Research On Thermoplastic Materials Temperature Dependency And Its Influence On The Vehicle Impact Protection

The mechanical properties of thermoplastic materials used on vehicles are temperature sensitive.While the existing research on vehicle impact protection is uaually carried out only at room temperature.There is lack of research corvering the temperature range of vehicle service.Therefore,it is necessary to study the temperature-dependent mechanical properties of thermoplastic materials and analysis the influence of temperature on vehicle impact protection.In this paper,ABS plastics and EPP foams are selected respectively as the typical thermoplastic materials inside and outside the vehicle,and their temperature-dependent mechanical properties are characterized.The material models for crash simulation at different temperatures are calibrated respectively.Crash simulations at different temperatures based on relevant vehicle impact regulations are carried out considering the temperature-dependent mechanical properties of thermoplastic materials.And the influence of temperature change on occupant head impact protection and pedestrian lower-leg impact protection is analyzed with the simulation results.Through the comprehensive experimental research including temperature,strain rate and stress state changing with ABS plastics,the mechanical properties of ABS plastics with temperature dependency,strain rate dependency and stress state dependency are obtained.The experimental results show that the yield behavior and fracture behavior of ABS plastics have a "time-temperature equivalence" relationship similar with modulus behavior in a certain temperature range.Based on this relationship,the dynamic mechanical properties of ABS plastics can be generalized at a wide range of temperature.At the same time,it is found that the yield behavior of ABS plastics does not meet the von-Mises yield criterion,which indicates that it is necessary to accurately characterize the yield behavior in the crash simulation at different temperatures and can not ignore the stress state dependent yield behavior of ABS plastics.Meanwhile,uniaxial compression tests are carried out under a wide range of temperature and loading speed with three different expansion ratio EPP foams.The mechanical properties of EPP foam with temperature dependency,strain rate dependency and density dependency are obtained based these tests.It is identified that there is almost no coupling between the temperature dependency and the strain rate dependency of the mechanical properties of EPP foam.Based on this phenomenon,the dynamic mechanical properties of EPP foam can be generalized at a wide range of temperature by using Sherwood constitutive model.The Gibson model which is based on foam's mesoscopic structure is modified.The modified Gibson model can well characterize the temperature and strain rate dependency of EPP foam's compression stress level.Then,the material models for crash simulation at different temperatures are calibrated respectively in LS-DYNA software.By comparing the simulation results of MAT_SAMP-1 material model with experimental results,the MAT_SAMP-1 is chosen for ABS plastics in impact simulation.And the input parameters of MAT_SAMP-1 material model at different temperatures were calibrated.The MAT_Fu Chang Foam material model was chosen to characterize the EPP foam,and the Sherwood constitutive equation was used to calibrate the material model input parameters at different temperatures.Based on the calibrated ABS plastics material model and EPP foam model at different temperatures,the typical impact situations between occupant head or pedestrian leg-form and vehicle based on relevant regulations are carried out respectively.The simulation results show that the thermoplastic material's temperature related mechanics feature makes the collision response not under room temperature deviates from the design conditions(under room temperature).The research reveals the risk of body injuries and the mechanical mechanism of changing rule.And the research provides basis for a more comprehensive crash protection design inside and outside the vehicle under different environment temperatures.