Role of Impurities and PSBs on Microcracking of Polycrystalline Copper at Very High Numbers of Cycles

Abstract

. Fatigue cracks in polycrystalline copper may originate from PSBs or grain boundaries. They usually form at the specimen surfaces, but also internal small stage I (shear) cracks have been observed with the ECC/SEM technique. They are formed together with a strongly elongated dislocation cell structure, which is reflecting in many cases localized deformation in "slip lamellae" with eventual ladder-like features, being typical of PSBs. Both, PSBs and small non-propagating cracks are initiated at cyclic stress/plastic strain amplitudes below the conventionally reported PSB threshold values, if the number of cycles exceeds a minimum, e.g. approximately 5x108 in the VHCF range. The internal small cracks are formed not only in polycrystalline electrolytic copper of 99.98% purity but also in high purity (99.999%) material.