The Study Of Time-dependent Springback And The Novel Prediction Model Based On Viscoelastic Theory For The Typical Metals At Room Temperature

The three basic crystal structures include body centered cubic structure(bcc),face centered cubic structure(fcc)and close packed hexagonal structure(hcp).High strength steel of DP600 and stainless steel tube of 1Cr18Ni9 Ti have bcc structure;aluminium alloys of AC600 PX and AC170 PX have fcc structure;commercial pure titanium of TA2 and titanium alloy of TC4 have hcp structure.The above typical metals were selected as the objectives to investigate their anelastic behaviors that were explored under different loading histories.The loading variables included pre-strain,loading strain rate,soaking time,and plastic deformation zone.The conclusions were drawn as follows.Two models based on viscoelastic theory and the non-linear behaviors of typical metals at room temperature for predicting time-dependent springback were provided.The first method was as follows.Based on the stress-strain curves of high strength steel of DP600,aluminium alloy of AC600 PX,and titaniumalloy of TA2,the creep compliances were obtained from the creep tests under the specified constant stresses.According to the viscous behaviors from the elastic and plastic loading tests,the lower limit of integration in the constitutive equation of linear viscoelasticity was modified.Each predicting curve was acquired by using the superposition of the unloading impulse and the historical loading curve.The second method was as follows.The limited Prony series based on Kelvin model was applied to fit the experimental curves of time-dependent springback of AC170 PX aluminium sheets.Then the best fit constants were fitted to obtain the change laws between the best fit constants and the loading strain rates.Therefore,the cases that were not tested could be predicted.The above typical metals in the tests showed high recovery rates in the initial time-dependent response and the recovery rates gradually decreased following the initial stage at room temperature.As the pre-strain went up,the absolute values of anelastic strain and the ratio of anelastic springback strain to total springback strain were both increased.In the same springback period,the ratio of the unloadingstress to the creep springback strain tended to vary more linearly with the pre-strain than those obtained from the immediate unloading.The time-dependent-springback behaviors were similar between the DP600 steel sheet and AC600 PX aluminium sheet.That was mainly associated with the behaviors of initial springback and plastic flow at room temperature.There exists an anisotropy in recovery behavior with respect to lattice plane for AC170 PX aluminium sheet.The anelastic strain was sensitive to the loading strain rate in the tests.The absolute values of time-dependent-springback strain(TDSS)for titanium alloy of TC4 and commercial pure titanium of TA2 both showed different exponential growths in the time-dependent response,as the pre-strain and the loading strain rate increased.The pre-strains that were selected for TA2 titanium distributed in the uniform plastic deformation zone,the diffuse necking zone,and the localized necking zone.The results showed the change law in the uniform plastic deformation zone was different from the case in the necking zone.The microstructure analysis indicated that the higher pre-strain led to more deformation twins in the relaxing microstructure of the specimen of TA2.The OM and TEM results of TC4 titanium alloy showed that the evolutions of “streamline”microstructure,dislocation pile-up,and deformation twin had positive correlation with the change law of TDSS.