Temperature Based Fatigue Damage Entropy for Assessment of High-Cycle Fatigue in Laser-Welded Joints

To quickly predict the fatigue strength of welded joints in high-cycle fatigue tests and fit the S-N curve, this paper proposes a new model based on infrared thermal imaging technology. High-cycle fatigue tests were conducted on laser-welded joints of weathering steel Q450NQR1 and separately, on joints made of stainless steel T4003, while local temperature variations in the joints were monitored. Based on the experimentally observed temperature drop behavior, a novel Temperature-Drop-Curve-Based Fatigue Damage Entropy (TDC-FDE) model was developed to rapidly estimate the fatigue life and fatigue limit of welded joints. The model quantifies the entropy generated during fatigue damage evolution based on the temperature-decrease slope and establishes a direct relationship between entropy and the fatigue performance of the joint using this slope as the linking parameter. Experimental results indicate that a material’s specific heat capacity, density, elastic modulus, and applied stress level directly influence fatigue damage entropy generation. The entropy increase associated with purely elastic deformation does not contribute to fatigue damage in high-cycle fatigue; therefore, this portion should be excluded from the fatigue damage entropy calculation. The fatigue damage entropy of a given weld joint tends to remain nearly constant under different stress levels and loading frequencies. Finally, traditional fatigue tests demonstrated that the maximum deviation between the fatigue strength predicted by the proposed model and the experimentally measured values does not exceed 3.4%, thereby verifying the model’s accuracy and effectiveness in evaluating fatigue performance.