Research On Multiaxial Low-Cycle Fatigue And Life Evaluation For Q235Steel

Most components in the engineering are usually subject to multiaxial low-cycle loading. Because of the complexity of the material fatigue properties and load conditions, the multiaxial low-cycle fatigue theory needs to be further investigated, which has important theoretical and practical significance. In this thesis, the researches were performed by the methods of experiment combined with theory. The various uniaxial and multiaxial low-cycle fatigue tests controlled under constant strain amplitude were conducted adopting the thin-walled tubular specimen of Q235steel. Furthermore, in order to investigate the law of accumulative damage in fatigue, a series of uniaxial and multiaxial two-stage variable strain amplitude low-cycle fatigue tests and two-stage variable path combining fatigue tests were carried out. On the basis of the existing fatigue theory, further study of multiaxial low-cycle fatigue problem was performed to improve fatigue theory.The predictive capabilities of several prediction models for multiaxial low-cycle fatigue life were checked against the experimental data of Q235steel by different loading paths of uniaxial and multiaxial low-cycle fatigue tests under constant strain amplitude control. According to the analysis of the relationship between the critical plane strain parameters and fatigue life in different loading paths, a new model for predicting multiaxial low-cycle fatigue life was proposed based on critical plane concept, which has specific physical meaning and convenient application. The capability to estimating fatigue life for the presented model was verified by the test data of Q235steel under uniaxial and multiaxial loading. It is proved that the new model which is introduced the tensile impact factor is in good agreement with the results. Furthermore, the existing typical accumulative damage models were inspected by the experimental data. A new nonlinear accumulative damage model was proposed based on the present multiaxial fatigue life prediction model. This model was applied to evaluate the residual life of Q235steel under two-stage variable strain amplitude and two-stage variable path loading, and it gives better satisfied with the results.