Plastic Deformation Of Metal Under Low-Stress/Low-Energy Repeated Impacts

Some key parts,moving components,and control components that are used in the chemical,nuclear power,aerospace,mining,and electrical industries work under multi-impact loads for a long time.The contact surfaces are constantly subjected to repeated impacts,and the contact stress is very small and generally far below the yield limit of the material.However,workpieces show a macroscopic plastic deformation from the top surface to the very inside,which causes failure of the components mentioned above.Therefore,it is very important to study such failure behavior under low-stress/low-energy multiple impacts.Low-stress/low-energy multi-impact plastic deformation is significantly different from static and quasi-static deformation.The yield stress is much lower than the static yield limit,but significant plastic deformation still occurs.The speed of low stress and multiple impacts ranges from approximately 5 to 20 m/s,which is between ratcheting and strong impact.This kind of plastic deformation caused by low-stress/low-energy load is a mechanical phenomenon that usually does not attract much attention from researchers.In this paper,Experiments on the typical metal material YT01 that is often used in electromagnetic mechanism,and the multi-dimensional parameters of plastic deformation under low-stress/low-energy multiple impacts are studied based on experimental analysis.Under low-stress and multi-impact load,the effects of impact peak stress on hardening degree,deformation amount,layer deformation rate,and relative strain were studied.The analysis of the experimental results shows that the cumulative deformation of the sample is approximately logarithmically related to the number of multiple impacts.With increasing impact peak stress,the cumulative deformation increases.The relationship between impact peak stress and layer deformation rate is an approximately exponential curve.As the impact peak stress increases,the layer deformation rate also increases.After the sample is impacted more,the hardness difference gradually decreases with increasing distance from the multi-impact surface layer.During the entire multi-impact process,the microstructure of the sample is continuously refined,and the average grain size continuously decreases as the peak stress increases.Under the action of low energy and multiple loads,the effects of hardening degree,cumulative deformation,layer deformation rate,and relative strain of the specimen were studied.The experimental results show that under different impact energy loads,the impact energy increase brings about an increased cumulative deformation of the sample.The layer deformation rate under multiple plastic deformation decreases when the distance between the check point and top surface increases.Multi-impact deformation occurs only at a certain distance away from the impact top surface.The deformation termination position is independent of the impact energy,but it is related to the impact peak stress.The larger the impact peak stress,the further the termination position is from the impact top surface.Under different impact energy loads,the larger the impact energy,the smaller the sub-grain size after the sample,and the larger the microscopic strain becomes.The impact energy and peak stress are important factors affecting the cumulative deformation of multiple impacts.The effects of impact frequency on the hardening degree,deformation amount,layer deformation rate,and relative strain of the sample under low stress loading were studied.The experimental results show that the impact frequency has a greater impact on the hardening degree of multi-impact plastic deformation than that of impact stress.With increasing impact frequency,the cumulative deformation after multiple impacts first increases and later decrease,while the number of impacts increases.The greater the impact frequency and impact peak stress,the greater the distance from the impact top surface to the strain termination position.The larger the impact frequency,the larger the average time the impact is applied to the sample,the more obvious the impact hardening effect,the greater the influence on the microstructure of the sample,and the finer the grain size becomes.Based on the John-Cook dynamics model,the constitutive relationship between the parameters at different dimensions and the strain after multiple impacts is established.Based on the model,the parameter values A,B,and n are determined and the yield constitutive relationship of low-stress multiple impacts is set.Meanwhile,the dynamics analysis method is used to simulate low-stress/low-energy multiple impacts by ABAQUS/Explicit finite-element software.The simulation results show that the equivalent plastic strain increases with increasing impact peak stress and impact energy.The equivalent plastic strain increases with increasing impact frequency.At the same time,the maximum equivalent plastic strain is close to the multi-impact top surface.The equivalent plastic strain has a tangible effect.The constitutive model and simulation analysis established in this paper provide a basis for predicting plastic deformation after multiple impacts.