Resin transfer molding (RTM) technology, as a typical representative of composite liquid molding process, has become one of the mainstream technologies for low-cost manufacturing of composite materials. Since the RTM process can be integrally formed, it is possible to produce composite components with complex shape and structure, high dimensional accuracy and good shape and dimensional stability, which reduces the number of fasteners, improves assembly accuracy and reduces assembly costs. It is widely used in the field of cost manufacturing.
In view of the excellent mechanical properties of domestic T800 grade carbon fiber and the low-cost manufacturing technology of RTM process, this paper mainly studies the composite material formed by the domestic T800 grade carbon fiber unidirectional cloth as the reinforcement and the RTM liquid molding epoxy resin as the matrix.
The mechanical properties can provide a reference for the promotion and application of domestic T800 carbon fiber composite materials.
Put the mold, plastic injection tank and piping system into the blast drying oven for preheating, the preheating temperature is 60~90°C, and the RTM resin is added to the plastic injection tank (preheated to 60~90°C) for pumping. Vacuum (vacuum degree not lower than -0.08MPa) for 40~60min to remove air bubbles in the resin. Open the glue injection valve to inject RTM resin through compressed air, and observe the glue discharge from the glue outlet. If there are bubbles in the resin overflowing from the glue outlet, keep the glue flowing, and close the glue outlet until all the glue outlets are closed after the glue has no bubbles. Injection is complete. Remove the glue injection system and start heating and curing: curing time 180C×3h. After curing, the heating and blasting were turned off, the mold was cooled with the furnace, and the flat test piece was taken out after demoulding.
The processing and testing of 0°, 90° tensile test pieces refer to ASTMD3039-2000, 0° compression refer to SACAMR1-1994, 90° compression test pieces refer to ASTMD6641-2009, and the processing and testing of open-hole tensile test pieces refer to ASTMD. 5766-2011, the processing and testing of open-hole compression test pieces are carried out according to ASTMD6484-2009.
Internal quality of T800 carbon fiber composites with 4 thicknesses prepared by RTM process For NDT images, in the red-white, red and orange regions, the fiber-resin interface is well bonded without pores and delamination, in yellow, green or blue area, indicating that there are different degrees of dense pores, holes or delamination defects in the interior of the sheet. From the C-scan results of the plates, it can be seen that the internal quality of the composite material plates with a thickness of 1.25mm, 1.16mm and 0.98mm is good, and the internal quality of the composite plate with a thickness of 0.94mm is poor.
The results of the 0° tensile strength test of the sheet with no internal defects show that the tensile strength of the sample with a thickness of 1.25mm is 1857MPa, the tensile strength of the sample with a thickness of 1.16mm is 2336MPa, and the tensile strength of the sample with a thickness of 0.98mm The strength is 2467MPa. This indicates that under the same layup state, the smaller the thickness of the specimen, the higher the 0° tensile strength. Because the smaller the sample thickness and the higher the dimensional volume content, the O tensile strength of the composite is mainly controlled by the fiber reinforcement, so as the fiber volume content increases, the 0° tensile strength is higher.
The tensile failure mode of the sample thickness is 1.25mm and 0.98mm, the fracture is relatively neat, and the fiber volume content of the thickness of 1.25mm is 51.3%. At this time, the resin in the fiber bundle and between the bundles occupies a large space, and the fiber bulk lower, the tensile strength is lower. In the tensile failure mode of 0.98mm, the fractures are uneven, and the fibers are pulled out, which is more conducive to exerting the tensile properties of the reinforcing fibers. The fiber volume content of the 0.98mm thickness sample is 65.3%, and the tensile strength is high. . Further microscopic analysis of the samples with thicknesses of 1.25mm and 0.98mm was carried out. The distribution state of the fiber reinforcements of the two groups of samples in the composite material. When the thickness of the sample is larger, the fiber occupies a relatively low volume in the composite material, and the distribution is relatively Sparse: Fibers with a smaller thickness have a higher volume; the arrangement is denser. It is also shown that when the fiber volume is high, it is beneficial to exert fiber strength.
The upper limit of fiber volume content of domestic T800 composite materials formed by RTM process is 68%.
After reaching the upper limit of fiber volume fraction, defects such as delamination, dense pores or holes will appear inside. With the increase of fiber volume content, the 0° tensile strength and opening tensile strength of T800 carbon fiber composites also increase accordingly. By normalizing the test results of T800 carbon fiber composites, it can be seen that the thickness has little effect on the 0° compressive strength; the 90° tensile strength is mainly controlled by the resin matrix, and is less affected by the thickness.
