Mean Stress Integration–Based Fatigue Life Prediction Model for Fluctuating High‐ and Low‐Frequency Superimposed Loading

Fluctuating high‐ and low‐frequency superimposed fatigue loadings (F‐HLFS) commonly occur during the service of engineering structures. The superposition of load components with different frequencies produces complex mechanical responses at fatigue‐critical details, posing risks to structural safety and reliability. This study develops a novel fatigue life prediction model for F‐HLFS based on the mean stress integration (MSI) approach, which addresses the challenge of quantifying fatigue damage under cycles with continuously varying mean stress. Constant‐amplitude and superimposed fatigue tests on notched low‐carbon steel specimens are conducted for parameter calibration and model validation. A comparative analysis is performed between the proposed MSI model and several existing models. The results show that within the medium‐ to high‐cycle fatigue regime, the proposed MSI model demonstrates a favorable balance of high accuracy, stable performance, and computational simplicity. This work provides theoretical support for fatigue‐resistant design and damage assessment of engineering structures under complex time‐varying loads.