Carbon fiber (CF) is a new type of fibrous material with a carbon content of more than 95% high strength and high modulus fiber. It is composed of flake graphite microcrystalline and other organic fibers in the direction of the fiber axial direction, and obtained by carbonization and graphitization. Carbon fiber "soft inside", the quality is lighter than the metal aluminum, but the strength is higher than the steel, and has the characteristics of corrosion resistance, high modulus, is important material in the defense military industry and civilian aspect. It is not only the intrinsic characteristic of carbon materials, but also the soft and processability of textile fiber, which is a new generation of reinforcing fiber.
Carbon fiber has many excellent properties, carbon fiber axial strength and high modulus, low density, high performance than, no creep, not resistant to ultra high temperature oxidation environment, good fatigue resistance, heat and electrical conductivity between non-metal and metal, small thermal expansion coefficient and anisotropy, good corrosion resistance, permeability good X-ray. Good thermal conductivity, electromagnetic shielding, etc.
The young's modulus is more than three times that of traditional fiberglass. Compared with kevlar, the young's modulus is about twice as large as that in the organic solvent, acid and alkali, and the corrosion resistance is prominent.
Carbon fiber composition:
Carbon fiber is inorganic polymer fiber with carbon content above 90%. The carbon content is higher than 99 percent of the graphite fiber. The microstructure of carbon fiber is similar to artificial graphite. The spacing between the layers of carbon fiber is about 3.39 to 3.42A, and each carbon atom in the parallel layers is not as neatly arranged as graphite, which is connected by van der Waals force between the layers.
The structure of carbon fiber is generally regarded as the structure of two - dimensional ordered crystals and holes. The content, size and distribution of holes have a great influence on the performance of carbon fiber.
When the porosity is lower than a certain threshold, the porosity has no obvious effect on the shear strength, bending strength and tensile strength of the carbon fiber composite. Some studies have pointed out that the critical porosity that causes material mechanical properties to decline is 1% to 4%. When the pore volume content is within the range of 0-4%, the pore volume content increases by 1% and the shear strength of the interlayer decreases by about 7%. According to the study of carbon fiber epoxy resin and carbon fiber bimaline resin laminates, the shear strength of interlaminates began to decrease when the porosity exceeded 0.9%. It is known from the experiment that the pore is mainly distributed between the fiber bundle and the interlayer interface. Moreover, the higher the pore content, the larger the pore size, and significantly reduce the area of the interlayer interface between laminated plates. When the material is stressed, it is easy to break between layers, which is the reason why the shear strength between layers is relatively sensitive to pores. In addition, the pore area is the stress concentration area, and the bearing capacity is weak. When the force is stressed, the pore expands to form a long crack, which can be destroyed.
Even if two laminates with the same porosity (using different presoaking methods and manufacturing methods in the same maintenance cycle), they also exhibit a completely different mechanical behavior. The mechanical property decreases with the increase of porosity, and the effect of porosity on mechanical properties is large and repetitive. Due to a large number of variables, the influence of pore on mechanical properties of laminated composite laminate is a very complicated problem. These factors include the shape, size and location of the pore. Mechanical properties of fiber, matrix and interface; Static or dynamic load.
The effect of pore size and distribution on mechanical properties is greater than that of porosity and porosity. It is found that large pore (area >0.03mm2) has adverse effects on mechanical properties, which is attributed to the effect of porosity on the crack propagation in the interlayer zone.