Research On The Multiaxial Fatigue Damage Accumulation Theory

The problem of multiaxial fatigue is still a difficulty in the study of metal fatigue. Although the research on multiaxial fatigue has obtained some progress, many problems of the theory are not solved completely, and the datum of the experimental are extremely limited. The requirement of life estimate becomes higher in engineering problems, with the degree of industrialization increasing. How to solve the problem of multiaxial fatigue damage accumulation and found the constitutive relation have both the important significance and practical value.The studies of existing multiaxial fatigue damage accumulation theory aim at the effect of multi-level variable amplitude loading on damage accumulation mostly, with that of multi-level variable mode loading less. Many components used in projects undergo multiaxial loading, not only the variable amplitude loading, but also variable mode loading. So the effect of variation of amplitude loading and variation of mode loading on damage accumulation that we should reflect is necessary.Experiments of LY12CZ aluminum alloy thin-walled tubular specimen are conducted under combined tension-torsion multi-level loading including variable amplitude and variable mode loading. The fatigue damage accumulation rules under two-level loading in different loading modes are analyzed. And, a new multiaxial fatigue damage accumulation model is proposed based on Marco-Starkey model. The new multiaxial fatigue damage accumulation model only contains two parameters, cycle ratio and phase angle, and evolved into the situation of multi-level loading with variable amplitude and mode. The accuracy of fatigue life prediction of the multiaxial multi-level loading with variable amplitude and mode for the proposed model is validated by the existing of LY12CZ aluminum alloy and 45 steel, the predicted results show good agreement with experimental results. It is demonstrated that the new proposed model has the capability of fatigue life prediction of multi-level loading under uniaxial and multiaxial non-proportional. The test of typical bolt joints is analyzed as an example and the predicted fatigue life is in good agreement with the experimental results.