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What are the differences between the mechanical properties of carbon fiber molding and hand lay-up?

Views:140 Author:Linda Publish Time: 2022-10-31 Origin:

          Carbon fiber reinforced composite materials have many advantages such as light weight, high strength, excellent fatigue resistance and vibration damping performance, and their applications in the fields of automobile and aerospace are increasing. There are many molding processes, and the widely used processes include hand lay-up molding, vacuum bag molding, compression molding, RTM molding and autoclave molding. Since the reinforced composite material is composed of the reinforcement material and the matrix material, different manufacturing processes will lead to great differences in the internal structure and mechanical properties of the material.                           Therefore,  it is of great guiding significance for the design of the structure to carry out research on the mechanics and performance laws of components with different forming processes.

          In the research, three economical processes of hand lay-up, vacuum bagging and molding are used to prepare samples. The material is Toray T300 plain woven carbon fiber cloth. Hand lay-up and vacuum bagging belong to the normal temperature forming process. The resin is made of epoxy resin, and each layer of carbon fiber cloth is laid at 0°.

          The hand lay-up process is to place reinforcement and resin in turn on a mold coated with release agent and manually operate with a brush or roller to impregnate the reinforcement with the resin, and repeat the operation until the composite product reaches the thickness requirement. The technological process is: material cutting, preparation of resin glue, mold preparation, mold release agent application, reinforcement material placement, resin brushing, curing, and mold release. The process is characterized by convenient operation, simple equipment and low cost, but low production efficiency, high labor intensity, quality and experience related, and poor production environment.

          The vacuum bag press molding process is to use a single-sided mold, lay a fiber cloth and a flexible vacuum bag on the mold, use the negative pressure of the sealed vacuum bag to pressurize the product, and use the vacuum negative pressure to inject resin, so that the resin impregnates the reinforcing material, and then solidifies forming. The process is as follows: mold preparation, coating release agent, laying reinforcing material, laying release cloth, laying out pipelines, rubber strips, laying vacuum bag sealing, vacuuming, resin inhalation, curing, and demoulding. The process is characterized by low cost, high material utilization rate, small product porosity, and good resin permeability, but additional molding auxiliary equipment and auxiliary materials are required.

         The molding process is to put the prepreg into the mold, fill the mold cavity with the prepreg through the pressure of the air bag inside the pipe, heat the mold, and cure the product under heat preservation and pressure. The process is: mold making, laying prepreg, putting in air bag, closing the mold, entering heating equipment, inflating and pressurizing, heating and curing, and demoulding. The characteristics of the process are that the integrated molding of the components can be realized, the overall strength of the components is good, and the surface performance is good, but the mold making is relatively complicated and the cost is high.

          Uniaxial tensile and three-point bending tests were carried out on samples of carbon fiber reinforced composite laminates prepared by hand paste molding and compression molding. The test is carried out on a microcomputer-controlled electro-hydraulic servo universal testing machine. The tensile test refers to the standard GB/T1447-2005 "Test Method for Tensile Properties of Fiber Reinforced Plastics". The three-point bending test reference standard GB/T1449-2005 "fiber-reinforced plastic bending performance test method", the test rate is 2mm/min, the three-point bending test span of laminates is 115mm, and the three-point bending test span of pipe fittings is 280mm. The tensile modulus and tensile strength of the hand lay-up laminate samples were 48.60GPa and 524.26MPa, respectively, and the tensile modulus and tensile strength of the molded samples were 56.34GPa and 599.69MPa, respectively. Compared with the former, the tensile modulus and tensile strength of the molded samples were increased by 15.93% and 14.39%, respectively, and the dispersion coefficients of tensile modulus and tensile strength among the latter samples were smaller than those of the former.

          The flexural modulus and flexural strength of the hand lay-up laminate samples were 31.24GPa and 562.68MPa, respectively, and the flexural modulus and flexural strength of the molded samples were 57.34GPa and 759.38MPa, respectively. Compared with the former, the flexural modulus and flexural strength of the molded samples were increased by 83.55% and 34.96%, respectively, and the performance dispersion coefficient between the latter samples was smaller than that of the former, indicating that the process consistency was better.

           There are a lot of bubbles on the surface of the hand lay-up tube, the thickness is uneven, and there is too much resin remaining. The excess resin will reduce the overall performance of the composite material. There are still a small amount of air bubbles locally: the molded product is molded under heating and pressure conditions, and the resin can penetrate into the fibers more uniformly, so that the matrix material and the reinforcing material are well integrated. Therefore, the surface of the molded pipe fittings High quality, almost no air bubbles, the most uniform thickness. The properties of composite materials depend on the properties, specific gravity and degree of bonding of the component materials. The higher the content of the component material with higher performance, the higher the performance of the composite material. The fiber volume content of the hand lay-up sample is 42%, the volume content of the vacuum bag molding sample is 46%, and the fiber volume content of the molded sample is 46%. The highest, reaching 55%, which is the reason for the higher performance of the molded samples. The performance of the resin is also one of the reasons. The strength of the high temperature resin is higher than that of the normal temperature resin, and the high temperature resin has better wettability to the fibers, so that the final composite product has higher performance.

         The failure of hand lay-up molding and vacuum bagging molding is first manifested as the cracking failure of the matrix and the separation failure of resin and fiber. This is because the uneven and insufficient pressure in the two processes makes the combination of resin and fiber without molding process. The compactness and redundancy of the resin reduce the overall performance of the component, and there is no matrix cracking in the molded component, and its failure form is mainly fiber fracture.

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