EXPERIMENTAL STUDY ON UNIAXIAL RATCHETING OF LZ50 STEEL UNDER TRUE STRESS-CONTROLLED CYCLIC LOADING

An experimental study was carried out on the cyclic deformation of LZ50 steel under a uniaxial strain and stress cycle at room temperature. The cyclic softening/hardening feature of LZ50 steel was first observed, and then the effects of mean stress, stress amplitude, stress-ratio and loading waveform on the ratcheting of the material were discussed. The main attention was paid to discern the different evolutions of the ratcheting presented in the material under the traditional nominal stress-controlled and improved the true stress-controlled cyclic loading. It is shown that: LZ50 steel presents no apparent time-dependent deformation, and can be simplified as a cyclically stable material. Both the ratcheting strain and ratcheting strain rate produced under the true stress-controlled cyclic loading are lower than those obtained from the nominal stress-controlled ones, and the extent of the difference greatly depends on the amount of total strain, and the difference was enlarged when the strain becomes larger and larger. The ratcheting strain increases monotonically with the increases of applied mean stress and stress amplitude, and decreases with the increase of stress ratio basically. However, it is not monotonic, and the most significant ratcheting occurs in the cycling with stress ratio of -0.659.