Fatigue failures of driveline and suspensions components for ground vehicles under multiaxial loading conditions are common, since most those components are subjected to complex multiaxial loadings in service. In addition to the multiaxial loadings, many of those components contain notches and geometrical irregularities where the fatigue failure often occurs due to stress concentrations. Therefore, the origins of the multiaxiality can be related to various combinations of external loadings and notch geometries. A computational fatigue analysis methodology has been proposed here for performing multiaxial fatigue life prediction for notched components using analytical and numerical methods. The proposed multiaxial fatigue analysis methodology consists of an elastic-plastic stress/strain model and a multiaxial fatigue damage parameter. The multiaxial stress-strain notch analysis method originally proposed by Buczynski and Glinka is adapted to develop the elastic-plastic stress/strain model to compute local stress-strain responses using linear elastic FE results of notched components. An original multiaxial fatigue damage parameter based on the maximum fatigue damage plane is proposed to predict the fatigue life for notched components under multiaxial loadings. Results of the proposed multiaxial fatigue analysis methodology are compared to sets of experimental data published in the literature to verify the prediction capability of the elastic-plastic stress/strain model and the multiaxial fatigue damage parameter. Based on the comparison between calculated results and experimental data, it is found that the multiaxial elastic-plastic stress/strain model correlates well with experimental strain data for SAE 1070 steel notched shafts subjected to several non-proportional load paths. The proposed multiaxial fatigue damage parameter, when applied to the uniaxial loading to account for the mean stress effect on fatigue life, is found to correlate very well with four sets of experimental uniaxial mean stress fatigue data. In the case of multiaxial loadings, the proposed multiaxial fatigue damage parameter provides very good correlation with experimental fatigue data of thin-walled tube specimens of 1045 steel and Inconel 718. In addition, the proposed fatigue damage parameter is found to correlate reasonably well with experimental fatigue data of SAE 1045 steel notched shafts subjected to proportional and non-proportional loadings. The proposed multiaxial fatigue analysis methodology enables rapid durability evaluation for notched components design. The effect of changes in material, geometry and loads on the fatigue life can then be assessed in a short time frame. The proposed multiaxial fatigue analysis methodology provides more efficient and appropriate analysis methods preferable to very expensive experimental durability tests and more complex and time consuming life prediction methods using non-linear FE stress-strain analysis.

Fatigue failures of driveline and suspensions components for ground vehicles under multiaxial loading conditions are common, since most those components are subjected to complex multiaxial loadings in service. In addition to the multiaxial loadings, many of those components contain notches and geometrical irregularities where the fatigue failure often occurs due to stress concentrations. Therefore, the origins of the multiaxiality can be related to various combinations of external loadings and notch geometries. A computational fatigue analysis methodology has been proposed here for performing multiaxial fatigue life prediction for notched components using analytical and numerical methods. The proposed multiaxial fatigue analysis methodology consists of an elastic-plastic stress/strain model and a multiaxial fatigue damage parameter. The multiaxial stress-strain notch analysis method originally proposed by Buczynski and Glinka is adapted to develop the elastic-plastic stress/strain model to compute local stress-strain responses using linear elastic FE results of notched components. An original multiaxial fatigue damage parameter based on the maximum fatigue damage plane is proposed to predict the fatigue life for notched components under multiaxial loadings. Results of the proposed multiaxial fatigue analysis methodology are compared to sets of experimental data published in the literature to verify the prediction capability of the elastic-plastic stress/strain model and the multiaxial fatigue damage parameter. Based on the comparison between calculated results and experimental data, it is found that the multiaxial elastic-plastic stress/strain model correlates well with experimental strain data for SAE 1070 steel notched shafts subjected to several non-proportional load paths. The proposed multiaxial fatigue damage parameter, when applied to the uniaxial loading to account for the mean stress effect on fatigue life, is found to correlate very well with four sets of experimental uniaxial mean stress fatigue data. In the case of multiaxial loadings, the proposed multiaxial fatigue damage parameter provides very good correlation with experimental fatigue data of thin-walled tube specimens of 1045 steel and Inconel 718. In addition, the proposed fatigue damage parameter is found to correlate reasonably well with experimental fatigue data of SAE 1045 steel notched shafts subjected to proportional and non-proportional loadings. The proposed multiaxial fatigue analysis methodology enables rapid durability evaluation for notched components design. The effect of changes in material, geometry and loads on the fatigue life can then be assessed in a short time frame. The proposed multiaxial fatigue analysis methodology provides more efficient and appropriate analysis methods preferable to very expensive experimental durability tests and more complex and time consuming life prediction methods using non-linear FE stress-strain analysis.

Mechanical components often possess notches. These notches give rise to stress concentrations, which in turn increases the likelihood that the material will undergo yielding. The finite element method (FEM) can be used to calculate transient stress and strain to be used in fatigue analyses. However, since yielding occurs, an elastic-plastic finite element analysis (FEA) must be performed. If the loading sequence to be analysed with respect to fatigue is long, the elastic-plastic FEA is often not a viable option because of its high computational requirements. In this thesis, a method that estimates the elastic-plastic stress and strain response as a result of input elastic stress and strain using plasticity modelling with the incremental Neuber rule has been derived and implemented. A numerical methodology to increase the accuracy when using the Neuber rule with cyclic loading has been proposed and validated for proportional loading. The results show fair albeit not ideal accuracy when compared to elastic-plastic finite element analysis. Different types of loading have been tested, including proportional and non-proportional as well as complex loadings with several load reversions. Based on the computed elastic-plastic stresses and strains, fatigue life is predicted by the critical plane method. Such a method has been reviewed, implemented and tested in this thesis. A comparison has been made between using a new damage parameter by Ince and an established damage parameter by Fatemi and Socie (FS). The implemented algorithm and damage parameters were evaluated by comparing the results of the program using either damage parameter to fatigue experiments of several different load cases, including non-proportional loading. The results are fairly accurate for both damage parameters, but the one by Ince tend to be slightly more accurate, if no fitted constant to use in the FS damage parameter can be obtained.

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