The Analysis Of Multiaxial Fatigue Life Considering The Dependence Of Loading Paths

Fatigue failure widely exists in the service process of mechanical structures and engineering components, and it is often shown as a more complex multiaxial fatigue failure. This is because on the one hand, structures often bear the role of complex multiaxial loadings; On the other hand, even under uniaxial loading, due to the presence of geometric discontinuities such as notches or defects, some partial regions of such structures will still in a multiaxial state of stress and strain, leading to the multiaxial fatigue fracture. Therefore, the multiaxial fatigue problems widely exist in the practical engineering problems, and it will have great theoretical significance and practical value if the prediction of fatigue life of such metal structures under complex multiaxial loadings can be properly solved.In this paper, some attemptions are made for the fatigue life prediction of metal smooth and notched specimens under multiaxial loadings, especially under multiaxial non-proportional loading conditions, and the main work is as follows:(1) The strain state of thin-walled tubular specimens under tension-torsion multiaxial loadings is numerically analyzed. It is found that in the presence of different phase angles of multiaxial non-proportional loadings, the shear strain amplitudes of different material planes reflect certain regularity. According to this finding, a new method is proposed to calculate the degree of non-proportionality of different multixial loading paths, in order to consider the influence of loading path on fatigue life reduction. The feasibility of this method is verified by the experimental fatigue datas of 6061 Al refering to the corresponding literature, and its accuracy is also compared with other methods.(2) The existing models based on the critical plane method of predicting low cycle fatigue life are reviewed and summarized, and a new LCF life prediction model which can describe well the effect of non-proportional additional damage is proposed. By introducing a non-proportional additional damage coefficient into the fatigue parameter, the influence of loading path and material hardening effect on fatigue life is considered. A large number of fatigue testing datas including 8 materials and 9 kinds of multiaxial loading paths are used to verify the effictiveness of this model.(3) According to the actual tension-torsion loading conditions which notched specimens bear under tests, the elastic-plastic finite element analysis of LY12 CZ notched specimen is implemented by using the ANSYS Work Bench software. Based on the information of the strain state at fatigue dangerous point of the notched specimen, the fatigue life prediction of LY12 CZ notched specimen under different loading paths is carried out by utilizing the proposed LCF life prediction model combined with the basic idea of local stress-strain method. The TCD method is then took into consideration to modify the extremely conservative prediction results of the local stress-strain method.