Comparative Evaluation of Fatigue Crack Propagation in High-Strength Steels S460, S690, and S960 Using Optical and CMOD Methods at Stress Ratio R = 0.1

The present study compares the fatigue crack propagation behavior of three high-strength steels—S460, S690, and S960—tested at a stress ratio R = 0.1 and at room temperature. The measurement of fatigue crack growth was accomplished through the implementation of two complementary techniques. The first technique involved the assessment of crack mouth opening displacement (CMOD), which was recorded using a clip gauge on an MTS 100 kN testing machine. The second technique entailed the use of an optical crack tracking resonant electromagnetic pulsator Amsler 20 HFP 5100. The material characterization process entailed the execution of tensile testing, Vickers hardness measurements, and microstructural analysis. The propagation rates of fatigue cracks were evaluated using Paris-law fitting, which revealed that S460 exhibited the lowest crack growth rate, followed by S690 and S960. Scanning electron microscopy (SEM) was utilized to confirm the presence of Mode I fatigue striations in all materials. The novelty of the work lies in the controlled replicate-specimen comparison of three HSS grades under identical CT geometry, stress ratio, laboratory conditions, and Paris-law evaluation procedure, while transparently defining the CMOD–optical comparison as a method-aware comparison rather than a simultaneous within-specimen validation.