Analysis of Peculiar Fatigue Fracture Behavior of Shot Peened Steels Focusing on Threshold Stress Intensity Factor Range

In the previous report, the authors focused on the plastic behavior of fatigue crack tip and proposed a new equation of threshold stress intensity factor range for fatigue crack propagation including the stress ratio (R). The equation showed high accuracy on the fatigue limit prediction of high-strength steel with micro cracks regardless of the R. Using the equation, the following peculiar fatigue fracture behaviors of the shot peened materials were analyzed and discussed. (1) The maximum surface crack size that can be rendered harmless by shot peening, (2) The fatigue limit of cracked materials that cannot be rendered harmless, (3) The reason why most of the fracture origins of peened materials with harmless pre-crack are outside the pre-crack, (4) The reason why non-propagating cracks of stage II occur in most of peened materials. As a result, (1)-(3) was evaluated quantitatively, whereas (4) was evaluated qualitatively using the microscopic residual stress distribution after shot peening.