| Polycarbonate(PC)is widely used in machinery,transportation,construction and other fields.As a typical viscoelastic material,its nonlinear viscoelastic behavior and rheological damage characteristics have important theoretical and research value for its mechanical performance design and structural optimization,safety performance evaluation and service life prediction.For a long time,which is a significant subject and have captured extensive attention of researchers in polymer material science and polymer mechanics.1.The ideal creep is the strain response under the step stress.In the actual creep test,the stress can't reach the preset value instantaneously,and the ramp loading time before the preset constant value is finite ramp time.It will have an impact on creep behavior.Based on the Findley model,the influence of finite ramp time on the creep behavior of viscoelastic materials is investigated by numerical simulation.The results show that the Findley model is used to describe the linear viscoelastic behavior when the stress is below the critical value.The longer the ramp time is,the greater the difference between the corresponding creep curve and the ideal creep curve.This difference is reflected in the early creep In particular,with the creep time prolonged,the difference is getting smaller and smaller,the creep time is 10 times the finite ramp time,and the error caused by the finite ramp time is about one tenth of the initial time of the creep.When the stress is above the critical value,the Findley model describes the nonlinear viscoelastic behavior,where the finite ramp time causes greater error at the initial creep.Then,the short-term creep test is carried out on PC,and the effect of finite ramp time on PC creep behavior is discussed.The experimental results verify the effectiveness of numerical simulation.Finally,according to the temporal correlation of viscoelastic materials,historical correlation of load and "memory effect",the influence of finite ramp time is explained theoretically.2.The effects of thermal aging temperature and strain rate on the nonlinear viscoelastic behavior of PC were investigated.The mechanical tests and analyzes of PC materials with 12 h thermal oxygen aging at different temperatures were carried out under various loading cycles.The results show that:(1)the stress-strain behavior of PC has obvious characteristics of "strain softening","necking" and "strain hardening";(2)increasing the thermal aging temperature and increasing the strain rate,(3)Under the process of multi-step creep and multi-step stress relaxation loading,the effect of aging on the material is reflected by the increase of the loading time.The higher the thermal aging temperature is,the higher the temperature of the PC,the stronger the creep resistance of the material.3.The rheological damage characteristics of PC were studied by AE(AE)technique.The results show that the total amount of acoustic emission signal is less in the process of tensile failure,but it follows a certain development law.At the initial stage of stretching,almost no AE signal is generated,that is,in the lossless stage,the AE signal is gradually released with the increase of the load.When the yield is near,the signal is denser degree.After yielding,after the strain softening and necking process,Almost no AE signal appears,which is related to the relative elongation of the molecular chain;with the material further stretching,AE signal accumulation,which is crack initiation,molecular chain fracture occurred.The AE behavior at different strain rates is somewhat different.The larger the strain rate is,the less the AE signal appears.This is due to the fact that the damage of the viscoelastic material has rheological properties,and the initiation and growth of microcracks,The strain rate is large when the corresponding tensile loading time is short,the micro-damage in the material too late and the formation of the formation of the intensity is not enough to break,only in the fracture before the dense AE signal. |
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