Study Of Shot Peening Simulation And Fatigue Crack Growth Behavior

The fatigue has always been the one of the problems widely concerned by the academic and engineer circles in industrial engineering.The fatigue fracture of metals parts and engineering structures would result in enormous economic loss,even endanger the worker's life.Study on fatigue crack growth behavior has not only engineering practical value but also social economy significance.Since most of fatigue cracks initiate from the surface and propagate to the interior,the surface strengthening treatment is generally carried out to stop or delay the initiation and propagation of cracks.Shot peening is one of the surface strengthening technologies which is most widely used in the industrial production.The beneficial effects of shot peening on enhancing the resistance of fatigue and stress corrosion are attributed to compressive residual stress and refining microstructures.Recently,along with the rapid development of finite element method and computer technology,numerical simulation of shot peening,which is used for investigation of peening strengthening mechanism and optimization of peening parameters,has been the nowaday requirements and tends of further development of shot peening technology.The peening model parameters(such as element size,friction coefficient,step time and shot material performance)were studied comprehensively based on the single shot impact model.The peening residual stress resulted from symmetrical cell model was evaluated according to the evolution of dynamic stresses and distribution of residual stresses within in the peened regions.According to the modeling method of symmetrical cell model,the simulation of prestress shot peening was performed,and the results show that the peening residual stress becomes more and more sensitive to the tensile prestress with the increase of shot velocity.According to the real shot peening process and based on the statistical analysis of dimple distribution,a random-probability model was developed.The double shot peening process was simulated by using the random-probability model,and the simulated results demonstrate that double shot peening can improve the compressive residual stress and reduce the surface roughness.A 3D finite element model,which incorporates the temporal-spatial distribution of shock wave pressure and a unified material model charactering the Arrhenius and non-Arrhenius manners of flow stress,was developed to simulate the confined laser shock peening of the oxygen-free high conductivity(OFHC)copper.With the developed model,the interactive effects of laser power density and full width at half maximum of laser pulse,and the propagation properties of the induced stress wave were quantitatively investigated.The peening residual stress and surface deformation increase and tend to a saturated state with the increase of impacts,which is related to the hardening history of target material.The residual stress resulted from both-side laser shock peening is sensitive to the thickness of target,which is dependent on the interactive effect of shock wave induced by both-side impacts.In order to investigate the peening microstructures,the dislocation density-based material model was used to calculate the cell size and dislocation density,quantitatively.The cell size(dislocation density)within the peened region decreases(increases)significantly under the impact load.Moreover,the refining cell size and increasing dislocation density approach the saturated values with the increase of impacts.Additionly,an attempt was made to simulate the deformation of single crystal copper under shock pressure.Compact specimens were employed to study fatigue crack growth of 2024-T4 aluminum alloy under constant/variable amplitude loading.Apparent R-ratio effect under constant amplitude loading is identified with the nominal stress intensity factor range.Fatigue crack growth rates predicted by a unified model agree with the experimental data well.Single tensile overload results in significant retardation of crack growth which is fully recovered after propagating out of overload-affected zone.Retarded crack growth induced by three-step sequence loading is heavily dependent on two sequence loading parameters.The influence of variable amplitude loading on crack growth is reasonably characterized by Wheeler's model.