X-Ray Investigation on the Fatigue Fracture of Notched Specimen

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Abstract
In the present paper, the results of the X-ray study on fatigue fracture of notched specimen that has been performed as a preliminary examination on the mechanism of the fatigue fracture are described.The specimen used was 0.07%C carbon steel plate, having 60-deg. V notch, and was loaded under full reversed bending stress by the Nishihara's type fatigue testing machine. The change in micro-structure at the notch root of specimen was observed by optical micro-scope and the dimension of plastic region appeared in the vicinity of the notch root, that meant the region enveloping the slip band which came in sight until the macro-scopic crack appeared, was measured, by optical micro-scope as for cases with various stress concentration factors and various nominal stress magnitudes.On the other hand, the change of half-value breadth of X-ray diffraction line taken at the notch root in fatigue process was measured with notched specimen having a stress concentration factor of 2.0. The obtained results are summerized as follows:(1) At the very early stage of fatigue process, the slip lines appears in quite localized region close to the notch root, and as the number of stress repetitions increases this plastic region develops wider and longer progressively until macro-scopic crack is initiated.(2) When stress concentration factor is constant, the plastic region that comes in sight until the crack initiation occurs depends upon the magnitude of nominal stress applied. On the contrary, the plastic region decreases as stress concentration factor increases, when the maximum stress at the notch root is constant.(3) The plastic region at the tip of crack is much smaller than that of the notch root, and is nearly constant in the course of propagation of fatigue crack. The plastic region at the tip of crack depends on the magnitude of nominal stress, and the plastic region measured in the direction of crack propagation is likely to converge into about 0.02mm when nominal stress is small.(4) The half-value breadth of X-ray diffraction line at the notch root of specimen increases with the number of stress repetitions, corresponding to the structural change, that is, the halfvalue breadth increases rapidly at the very early stage of fatigue process and at the next stage it increases gradually, and nearing the point of crack initiation it increases rapidly again. After the crack initiation has occurred, the half-value breadth increases very slowly with the increase of the number of stress cycles.

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