In the process of producing FRP profiles, it is necessary to change the melting temperature of the glass fiber raw materials in the FRP profiles, the hole diameter of the bushing plate, the drawing temperature and the drawing speed, etc., so as to control the diameter of the glass fibers. In the past, glass fibers with a diameter of 9 to 10 μm were often used as reinforcing materials for composite FRP profiles, but now glass fibers with a diameter of 13 to 18 μm are gradually used.
In fact, theoretically speaking, the finer the diameter of the glass fiber, the higher its strength, but in actual production, the strength of the glass fiber is far lower than the theoretical value, which is due to the formation of many microcracks on the surface of the monofilament during the drawing process. The research data of Liang Zhongquan et al. show that when the glass fiber diameter changes in the range of 9-13 μm, the tensile fracture strength of high-strength glass fiber precursors remains basically stable. However, when the diameter of the glass fiber is further increased, the specific surface area of the glass fiber and its surface activity become smaller, so that the amount of adsorbed wetting agent on the surface of the glass fiber is greatly reduced, resulting in a substantial increase in the surface defects of the glass fiber and a decrease in strength.
According to the principle of fiber reinforcement, only when the fiber length is above the critical length can the reinforcement effect of the fiber be fully exerted. Theoretically speaking, the minimum length of the reinforcement fiber is 50 to 100 times the diameter. However, as the fiber length continued to increase, the tensile strength decreased significantly. The research shows that with the increase of the fiber length, the reinforcement efficiency of the fiber increases, and when the fiber length exceeds 12mm, the reinforcement effect of the fiber on various properties of the composite material basically reaches the best. Although as the fiber length increases, the microcracks of the fiber will increase accordingly, thereby reducing the strength of the fiber, but in terms of rigidity, compressive strength, bending strength, creep resistance, impact strength, etc., long fiber FRP is better than short fiber. FRP exhibits better performance. There are three ways for fiber composites to absorb impact strength: fiber breakage, fiber pullout, and resin breakage. As the fiber length increases, more energy is consumed in pulling out the fiber, which is beneficial to the improvement of the impact strength. In addition, the end of the fiber is easy to cause stress concentration, which is the initiation point of crack growth, so long fibers with a small number of ends can also improve the impact strength of the material.