Experimental Study on Fatigue Performance of Q355D Notched Steel Under High-Low Frequency Superimposed Loading

During the service life of steel bridges, the structural stress histories display combined cyclic characteristics due to the superposition of low-frequency thermal loading and high-frequency vehicle loading. To investigate the fatigue performance under such loading patterns, a series of constant-amplitude and high-low frequency superimposed loading fatigue (HLSF) tests were conducted on notched specimens fabricated from Q355D bridge steel. The influence of HLSF waveform parameters on fatigue life was systematically investigated. Based on the fracture evolution mechanism, a concept of low-frequency periodic damage acceleration factor was proposed to effectively model the block nonlinear damage effects, and the applicability of existing fatigue life prediction models was discussed. The results show that the effect of average stress on the fatigue life under HLSF can be effectively considered by Walker’s formula. Low-amplitude ratios and low-frequency ratios indicate unfavorable loading conditions that may accelerate the Q355D fatigue damage accumulation, and these conditions are not adequately accounted for in current life prediction models. Compared to constant amplitude loading, HLSF can lead to a 66% and 46% reduction in high-frequency life when the amplitude ratio reaches 0.12 and the frequency ratio reaches 100. Compared to Miner’s rule, the proposed damage correction method reduces the life prediction error for HLSF by 11%. These findings provide valuable references for the fatigue assessment of bridge steel structures under the coupled effects of temperature and vehicle loading.