Design And Multi-Objective Optimization Of Vehicle Bumper Beams Prepared In LFT-PP

Driven by the goal of achieving zero-emission of global automobiles by 2050,lightweight structures and materials of automobiles have gradually become the research focus of energy-saving and new energy vehicle technologies.Long fiber reinforced thermoplastic composites(LFT)have become a research hotspot of automotive lightweight composites due to their excellent strength,specific modulus,impact resistance,and good processability.Based on the experimental preparation of long glass fiber reinforced polypropylene(LFT-PP)and the design principle of equal stiffness and equivalent generation,the structural design and optimization research of LFT-PP automobile bumper anti-collision beam were carried out,with the focus on its collision safety performance,lightweight effect and pedestrian lower limb protection performance.This paper focuses on the application research of LFT-PP automotive lightweight materials in the following three aspects:(1)LFT-PP composites with a glass fiber content of 30%,40%,and 50%were prepared by hot-melt impregnation method.The microstructure and fiber distribution morphology of LFT-PP were studied by CT scanning and SEM electron microscopy,and the mechanical property parameters and Poisson’s ratio of LFT-PP were obtained.The results show that with the increase of fiber content,the failure mode of LFT-PP material is more fiber pullout than fiber fracture.The Poisson’s ratio of LFT-PP decreased with the increase of glass fiber content.The elastic modulus of LFT-PP with 30%glass fiber content reached 7.18 GPa and the tensile strength reached 129 MPa.The bending strength and modulus of LFT-PP samples increase with the increase of fiber content,and the resistance of LFT-PP to bending deformation and impact is improved.Based on the tensile test data and the Swift-Hockett hardening model,the finite element simulation analysis parameters suitable for LFT-PP materials were obtained by fitting and extrapolating,which ensured the effectiveness of the subsequent simulation work.(2)Based on the design principle of equal stiffness and equal generation,the finite element model of LFT-PP bumper beam with low-speed impact is established based on the prototype of a certain model T6 6061 aluminum alloy bumper beam.The results show that,compared with the AL anti-collision beam,the LFT-PP anti-collision beam reduces the mass by 17.4%,the cost by 69%,and the lightweight effect is remarkable.LFT-PP beam has higher specific energy absorption while ensuring crashworthiness.Based on the different sensitivity of the force and deformation of the anti-collision beam to the thickness variation under different collision conditions,a design of the anti-collision beam with continuous variable cross-section thickness LFT-PP bumper was proposed.The energy absorption effect and collision safety of composite beams are further improved by building a continuous anti-collision beam with variable section thickness.The optimal thickness parameter combination was obtained by combining the optimal Latin hypercube experimental design method with the second-order polynomial response surface proxy model.The results show that under the same impact time,the peak impact force of the bumper beam with continuous variable section thickness decreases by 12.5%and 19.7%under the longitudinal and angular impact conditions,respectively.Compared with the beam with uniform thickness,the maximum invasion of the beam with variable section thickness decreases by 5.3%,and the energy absorption ratio increases by 20.6%.The optimized continuous variable cross-section beam can better balance the crashworthiness and lightweight performance.(3)Through the establishment of the simplified vehicle model and the finite element collision simulation model of the rigid leg,the research on the protective performance of pedestrian lower limbs of LFT-PP automobile bumper anti-collision beam was carried out.The results show that the tibia acceleration at Y0 and the tibia acceleration at Y390 and the knee bending angle is out of standard.By introducing the rectangular EPP energy absorbing foam buffer structure to optimize the automobile front structure,pedestrian leg injury can be effectively reduced.After optimization,the peak acceleration of the tibia corresponding to the original vehicle model with the three materials of anti-collision beams at the collision position Y0 and Y390 was significantly reduced,and the bending angle of the knee was significantly reduced at the collision position Y390,the shear displacement of the knee was reduced to a certain extent,and the comprehensive injury value of the pedestrian’s leg was significantly reduced.Compared with Al and uniform thickness beams,LFT-PP cross-section beams with variable thickness have lower pedestrian injury risk and better pedestrian protection performance.