Deformation Behaviour of Asymmetrically Fatigue Cycled Metallic Materials

Abstract

This investigation aims to examine the influence of asymmetric fatigue cycling on the nature of strain accumulation and resultant variations in the post fatigue tensile properties of three different metallic materials like 6063 aluminum alloy, interstitial free steel and 70/30 -brass. In order to fulfill the aim, a series of stress-controlled fatigue tests have been carried out at room temperature (≈ 300 K) under various combinations of mean stress (m) and stress amplitude (a) using a ± 250 kN servo hydraulic Instron testing machine. All fatigue tests have been done up to 100 cycles. A 12.5 mm gauge length extensometer has been used for the strain measurement during cyclic deformation. Tensile tests have been done on the ratcheted samples to assess the effect of pre-ratcheting damage on the deformation behavior of the investigated materials. The results highlight that strain accumulation increases when mean stress is increased keeping stress amplitude constant or when stress amplitude is increased keeping mean stress. The results of tensile tests on the ratcheted samples indicate that accumulation of ratcheting strain provides improvement in yield and tensile strength of the investigated materials, associated with reduction in their %uniform elongation, compared to that of unratcheted samples; however, increase in stress values may remain associated without any significant degradation of %total elongation. Increase in strength values occurs due to the fact that ratcheted specimens possess accumulation of plastic strain and behave like prestrained materials. However, %total elongation is governed by characteristic post-necking elongation dictated by the substructural features of the ratcheted material.