TU-TESL

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In the automotive industry, biomaterials such as natural fiber reinforced thermoplastic composites have been used in the manufacturing of automotive parts. For example, the interior panel of golf VII is made of natural fiber. However, due to the limitation of strength, natural fiber composite materials are only used for interior decoration. In order to further expand the use of natural fiber composite in automobile structural parts, this paper studies and develops the composite of glass fiber and flax fiber. A new composite seat frame is designed by using new materials, and the mechanical properties and crash safety of the composite seat are tested and characterized by finite element simulation. In the manufacturing of fiber-reinforced composites, the selection of materials and composite technology are more critical. In this study, natural fiber and glass fiber bi-directional mixed woven fabric was selected as reinforcement, and then the semi-finished product with certain size was cut by applying pressure and impregnating resin. Then, the semi-finished product is heated in the molding mold to melt the resin, and then the short fiber reinforced PP resin is injected to form the coating. See Figure 1 for the specific forming process flow. In the manufacturing process, in order to achieve uniform impregnation of reinforcement materials, the setting of process temperature is very important. On the one hand, the temperature needs to be higher than the melting temperature of PP resin; on the other hand, due to the low thermal stability of natural fiber, the temperature cannot be set too high. Flax fiber is mainly composed of cellulose and biomass (such as hemicellulose, lignin, pectin, etc.), and its thermal performance is tested and analyzed through TGA, as shown in Figure 2. It can be seen from the figure that considering the melting point of recycled PP material and the thermal decomposition temperature of flax fiber, the process temperature is selected at 180 ℃ - 190 ℃. The reinforcing material used in this study is a mixture of flax fiber and glass fiber, and the polymer matrix is polypropylene resin with low viscosity and low melting point. Among them, polypropylene resin can improve its wettability by adding mold release agent, lubricant and coupling agent. In order to improve the volume fraction of the fiber in the composite, the fineness of the fiber and its combination with the resin are very important. Glass fiber has poor hygroscopicity, but flax fiber shows high hygroscopicity because of its more hydrophilic groups. 50 samples of flax fiber and glass fiber with the same fineness were selected for the test, and the tensile test was carried out according to DIN EN ISO 5079 standard. The results are shown in Figure 3. It can be seen from the figure that the tensile modulus of flax fiber is lower than that of glass fiber; the strain of common flax fiber is similar to that of dry flax fiber; while the strain of glass fiber is higher, reaching 3%. Test on the properties of organic sheet semi-finished product the organic sheet semi-finished product is composed of mixed fiber fabric and PP resin matrix, the volume fraction of fiber material is 52%. In order to evaluate the tensile properties of the bio composite, the tensile tests were carried out in the longitudinal (0 °) and latitudinal (90 °) directions, and the results are shown in Figure 4. The results show that although the recycled plastics are prone to molecular degradation, resulting in the degradation of mechanical properties, the organic sheets with recycled PP show a higher tensile modulus. The tensile test results of organic sheet are shown in Figure 5. Among them, figure a shows the glass fiber reinforced PP organic sheet. The mechanical properties of glass fiber reinforced composites are isotropic, and the stress-strain behaviors of samples at 0 ° and 90 ° are almost the same. Figure 5B shows the organic sheet reinforced by flax natural fiber and glass fiber. It can be seen from the figure that elastic deformation occurs at the initial stage of tension, and the load almost increases linearly. Different from pp-gf47, NF enhanced organic sheet showed different mechanical properties in different directions. The tensile strength of the samples tested in 90 ° direction is higher than that of the samples tested in 0 ° direction, which is mainly caused by the different weaving directions of the mixed fibers. Figure 6 shows the failure test results of NF organic sheet. Due to the thermo viscoelastic properties of organic sheet, its forming behavior is highly related to temperature and forming speed. Therefore, according to figure 7, the mechanical test of thermal cycle is carried out. The organic sheet is first heated to the process temperature using an infrared heater and then transferred to the mold. During the transfer process, the material will lose part of the heat. The die was then closed and the material underwent non isothermal cooling due to the heat transfer between the material and the die steel. In this process, the cooling rate of the material is very important, because the recrystallization of the matrix is highly dependent on the cooling rate. Because the change of fiber orientation during thermoforming is mainly affected by in-plane shear behavior, this study focuses on the characterization of shear behavior. At present, there are two widely used research methods: picture frame test and bias extension test. During the test, the fiber direction is ± 45 ° with the stretching direction, as shown in Figure 8. Because the shear rate has an important influence on the flow behavior of thermoplastic matrix, it also has an important influence on the occurrence of wrinkles. Therefore, two constant shear rates are selected for the experiment: 3 ° / s and 6.43 ° / s. The velocity distribution is shown in Figure 9. The shear angle in region C can be calculated from equation 1. Since the shear stiffness of thermoplastic PP matrix increases with the increase of shear rate, formula 1 is optimized to evaluate the effect of shear angle only on shear stiffness. Assuming that the shear rate is constant, the shear angle γ (T) changes linearly with time, as shown in formula 2. Further derivation is shown in Equations 3 and 4. In the process of forming, three temperatures were selected in the range of 140 ℃ - 190 ℃, and the temperature dependence of the material was studied. It can be predicted that the crystallization of thermoplastic polymer has an important influence on the forming behavior and the mechanical properties of the formed parts. During the test, the sample is fixed in the tensile tester and heated to 190 ℃, and then cooled in contact with the mold. Figure 10 shows the temperature change of the sample. The test results are shown in Figure 11. In the experiment, nf-pp organic sheet material was used to design and develop a seat shell, and Fe drape simulation software pam-form 2017 was used for finite element simulation. PAM form is used to model the parameters of the whole forming process including die, composite layer and so on. The structural design of the seat shell is shown in Figure 12. In the process of design simulation, the rectangular organic sheet is used first, and the 3D trimming curve of the final part shape is projected onto the organic sheet by forming simulation, and then the sheet is cut. Considering the deformation of gravity state, simulation deviation and so on, the simulation trimming is repeated until the required part shape is finally obtained. The optimization process is shown in Figure 13. Using PAM crash software for collision simulation, the simulation data obtained is compared with the experimental data as follows: using European seat collision ecer17 standard, dynamic collision simulation is carried out at the speed of 64 km / h, and the effective equivalent plastic strain of collision simulation is shown in Figure 15. The results show that the bottom of the seat shell shows obvious deformation, which is the result of the dummy moving forward. The cracks formed in the manufacturing process form the stress concentration, resulting in the final failure. As shown in Figure 16, there is a critical area in the seat shell where failure may occur. Conventional isotropic simulation of impact test is not enough to predict the plastic strain and failure behavior of finished parts. In order to improve the results, it is a very good choice to consider the anisotropic behavior. As a result, more accurate predictions have been obtained in many regions, while in others, the results have not improved. One of the reasons is the wide range of experimental data processing, part of which is based on non-standard test methods. In addition, the description of organic sheet has not been fully studied, so it is difficult to verify the compiled material card. There is still great potential in processing experimental data and determining input values in practice. The purpose of this study is to promote the application of natural fiber composite in automobile structural parts. In order to evaluate the mechanical properties of the composites, a series of characterization and finite element simulation were carried out from the aspects of forming temperature and strain rate. At the same time, the car seat frame is made of the natural fiber reinforced composite, and the suspension simulation and finite element simulation are carried out. The results show that the mechanical properties of natural fiber composite materials can not meet the requirements of structural parts, and the composite materials reinforced by natural fiber and glass fiber have better mechanical properties, which can be used in automobile structural parts.