A Computational Study On The Axial And Axial-torsional High-cycle Fatigue Of Metals With Nanograined Surface Layer

The failure of most engineering materials(fatigue,erosion fatigue,wear fatigue,and corrosion)is sensitive to the structure and properties of the material surface.In most cases,failure originates from the exterior surface of the specimen.Therefore,it is necessary to choose appropriate processing technology to enhance the properties o f the exterior surface.At present,surface mechanical attrition treatment is one of the widely recognized self-nanocrystallization methods for improving the surface properties of material.It repeatedly acts on the surface of the material by applying an external load,resulting in a large amount of plastic deformation.It refines coarse grains on the surface to nanograins so that the fatigue performance of the material can be improved.Specimens after sudden fatigue failure do not have large plastic deformation.Therefore,it is more dangerous and often causes serious accidents.In this paper,numerical methods have been applied to study the fatigue behavior of metals without and with a nanograined layer(NGL)under axial load and combined axial-torsional loads.A 3D cohesive finite element method combined with a local Monte Carlo method is proposed to analyze axial-fatigue crack initiation and propagation in metals without and with a NGL.The three-parameter Weibull distribution is used to characterize the spatially-random cohesive strength and fracture energy.For coarse-grained(CG)specimens,both random field and load level are found to have significant influences on the crack path and fatigue life.For specimens with NGL,the fatigue life also depends on the NGL thickness;it exhibits a marked increase as NGL thickness increases from 0 to 10 ?m.Furthermore,the combined axial-torsional fatigue life and damage evolution of both CG metals and nanostructured metals(metals with NGL)are modeled by a 3D cohesive finite element method.To account for the random nature of metal fatigue,Monte Carlo simulation is combined with the three-parameter Weibull statistical distribution function.For both CG and nanostructured metals,it has been found that the axial load levels have greater effects on the amplitude of specimen rotation.Compared with the CG metals,the nanostructured metals are found to exhibit an improved fatigue resistance,for the reason that their damage process initiates from the subsurface beneath the nanograined layer and then extends to the exterior surface.Good agreements between the numerical results and experimental data are observed.It shows the applicability of the 3D cohesive finite element method for the analysis of damage evolution and prediction of fatigue life in these two classes of metals.This work can provide some guidance for predicting fatigue life and fatigue damage evolution of metallic materials,and lays a foundation for the improvement of fatigue resistance and engineering application of metallic materials.