Study on the surface state induced by ultrasonic impact treatment and its influence on high-temperature tension-tension fatigue behavior

To reveal the influence of ultrasonic impact treatment (UIT) parameters on surface state and the influence of surface state on high-temperature tension-tension fatigue behavior, UIT tests were conducted and 400 ℃ high-temperature fatigue tests were performed on UIT specimens with different surface states. The relationship between surface state and process parameters was analyzed, and the fatigue failure mechanism of UIT specimens was revealed. The finite element method was used to explore the influence of different residual stress distributions on the distribution of stress intensity factors at the crack front and the evolution of crack shape. The results show that the surface roughness changes between 0.43 μm and 0.72 μm with the variation of UIT intensity. As the increase of UIT intensity, the maximum residual stress can reach −1085 MPa, located approximately 30 μm below the surface. And the corresponding maximum surface hardness can reach 561 HV_0.025. Microstructure elongation and extrusion intensification results in a maximum deformation layer depth of 9 μm, with a nanocrystalline layer on the outermost surface. The maximum average fatigue life is 6.431×10⁶ cycles and the cracks initiate under the surface with quasi-cleavage as the initiation mode. As the cracks deepen, the stress intensity factor increases, and the crack shape eventually tends to be circular. Alterations in residual stress amplitude and depth do not influence the final crack shape. However, increased amplitude and depth can accelerate the consistency of the stress intensity factor outside the compressive stress region.