius ρ = 5 mm), and sharply V-notched (circumferential V-notch, ρ = 0.375 mm). Six loading configurations were examined, including pure bending, pure torsion, and combined bending-torsion with two nominal biaxiality ratios (Λ = 1 and Λ = √3), each tested under in-phase (Φ = 0°) and out-of-phase (Φ = 90°) conditions. Crack initiation and subsequent propagation were monitored through the DCPD technique configured with two potential probes. Calibration curves correlating potential drop variation with propagating crack depth were obtained experimentally through dedicated tests on notched specimens subjected to pure bending and pure torsional loadings. A beach-marking procedure was adopted to identify crack fronts on fracture surfaces. With a sensitivity of approximately tenths of millimetre, the DCPD technique applied to this extensive test matrix enabled a systematic assessment of technical crack initiation and propagation mechanisms across a wide spectrum of stress states and notch severities, laying the groundwork for the application of local fatigue assessment methods. • The direct current potential drop (DCPD) technique is applied to multiaxial fatigue. • Proper DCPD calibration curves are derived from a dedicated beach-marking procedure. • Systematic investigation of crack initiation and propagation patterns is conduced. • Fracture maps for multiaxial stress states and notch severities are provided. • The DCPD may support non-destructive (NDT) inspections and predictive maintenance.