A Continuous Step‐Loading Fatigue Test Method Based on Thermodynamic Entropy

In this study, 2.5 mm low‐carbon steel Q350 was used to fabricate high‐quality, defect‐free, laser‐welded joints, and high‐cycle tensile fatigue tests were conducted. A continuous step‐loading fatigue test method based on thermodynamic entropy was proposed for evaluating the fatigue performance of the welded joints. Based on the steady‐state temperature of the specimen during the fatigue test and the temperature drop slope of the corresponding local region following fatigue failure, the fatigue damage entropy ( FDE ) is calculated, and the fatigue performance of the welded joint is assessed. The experimental results indicate that the relationship between steady‐state temperature , temperature drop slope , entropy generation rate, and stress level conforms to an power function, and the calculated FDE is confined to a narrow range. The results of traditional fatigue tests confirmed the validity and reliability of the continuous step‐loading fatigue test method based on thermodynamic entropy. The microscopic morphology of the fatigue failure fracture was examined, revealing that the fracture exhibited fatigue striations and secondary shear cracks, which were perpendicular to the crack propagation direction. Additionally, the crack initiation points were located at the weld toe.