X-Ray Investigation on the Fatigue Fracture of Notched Steel Specimens

In the previous papers the authers pointed out, using annealed low carbon steel plates, that slip lines appear in a quite localized region close to the notch root at a very early stage of fatigue process. In the next stage the slip lines develop wider and longer progressively until a macroscopic crack is initiated. On the other hand, the half-value breadth of X-ray diffraction line at the notch root increases rapidly at an early stage of fatigue, then gradually in the next stage and near the crack initiation it increases rapidly again. After the crack initiation the half-value breadth increases considerably. The correlation of the changes in micro-structure and the half-value breadth for the notched specimen are similar to the case of fatigue in the unnotched specimen.<br>Considering these experimental results, it is suggested that the fundamental difference of the mechanism of fatigue fracture is not found between the notched and unnotched specimens. However, it is reported by many investigators that the fatigue strength of the notched specimen which is charactarized by the initiation of the crack is larger as compared with the predicted value of strength of the unnotched specimen divided by stress concentration factor. In order to interpret the discrepancy of fatigue strength between the notched and unnotched specimens, it is intended to take the discrepancy to be attributed to the behaviour of residual stress in fatigue process.<br>Hence, one of the authers pointed out from the X-ray study on fatigue of the unnotched specimen of annealed low carbon steel plate that the compressive residual stress that appears on the surface increases rapidly at an early period of cyclic stressing, but in the next stage of fatigue it decreases suddenly, thereafter its diminuation proceeds gradually until fracture occurs. It is considered that such a change in residual stress may occur in the case of fatigue in the notched specimens, and stress gradient at the notch root should give an influence on the change in residual stress. In the present paper, the results of measurement of residual stress and applied stress at the notch root at several stages of fatigue process are presented, andthe in fluence of residual stress on fatigue endurance of the notched and unnotched specimens are discussed.<br>The specimens used were 0.16% C carbon steel plates having 90-deg. V notches, and were annealed and electropolished after machining. The condition of applied stress was a fully reversed bending stress. For determining the stress value by X-ray the so-called sin²ψ method was adopted.<br>Since the fatigue fracture occurs at a very localized region in the vicinity of the notch root, an application of X-ray method of stress measurement to such a notched specimen is very effective, because the X-ray method of stress measurement is eligible to measure such a localized stress.<br>The obtained results are summerized as follows:<br>(1) The value of the compressive residual stress that is generated during fatigue process increases as stress concentration factor of the specinen increases. And the residual stress acts on the actual stress condition as the mean stress.<br>(2) These results suggest that the residual stress makes an important contribution to the increase of fatigue endurance with the increase of stress concentration factor.<br>(3) The fatigue crack can support the compressive load, while it can not support the tensile load. Consequently, it is considered that the stress condition at the tip of crack is a repeated stress with the mean tensile stress.