Microscopic Observation of Fatigue Crack Initiation Process in Low-Carbon Steel Notched Specimens

Abstract

Sharply notched specimens of 0.03%C annealed steel were fatigued under completely reversed in-plane bending. Microstructural change in the vicinity of the notch root during fatigue process preceding crack initiation was examined by means of the X-ray microbeam diffraction technique and optical microscopy. The following are the summary of the results obtained.(1) The distribution of the excess dislocation density near the notch root just before crack initiation was measured with the X-ray microbeam technique and was used to evaluate the stress amplitude at the notch root, termed as the effective stress amplitude σeff. The relation between σeff and the number of stress cycles at crack initiation near the notch root Nnc was equal to that between the stress amplitude σ and the number of stress cycles at crack initiation Nc for plain specimens. A method of estimating Nnc of notched specimens from σ-Nc relation for plain specimens was proposed.(2) The formation of slip bands for notched specimens took place when the average value of elastic stress amplitude over one grain at the notch root was higher than the lower yield stress of the test material.(3) The initiation of fatigue cracks for notched specimens was thought to occur when the stress amplitude was high enough for the linked micro cracks to grow beyond the grain boundary, which were formed along the slip bands within a grain at the notch root. The critical stress amplitude can be correlated to the threshold value of the stress intensity factor in fatigue crack propagation laws proposed by Paris et al.