Non-destructive direct current potential drop assessment of forming-induced pre-damage in AISI 5115 steel

Due to efficient material utilization and reproducible high-quality level, metal forming processes, such as forward extrusion, meet the requirements of resource-saving and economical production. However, the performance of workpieces is considerably affected among others by the occurrence of forming process-induced damage. So far, there are only few investigations available on the influence of process-induced pre-damage on fatigue behavior. With the knowledge of damage influence on the component durability, components can be designed specifically and enable optimization of loading capability and lightweight construction. Also, the degree of process-induced pre-damage after forming is currently measured by time-demanding destructive analyses in scanning electron microscopy (SEM). Electrical resistance measurement (DCPD) could offer the opportunity for a time-efficient non-destructive assessment of ductile damage. The aim was to develop a method for damage characterization that allows conclusions to be drawn about the expected lifetime of components. Therefore, a highly accurate resistance-based sensor system for the evaluation of forming process-induced damage and loading-induced cyclic damage, and for the analysis of their interaction has been established and validated. The measurement technology required for damage detection was adapted and optimized to round specimens and should lead to highest possible reproducibility. The measurement parameters, such as applied current and duration of current pulse, were optimized with the aid of statistical experimental design. The new sensor system was tested on specimens of case-hardened steel AISI 5115 (16MnCrS5, 1.7139) with different damage states. The signal acquisition could be optimized by the measurement parameter variation by a factor of more than 100 compared to initial measurements. By using the delta mode, the thermoelectric effects could be eliminated during the measurement. A significant influence of deformation-induced damage on the electrical resistance measurement was validated. Specimens with less forming-induced damage showed lower electrical resistance. The results were validated with recordings by destructive testing in SEM. Thus, a cost-effective non-destructive method for forming-induced damage analysis was developed, providing the capability for process monitoring in future.