Research On Multiaxial Fatigue Strength Analysis And Life Prediction Method By Critical Plane Theory

In practical engineering,most components work under cyclic load conditions,which can be divided into uniaxial and multiaxial load cases depending on the type of load.The uniaxial load only reflects the force in a single direction of the component,while most actual components are subjected to multi-directional stresses,namely multiaxial loads.Therefore,in order to accurately describe the actual stresses on components,fatigue strength analysis and life prediction under multiaxial loading is essential for the design optimization and safety in service of major equipment.Critical plane theory is the main method of multiaxial fatigue strength analysis,which assumes that cracks sprout in a dangerous plane.It reveals the fatigue damage mechanism and is widely used in fatigue life prediction under multiaxial loading.Geometrically speaking,the actual components can be summarized as thin-walled circular tube specimens(smooth components)and geometrically discontinuous components(notched components).For thinwalled circular tube specimens,the fatigue damage mechanism is relatively simple.However,under multiaxial loading,the component is in a complex stress-strain state and the effects of material properties,loading forms,phase differences and non-proportional additional strengthening effect need to be taken into account when accurately predicting fatigue life.For geometrically discontinuous notched specimens,the crack sprouting and extension mechanisms are more complex due to the stress concentration at the notch.Moreover,the variation of the geometric parameters at the notch has a greater influence on the fatigue strength and life prediction of the notched component.Therefore,the proposed multiaxial fatigue life prediction method,which is applicable to the two different types of specimens mentioned above,can provide theoretical guidance for the designed fatigue resistance of the components.According to the problems mentioned above,this paper investigates the fatigue strength analysis and life prediction methods of thin-walled circular and notched specimens under different loading conditions based on the critical plane theory:(1)A multiaxial fatigue life prediction model for thin-walled circular tube specimens is proposed.Based on the critical plane theory,the stress/strain parameters and phase differences in the critical plane are considered for the thin-walled circular tube specimens.The non-proportional additional strengthening effect and the mechanism of crack sprouting and extension are investigated,and a multiaxial fatigue life prediction model is proposed for thin-walled circular tube specimens.(2)The direction of crack initiation in notched specimens is determined under multiaxial loading.The material plane with the highest strain energy density is defined as the crack initiation plane,and the stress-strain state at the root of the notch is analyzed by the coordinate transformation principle and critical plane theory to determine the crack initiation direction.The problem of determining the crack initiation plane by experimentation is solved so that the fatigue damage mechanism and damage evolution process of notched specimens can be studied.(3)The high stress influence zone is characterized.Based on the energycritical plane method,the six order multinomial stress function is used to explore the stress/strain distribution law on the critical plane,analyze the influence of the relative stress gradient value and fatigue damage zone on the fatigue damage evolution process,characterize the high stress affected zone and the whole fatigue damage zone,and propose the high stress affected zone index.(4)A multiaxial fatigue life prediction model for notched components considering stress gradients and size effects is established.In order to characterize the effects of different notched geometric parameters,a size effect influence factor is introduced to characterize the geometric size effect at the notch.The effect of relative stress gradient on fatigue life is also considered,and the Manson-Coffin equation is combined to establish a fatigue life equation considering the relative stress gradient and size effect,which is applied to the fatigue life prediction of notched specimens.