Near-threshold behaviour of polycrystalline copper

Abstract

Fatigue slip band accumulation, crack initiation, and crack-growth behaviour at low strain amplitudes in polycrystalline cylindrical and notched specimens of commercially pure Cu have been investigated. A high-frequency tension-eompression test method was used which permits cyclic loading of a significant amount of specimens at total strain amplitudes ranging from 1 × 10⁻⁴ to 8 × 10⁻⁴ for numbers of cycles in excess of 10¹⁰. The results indicate the existence of a plain specimen fatigue limit at ε_tot = 7.2 × 10⁻⁴ while a much lower limit for persistent slip band formation was found at ≃ 3 × 10⁻⁴. From crack-growth measurements, a threshold stress intensity was observed to occur at 2.14 MN m^−3/2 at growth rates approaching 10⁻¹³ m/cycle. A comparison of the values for the plain specimen fatigue limit and the threshold stress intensity reveals a limiting crack length of about 0.1 mm, above which the cracking condition can be described by the assumption of linear elastic fracture mechanics. For smaller crack sizes, the fatigue behaviour appears to be controlled by the plain specimen fatigue limit.