Research On Multiaxial Fatigue Life Prediction Method Of Notched Parts Based On Damage Mechanics

In engineering practice,a large number of engineering components are often subjected to complex multiaxial proportional and multiaxial non-proportional cyclic loading in service,and fatigue fracture often occurs first in the geometric discontinuity of the component.On the one hand,the existence of notches will lead to stress concentration,making the region near the notch produce non-uniform stress distribution field;on the other hand,when the notched specimens are loaded under non-proportional cyclic loads,non-proportional additional hardening effects will be generated,leading to a reduction in the fatigue life of the component.Thus,it is of great theoretical and engineering importance to perform multiaxial fatigue life prediction under the combination of stress gradient and non-proportional additional hardening.This paper takes notched specimens as the research object,and carries out research on the effects of stress gradient and non-proportional additional hardening on fatigue life through a combination of theoretical analysis,finite element numerical simulation and fatigue test,and carries out numerical simulation of crack initiation direction of notched specimens,fatigue damage parameter characterization method and fatigue life prediction,etc.At last,the correctness of the model in this paper is confirmed with the help of multiaxial fatigue test of notched specimens.The main research is in the following areas:(1)A critical plane calculation method for notched specimens based on the maximum shear strain criterion was proposed.The method uses finite element software to analyze the stress distribution state of the component and determine the location of the fatigue dangerous point;transforms the stress and strain components at the dangerous point under different working conditions into stress and strain components in any plane with the help of coordinate transformation principle;uses numerical analysis software to search for the plane where the maximum shear strain is located.(2)A multiaxial fatigue damage parameter considering the stress gradient effect was established.The stress-strain field near the dangerous point of the component is analyzed by finite element software and the equivalent force values of the specified paths on the critical plane are extracted;the equivalent force values extracted are fitted to the relative stress gradient function with the help of numerical analysis software and the effective distance is obtained by finding the minimum value of the relative stress gradient function;the multiaxial fatigue damage parameter considering the stress gradient effect is established based on the stress field intensity method.The results show that the multiaxial fatigue damage parameter can better characterize the stress gradient effect and can quantitatively describe the size of the fatigue damage area.(3)A new non-proportional additional hardening factor was defined.This factor is based on the FS model,which uses the maximum normal stress on the critical plane to characterize the non-proportional additional hardening effect and introduces the phase difference to reflect the strength of the non-proportional additional hardening effect.The results show that the factor can better characterize the non-proportional additional hardening effect and is applicable to both proportional and non-proportional loading situations.(4)A multiaxial fatigue life prediction model for notched specimens considering stress gradients and non-proportional additional hardening effects was established.The model is based on the damage mechanics theory,and the effective stress field intensity is used to construct the multiaxial fatigue damage parameters,and the additional hardening effects factor is used to characterize the non-proportional additional hardening effect.The model is validated and evaluated with the help of fatigue test of notched specimens,and the outcome shows that the predicted results of the model in this paper are in good accordance with the test outcomes and have better prediction precision,and the prediction results are on the safe side,which is more conducive to engineering applications.