Ratcheting Fatigue Behaviour and Post-fatigue Tensile Properties of Commercial Aluminium

Abstract

This investigation examines the ratcheting fatigue behaviour and post-fatigue tensile properties of commercial aluminium in annealed and normalised condition. Ratcheting is the phenomena of progressive accumulation of permanent deformation when a component is subjected to asymmetric cyclic loading which also causes variation in their mechanical properties. One set of commercial aluminium rods were annealed by soaking the as received rods at 200°C for 30 minutes prior to machining and other set were normalised up to same temperature with same soaking time. The microstructural examinations were carried out with the help of optical microscope as well as by scanning electron microscope. Hardness measurements were done using a Vickers hardness tester by applying 3kg load with dwell time of 15 s.
Specimen for fatigue test were designed with test section diameter of 10mm and reduced gauge length of 20mm. stress controlled fatigue test were carried out at room temperature up to 150 cycle under different combination of mean stress and stress amplitude in servo-hydraulic testing machine. In annealed condition mean stress is 40, 45, 50 MPa and stress amplitude is 175, 185 MPa, while in normalised condition mean stress is 30, 35, 40 MPa and stress amplitude is 170, 180 MPa. Fatigue test were done at a constant stress rate of 50 MPa/s. In addition a series of specimens were first subjected to 150 cycles of loading prior to carrying out tensile test on these specimens using nominal strain rate 0.83 x 10-5 s-1. Fractographic studies were carried out on the broken samples by using SEM. Hardness was measured after post-fatigue tensile test.
The results highlight that accumulation of ratcheting strain increases with increasing mean stress and stress amplitude in both annealed and normalised condition. In normalised condition strain accumulation is less in comparison to annealed. Strain accumulation get saturated after about 40 cycles in annealed condition and 30 cycles in normalised condition for any of the selected test parameters. Post-fatigue tensile strength increases with mean stress and stress amplitude but with associated reduced ductility. It is observed that dimple size decreases with increase in strain accumulation. Hardness also increases with increase in strain accumulation. The increase in strain accumulation is explained by the phenomenon of increased dislocation density during asymmetric cyclic loading, whereas the increase in tensile strength and decrease in ductility are attributed to cyclic hardening.