Growth behavior of micro-cracks in fatigued copper and low carbon steel

The annealed copper and JIS S15C steel were fatigue-tested under rotating bending and the growth behavior of microcracks was examined through an optical microscope. The distribution of crack size was found to be described by either a single straight line or a broken line on the Weibull plot. The appearance of the break is due to the coalescence of pretty large cracks of the size from about 100 to 200μm. The maximum crack size plotted against the ratio of a given cycle number to that to failure, N/Nf, increased discontinuously around N/Nf≅0.4 in copper. On the other hand, it rose continuously in the most part of fatigue process in S15C steel and a sudden ascent on this curve occurred around N/Nf≅0.6-0.8. Such a discontinuous increase in the maximum crack size is also due to the coalescence of pretty large cracks mentioned above. These results indicate that the fatigue process is controlled mainly by the gradual growth and connection of microcracks less than about 100-200μm in size in S15C steel, while the discontinuous joining of pretty large cracks dominates the fatigue process in copper.The relation between the maximum crack size and the cycle ratio, N/Nf, was almost independent of cyclic stress level in copper, while it was definitely influenced by this in S15C steel. The magnitude of cumulative cycle ratio in the double repeated fatigue tests under two step stress amplitudes, which had been found to be dependent on the kind of materials, was reasonably interpreted through such crack growth behaviors.The Frenche's damage lines were found to be located at N/Nf≅0.1 and 0.50 in copper and S15C steel, respectively. The cracks of about 120μm in size were formed at these lines in both materials.