Study On Multiaxial Fatigue Life Prediction Method Considering Notch Effect

Fatigue fracture is the most important failure mode of mechanical components and structural parts under cyclic loading.In practice,most engineering components work under complex external loading.Meanwhile,as the mechanical structures become more and more complex,there are many notches where the structure size changes suddenly in the components.The combination of external loading and notches results in multiaxial alternating stress.During the entire service period,the fatigue damage gradually accumulates,which leads to the deterioration of component performance until the fatigue fracture occurs.Nowadays,more fruitful results have been made in the research of multiaxial fatigue problems,and several types of multiaxial fatigue life prediction models have been formed.However,if these models proposed based on the smooth specimens are applied to analyze the fatigue life of notched components with more complex stress states,the results are not ideal because they do not consider the influence of the notch effect.In this paper,the Q345(A)steel solid round bar specimens with circular U-shaped notch were taken as the research object,in order to investigate the crack initiation orientation at the notch,and predict components' life under multiaxial loading.By combining theoretical analysis,finite element method and test,a fatigue life prediction model for multiaxial notched parts was proposed,and the feasibility and correctness of the model were verified by comparing with the test results.The main research contents include:(1)Fatigue tests under the proportional loading conditions are carried out,the influence of notch size,strain amplitude and strain ratio on fatigue life were studied,which provided experimental basis for further study of multiaxial fatigue theory.(2)Based on the energy criterion and the critical plane theory,the strain energy density function of any plane at the dangerous point was given by introducing the coordinate transformation matrix,and a method for determining the critical plane based on the energy criterion was proposed.(3)FEM was used to carry out mechanical analysis of the notched specimens subjected to tension-torsion loading.Based on the critical distance method,the function of equivalent stress in a certain range on the critical plane was fitted by MATLAB software.By calculating the stagnation point of the relative stress gradient function,a mathematical calculation method of critical distance was given.(4)Based on the stress field intensity approach,the effective field intensity within the critical distance was taken as the fatigue damage parameter.Meanwhile,a multiaxial fatigue life prediction method considering the notch effect was proposed and used to predict the fatigue life of notched specimens.The results show that the multiaxial fatigue life predicted by the new model is in good agreement with the test results,and the prediction accuracy is significantly improved compared with the traditional local stress strain approach.