VARTM Simulation And High Temperature Mechanical Properties Research Of Large-tow Carbon Fiber Reinforced Automotive Floor

The lightweight of automobile is a common problem all over the world,and carbon fiber reinforced resin matrix composite(CFRP)is an important substitute material to realize the lightweight structure of automobile.The traditional CFRP represented by small tow has been widely used in high-end models such as F1 car,BMW i3 and i8,while the relatively high price of carbon fiber greatly limits its use in middle and low-end models.The important way to solve the problem is to use large tow carbon fiber with more cost advantage.However,too much single fiber in the large tow carbon fiber usually leads to poor yarn spreading effect in the bundle,and there is slow infiltration in the fiber bundle during liquid forming,which is easy to produce bubbles,pores and other manufacturing defects.At the same time,in the traditional automobile assembly process,the car body structure represented by the automobile floor has to go through the electrophoretic drying process,which means that the carbon fiber parts can not have obvious plastic deformation at high drying temperature.and in the assembly state should have a certain strength and stiffness.In order to solve the above problems,firstly,the comparative study on the mechanical properties of large tow carbon fiber reinforced high temperature epoxy composites at room temperature and high temperature was carried out,and the numerical simulation and optimization of vacuum assisted resin transfer molding(VARTM),for automobile floor were carried out.The main research work is as follows:(1)Large tow carbon fiber reinforced high temperature resistant epoxy composites were prepared by vacuum-assisted resin transfer molding process using0/90^? biaxially stitched large tow carbon fiber cloth(50 K)as reinforced material and high temperature resistant epoxy resin(glass transition temperature 185 ^?C after curing).The tensile,bending and compression experiments at room temperature were carried out.The results show that the average tensile strength and modulus of the material are 502.72 MPa and 55.4 GPa respectively,and the average bending strength and modulus are 674.56 MPa and 21.82 GPa respectively,showing good tensile and bending properties.The compressive strength is only 146.44 MPa,which is much lower than the tensile strength,indicating that the compression resistance of the material is limited,which fully reflects the anisotropy of the composites.(2)The on-line tensile experiments at 80 ^?C,140 ^?C and 180 ^?C were carried out by using Shimadzu high and low temperature testing machine.The results show that the tensile strength of 491.19 MPa,486.54 MPa and 472.24 MPa,at three temperatures decreases gradually,but all of them are brittle fracture.The SEM observation of tensile section shows that there is more resin adhesion on the surface of carbon fiber at room temperature,while the surface of carbon fiber broken at 180^?C is smooth,indicating that the increase of temperature results in the decrease of interfacial adhesion between fiber and matrix,which leads to the decrease of sample strength.The above results show that the composites still maintain high strength at the usual temperature of 180 ^?C.The results have important guiding significance for guiding the application of composites in traditional automobile assembly process.(3)The strain change and strain recovery behavior of the composites at 25 ^?C,60^?C,100 ^?C,140 ^?C and 180 ^?C were studied by single cantilever bending test.The experimental results show that the maximum strain of the material increases gradually with the increase of temperature under the same stress.At 1800 s and at 180 ^?C,the stress is 0.84% and 1.58% of the maximum strain of 20 MPa and 40 MPa,respectively,and the creep is 0.49% and 0.73%,respectively.Relative to the initial strain,92% and70% of the material strain in 30 min can be recovered at 25 ^?C and 180 ^?C.The results show that under specific stress conditions,the materials show good creep resistance and strain recovery at different temperatures,which plays a guiding role in evaluating the plastic deformation of composite parts at high temperature.(4)Based on Darcy law,the calculation formulas of one-dimensional and two-dimensional fiber permeability are derived.Combined with VARTM experiments,the permeability of large tow carbon fiber is calculated by fitting the displacement-time curve parameters of resin flow front.On this basis,the numerical simulation of VARTM forming of automobile floor is carried out,and it is obtained that the molding scheme of simultaneous injection of glue on four sides and vacuum in the center is the best,which can not only ensure the mold filling quality,but also greatly improve the molding efficiency.(5)According to the optimal scheme of simulation,the VARTM molding mold of automobile floor was designed and manufactured,and the large tow carbon fiber reinforced high temperature resistant epoxy resin based automobile floor was successfully produced.The quality of the product is good,and the cross section of the product observed by three-dimensional video microscope system shows that the infiltration in the fiber bundle and between layers is good,and there are no defects such as dry spots and gas wrapping.In this paper,the mechanical properties of composites at high temperature are studied,and VARTM moulds and molded products are designed and manufactured based on the numerical simulation results,which is of great significance to guide the liquid molding of large tow carbon fiber and evaluate the service behavior of composite products at high temperature.