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To investigate the residual stress relaxation behaviors and fatigue strength, EA4T steel specimens are subjected to micro-shot peening (MSP), and part of them are isothermally annealed at 250 °C for 2 hours. Fatigue experiments are conducted on a four-point rotating bending machine under constant and variable loads. Surface compressive residual stress (CRS) and full width at half maximum (FWHM) before the experiment and during fatigue loading are analyzed using the x-ray diffraction method. The results indicate that the CRS of the MSP specimens are relaxed after the isothermal annealing, leading to a decrease in their fatigue strengths. Variable loading slightly increased the fatigue life of the un-peened specimen, but significantly reduced that of the MSP specimens. The effect of CRS relaxation on fatigue strength is analyzed, and the relationship between CRS and FWHM is also discussed. [ABSTRACT FROM AUTHOR]

The effects of micro-shot peening (MSP) and artificial surface defects were investigated in relation to the defect tolerance of Al-Si-0.6Mg alloy in the framework of fracture mechanics. The results indicated that coral-shaped casting defects inside the alloy could be effectively quantified using the X-ray synchrotron radiation technique. An affected layer with a surface roughness (Ra) of 1.25 and residual stress of − 125 MPa was introduced on the MSP specimen compared to the un-peened (UP) specimen. The fatigue strengths of the UP and MSP specimens containing the artificial defects decreased as the defect size increased under rotating bending fatigue tests, except in the case of defects with the equivalent size of 395 and 490 µm, respectively. MSP could shift the crack initiation site toward the subsurface, while all the cracks of the specimens with the artificial defects originated from the surface and maintained an aspect ratio of 0.66 during the crack growth process. The relationship between the defect sizes and the fatigue strengths could be reasonably assessed by the modified Kitagawa-Takahashi (K-T) diagrams. The critical defect size of the MSP specimens was increased from 450 to 542 µm compared with that of the UP specimens, owing to the beneficial effect of the compressive residual stress. The defect tolerance of the Al-Si-0.6Mg alloy was improved by the MSP process. • Effects of micro-shot peening (MSP) on the fatigue properties of Al-Si-0.6Mg alloy are explored. • Casting defect inside the alloy was identified using synchrotron radiation X-ray computed microtomography (SR-μCT) technique. • The influence of MSP on the fatigue tolerance was characterized by Kitagawa-Takahashi (K-T) diagrams. • The critical defect size harmless to fatigue strength was evaluated using fracture mechanics. [ABSTRACT FROM AUTHOR]