As one of the investigations to reveal the effect of impact load on the fatigue behavior, i.e., fatigue life, fatigue crack initiation and propagation, a series of fatigue tests was conducted with a cantilever type rotating bending fatigue testing machine equipped with an impact loading device under two sorts of program load patterns; one is the impact program load condition in which the maximum bending stress followed by the transitional decaying vibrational load appears by an impact load at the rise time of the primary stress and another the ordinary periodical 2-step program load condition.
Here, annealed 0.23%C carbon steel specimens with a small hole on the plane of the minimum sectional area were used to observe the behavior of fatigue crack initiation and propagation during the fatigue process. The experimental results were discussed from two points of view; the equivalent number of overstressing cycles and the cumulative damage in the modified Miner's rule concept.
From the comparison of the equivalent number of overstressing cycles for crack initiation with that for fatigue failure, it was concluded that the impact program load pattern like the one in this experiment is more detrimental to propagate fatigue cracks than to initiate those. Such a fatigue behavior in the impact program condition is also recognized from the discussion based on the cumulative damage in the modified Miner's rule concept, that is, the ratio of the cumulative damage for crack initiation to that for fatigue failure takes generally a higher value than the value in nonimpact fatigue load conditions.
