Fatigue crack initiation and propagation pose critical challenges to structural integrity assessment, particularly in safety-critical applications where reliable in-service monitoring is essential. Previous studies have demonstrated the feasibility of baseline-free crack detection using higher-order harmonic parameters, specifically the second harmonic parameter (β′) and the third harmonic parameter (γ′). While these parameters showed the potential for online in-service crack detection, their fluctuations and the dependence of the first stage on sensor bonding conditions, thereby limiting robustness and interpretability. To address these limitations, the present study introduces an alternative feature based on the root mean square (RMS) value of time-domain signals for baseline-free in-service crack monitoring. owing to its direct reflection of the signal content, the RMS features demonstrates greater stability and reliability compared with harmonic-based parameters. The Dynamic Piecewise Linear (DPL) method was employed to analyze RMS data obtained from fatigue experiments conducted on multiple specimens. Results reveal that the RMS change can be clearly divided into two stages, with the critical transition point closely coinciding with the experimentally observed crack initiation. Furthermore, the proposed approach successfully identified cracks smaller than 2 mm, yielding consistent detection outcomes across different specimens and sensor configurations. Most importantly, the RMS-based method exhibited insensitivity to sensor bonding conditions, thereby addressing one of the primary shortcomings of harmonic-based approaches. This study confirms the feasibility of using RMS as a robust and interpretable feature for baseline-free online crack detection. The observed two-stage evolution and the consistency of detection across varying test conditions provide a solid foundation for the practical implementation of structural health monitoring (SHM) systems in engineering applications.

The fatigue capability of components is considerably affected, among others, by the occurrence of forming-induced damage. Understanding how forming-induced damage affects the fatigue behavior allows for optimized component design that enhances service performance and promotes lightweight construction. Electrical resistance measurements offer the opportunity for a non-destructive fatigue damage assessment. In contrast to common ex-situ measurements, in-situ measurements provide the advantage of continuous acquisition, but also require complex compensation of influences. Therefore, the fatigue damage evolution was determined through a coupled measurement setup on the basis of the electrical resistance. The resistance was measured with a self-developed in-situ experimental setup combined with the strain, diameter and temperature. This allows a compensation of geometrical and thermal influences on the electrical resistance, so that microstructural changes under fatigue loading can be evaluated. Additionally, the individual influences of geometry and temperature on the electrical resistance were quantified and compared with the measured electrical resistance change.