Study On Mechanical Response Of Q235 Steel And Constitutive Model Under Cyclic Loading

Large hydraulic press is a type of mechanical equipment used widely.Beam is the main structural components which bears all the working loading.Due to the discontinuity of beam structure,there is often a local high stress zone inside,and even local plastic deformation occurs.Ratcheting effect,Bauschinger effect,cyclic hardening and softening phenomenon will probably occur in local plastic deformation area because of the cyclic and alternating loading on the beam during the working process.The ratcheting effect can cause the plastic deformation to accumulate continuously and aggravate the failure process of the equipment,which must not be ignored in the structural design and life evaluation.Therefore,it makes important theoretical significance and engineering application value for improving the design level of hydraulic press body to deeply study the mechanical response characteristics of common structural materials of hydraulic beam under cyclic loading and accurately predict their deformation behavior and evolution trend.Multi-level single-stage symmetrical strain and asymmetric strain cyclic loading experiments,as well as multi-stage symmetrical strain cyclic loading experiments with increasing and decreasing amplitude were conducted by using Instron8801electro-hydraulic servo fatigue test machine.The mechanical response characteristics of Q235 steel under the strain cyclic control mode were explored,and the evolution law under different loading modes were analyzed,such as yield platform,yield stress,hysteresis curve,Bauschinger effect,and cyclic softening/hardening.The corresponding prediction model was established.The results show that Q235 steel exhibits different cyclic softening and hardening characteristics under different strain cyclic strain conditions.The cyclic softening/hardening rate will reach a stable value after a certain cycle,and the value is basically unchanged under forward and reverse loading.The Bauschinger effect increases with the decrease of strain amplitude and tends to be stable after the cycle reaches a certain value.The single-stage asymmetric stress cyclic loading experiments with positive and negative mean values,the single-stage stress cyclic tensile-unloading experiments,experiments with loading holding time and pre-deformation,and the multi-stage stress cyclic loading experiments with historical loading were carried out.The effects of stress amplitude,mean stress,deformation history and loading time were studied.The evolution laws of the hysteresis curve,ratcheting effect,Bauschinger effect,softening/hardening properties with the stress cyclic loading were analyzed,and the corresponding prediction model was established.The study shows that the ratcheting effect occurs when stress peak is greater than the lower yield limit.The cycle numbers of ratcheting occrence is related to the peak loading.The loading history has a significant influence on the cyclic response characteristics,but the influence gradually fades with the increase of the cycle numbers.Based on the systematic analysis about hysteresis curve and ratcheting effect evolution characteristics of Q235 steel,an elastic-plastic cyclic constitutive model is constructed which can describe the stress-strain hysteresis curve for any shape and accurately predict the ratcheting strain evolution process.The model can be reduced to Prager model,A-F model,Bower model and A-V model,and different combinations of the above models can also be formed.The full analytical formula of the back stress in the model is given,the solving method of material parameters is proposed,and the influence law of material parameters on the model prediction results is discussed.By embedding the constitutive model into commercial finite element software,cyclic mechanical response analysis of complex structures can be performed.The accuracy of the constitutive model in predicting material ratcheting effect is verified by the simulation analysis and comparison of single-axis cyclic loading experiments under various conditions.Taking the lower beam of a large hydraulic press as the prototype,a 1:14 simplified box structure with local high stress was designed.A cyclic loading test system for complex box structure was built based using the electrical and the speckle measurement methods.Through the comparison of the numerical simulation and the test data,the predictive ability of the constitutive model to the mechanical characteristics response of box structures under cyclic loading is further verified.