Abstract
This paper aims at investigating the strain ratio effect on cyclic deformation behaviour of 7050-T6 aluminium alloy. Low-cycle fatigue tests are conducted under strain-controlled conditions at three strain ratios (−1, 0, and 0.5) with strain amplitudes in the range 0.60–1.75%. Microstructure is analysed by optical microscopy, transmission electron microscopy and X-ray diffraction, and fracture surfaces are examined by scanning electron microscopy and transmission electron microscopy. The results show that the material exhibits a cyclic strain-softening behaviour whose degree increases with increasing values of strain ratio and decreasing values of strain amplitude. Under non-zero mean strain, full relaxation of mean stress is only observed at higher strain amplitudes. A second order logarithmic model is proposed to account for the mean stress relaxation rates. Lifetime expectancy is reduced as the strain ratio raises which is consistent with the progressive increment of fatigue striation spacing observed in the fractographic analyses.
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