This paper investigates the effect of stress gradient on the fatigue life of U‐notched specimens under multiaxial proportional loading by studying the shear stress gradient and normal stress gradient on the crack initiation plane separately. First, based on the energy method and critical plane method, an energy‐critical plane method for determining the direction of crack initiation is proposed. Second, with the help of the concept of stress field intensity, the shear stress gradient and normal stress gradient on the critical plane are studied. Finally, a multiaxial fatigue life prediction model is established by combining the equivalent stress field intensity, composed of the shear stress field intensity and normal stress field intensity, with the Manson–Coffin equation. Theoretical analysis is verified by the experiments of Q345 steel notched specimens, and the results show that the predicted accuracy of the proposed method is not conservative and is superior to the local stress–strain method.
To investigate the effects of notched root stress gradient and geometric size parameters on the fatigue performance of engineering components, a new multiaxial fatigue life prediction model is established. First, based on the energy-critical plane method, the location of the dangerous plane is determined with the help of finite element (FE) analysis. Second, the sixth-order multinomial stress function is used to study the stress/strain distribution state on the critical plane, and the relative stress gradient (RSG) function is used to find the stationary point to determine the RSG value and the magnitude of the fatigue damage zone boundary value. Specifically, for specimens with different notch types, the indexes to determine the high-stress influence zone and the size influence factor are proposed. Finally, a multiaxial fatigue life prediction model considering RSG and size effect is established, and the compared resultsshow that the new model has a high fatigue life prediction capability.
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