Multiaxial Fatigue Life Prediction Models Of Metal Materials

In the field of engineering,fatigue has become one of the most common failure modes of structures subjected to cyclic loads.Therefore,fatigue research has always been an important direction of strength research.For simple uniaxial fatigue,researchers have performed extensive investigation and achieved satisfactory results.However,there is no fatigue life prediction model accepted by all researchers for the more common multiaxial loading cases.In particular,fatigue assessment becomes more difficult when the multi-axis load coupled with mean stress,non-proportional loading,notch geometry and other complex situations.Inspired by MWCM using interpolation method to deal with multiaxiality effect,the author proposed 4corresponding fatigue life prediction models for R=-1 axial-torsional proportional loading,multiaxial loading with mean stress,non-proportional loading and notch fatigue situation.The paper mainly includes the following parts:Firstly,in this paper,the MWCM in terms of von Mises stress is modified based on the characteristic that the interpolation accuracy can be improved by narrowing the interpolation range.A modified von Mises stress is obtained by introducing a ratio parameter?,which can shorten the distance between the axial and torsional fatigue curves near the predicted point,thus effectively reducing the sensitivity of predicted life to multiaxial parameter.Meanwhile,the iterative algorithm is used to obtain more accurate ratio parameter?.Through a lot of data validation,the life prediction accuracy of the modified von Mises stress model is higher than that of the original von Mises stress method.Secondly,based on the MWCM,a new general multiaxial fatigue life prediction model that can consider the effect of mean stress is proposed.Different from the MWCM,the expressions of multiaxiality influence and mean stress effect are located separately in the proposed fatigue equation,so that the new model can consider the impacts of both axial and torsional mean stresses,and the equation form possesses excellent extendibility and variability.The wildly used von Mises equivalent stress is adopted as the fatigue parameter to improve computational efficiency.Finally,in conjunction with the Itoh criterion,the model can be trivially extended to perform non-proportional fatigue prediction with different mean stresses.Compared with MWCM,the new mean stress model can be applied to metal materials with different sensitivity to shear mean stress.Thirdly,in order to analyze the multiaxiality effect on non-proportional fatigue life,a new method for predicting low-cycle non-proportional fatigue life is proposed.A multiaxial model in terms of ASME strain is proposed to calculate the fatigue damage under low cycle multiaxial proportional loading.Based on this multiaxial ASME strain model,the influence of multiaxial strain state on determining the reference proportional path and calculating the non-proportionality factor is considered.For the first time,a viewpoint of inhomogeneous integral path for calculating factor F_np is proposed,and a path-dependent weight factor is defined to describe this heterogeneity.The prediction results show that the proposed non-proportional factor model can more accurately quantify the non-proportionality degree of loading path.Finally,for the purpose of considering the multiaxiality effect on notch fatigue life prediction,this paper makes an extension research on the theory of critical distances?TCD?and proposes a new multiaxial notch fatigue life prediction method.The method for determining critical distance shown in Kitagawa-Takahashi diagram is extended from the field of fatigue limit to the high cycle finite life interval.The relationship between axial and torsional notch fatigue life is established by using Sih mixed-mode crack growth theory,and the torsional notch fatigue life can be estimated.Furthermore,a new notch multiaxial parameter?_n is defined,and the interpolation method similar to MWCM is used to deal with the multiaxiality effect on multiaxial notch fatigue life.In this way,the computational resources required by the present method are consistent with those of the original TCD method.In addition,the new notch method preserves the advantage of simple calculation of TCD method by using linear elastic stress analysis.The predicted results show that the multiaxiality effect has a significant influence on the high-cycle notch fatigue life,and the new notch model can properly describe this effect.However,if such influence is not fully considered as the original TCD method,the predicted result will be too conservative.