Investigation of Stress State at Fatigue Crack Tip by Means of X-Ray Microbeam

Abstract

The stress-strain relation at the vicinity of fatigue crack tip is one of the most demanded points in the study of fatigue crack propagation. Recently, the oscillating crystal X-ray microbeam diffraction technique which enables to measure the stress in such a minute domain as the vicinity of fatigue crack tip has been devised by K. Honda and T. Konaga.In the present study, the stress condition at the crack tip was studied by this technique. An alternating tension-compression load was applied to the annealed and recovered plate specimens of 0.05%C low carbon steel with notches at the both sides. The stresses at the vicinity of fatigue crack tip in the area of 180μ in diameter were measured under several loading conditions. The results obtained are summarized as follows;(1) The stress concentration factor, which is the ratio of the stress at the maximum load or the minimum load to the applied nominal stress, is approximately the same for both annealed and recovered specimens: it's value is nearly unit for compression load, nearly 2.5 for tension load in the case of annealed specimens and nearly 2.2 in the case of recovered specimens.(2) The mean value of cyclic stress at the vicinity of fatigue crack tip is an almost zero as a consequence of compressive residual stress when the applied mean stress is zero.(3) No remarkable distribution of residual stress is measured in the vicinity of crack tip.(4) The result mentioned in (1) suggests that the cyclic stress-strain relation at the tip of crack having a relatively low propagation rate is similar to the relation obtainable after the saturation of cyclic hardening or softening.(5) The residual stress at the crack tip tends to change in reverse direction to the applied mean stress so that the cyclic stress at crack tip is almost in a fully reversed stress condition almost irrespective of the mean applied stress.