Multi-axial Fatigue Life Model And Fatigue Test Spectrum Formation

The fatigue life prediction model and the fatigue test spectrum formation of structures under multi-parameter and non-proportion loading are studied in this paper.The stress distributions of multi-hole thin cylinders and aeroengine cases under multi-parameter loading are analyzed by finite element method. The fatigue damage distributions of structures under multi-parameter loadings are calculated. The results show that the maximum stress point and the maximum damage position are changed with the multi-parameter loadings.Based on the damage equivalent principle, a new method which can be applied to structures with uniaxial stress failure modes under multi-parameter loadings is developed to create the multi-parameter fatigue test spectrum. At first the strain histories of the initial crack position are decomposed into a series of simple individual cycles by rain-flow counting method. The individual cycles are divided into several groups by strain amplitudes. In each group, the number of cycles is counted and the average damage value is computed. Then the multi-parameter cyclic loadings are searched in each group so that the maximum damage location under the loading is accord with the results under the effect of the original spectrum and the cyclic damage is equivalent with the average damage. The fatigue test spectrum is developed by linking these constant amplitude multi-parameter loadings. The experiment results show that the test life scatter band under the fatigue test spectrum which is compiled by this method is within 2 times compared with the original one, which proves the efficient of this method.A new multi-axial fatigue life prediction model, in which the parameters of the maximum shear strain amplitude and normal strain excursion are included and the influence of the additional cyclic hardening of maximum equivalent stresses are considered, is developed. The life prediction model developed can be degenerated to the traditional strain based life prediction model under uniaxial tension or torsion loadings. The experiment results show that the new life prediction model can be applied to both proportional and non-proportional loadings in different materials with smaller life scatter band and higher precision.Based on damage equivalent principle, a new method of creating the fatigue test spectrum is developed for the multi-axial stress destroying mode with single danger position under multi-parameter loadings. At first, the failure mode under multi-axial fatigue loadings is studied. The main damage parameters are counted by the multi-axial rain-flow counting method and divided into several groups. The damage value and the main damage parameters are searched in each group, so that these values under multi-parameter cyclic loadings are accord with the statistic average values of each group. The fatigue test spectrum could be created by linking these cycles with constant amplitudes. The experiment results show that the life scatter band under the fatigue test spectrum which is compiled by this method is within 2 times compared with the original one, which proves the efficient of the method developed for compiling the fatigue test specrum.