What is the standard for premium carbon fiber?
Popular science concepts, what is carbonization and graphitization? What are strength and modulus? In fact, many people do not understand these concepts, including material majors. Carbonization is a process in which organic fibers are cracked at high temperature to escape small molecules such as hydrogen, oxygen, and nitrogen, and finally leave carbon elements. This is a process of increasing the proportion of carbon elements. Some literatures increase the proportion of carbon elements to 92% before they are called carbon fibers; graphitization is the rearrangement of carbon atoms in carbon materials at high temperatures, gradually tending to perfect six A process of meta-mesh. Inside the carbon fiber is the process in which the original disordered arrangement of carbon atoms gradually tends to a disordered graphite structure. Some people use temperature to distinguish carbonization and graphitization. In fact, this is not rigorous. Both carbonization and graphitization are a process, not the final result. The carbon fiber is heated to 1500 ℃. If this process is a process of carbon aggregation, then it is called carbonization. If at this temperature the carbon elements are no longer aggregated and maintained in a fixed proportion, but the arrangement of carbon atoms tends to a regular six-element grid structure, then I call it a graphitization process. Therefore, carbonization and graphitization have no obvious temperature limit, whichever trend is more obvious is called which process. Different raw materials have different trends at different temperatures, so don't differentiate between carbonization and graphitization by temperature.
Regarding strength and modulus. In layman's terms, the maximum force used to damage a material is strength. If it is tensile damage, it is tensile strength, if it is bending damage, it is flexural strength, also called flexural strength, and if it is crushed, it is compressive strength. Modulus is simply the force used to deform a material to a certain size by applying an external force. The force used to stretch a material a certain size is the tensile modulus, as well as the flexural and compressive modulus. For example, the maximum force I use to break a steel bar is the tensile strength, and if I don't break it, the force it takes to stretch just n mm is the tensile modulus. Strength refers to how much external force the material can withstand, and modulus refers to whether the material is easily deformed under external force.
So what is good carbon fiber?
If you want to know why carbon fiber is bad, you must first know what is good carbon fiber. First of all, a fiber should have a uniform diameter and a uniform phase. Each fiber segment is the same as the entire fiber, and its strength is within a certain range. A fiber is cut at different places on a fiber, and if they are relatively uniform in strength, it is a good fiber. Secondly, there are more than n fibers in the same bundle of fibers, and the strength of these fibers is relatively uniform, which is a good fiber. Finally, a good fiber is a fiber whose properties such as strength, modulus and size are relatively uniform in all parts and reach the nominal standard. For carbon fibers with a nominal strength of 3500MPa, each fiber and each segment of the fiber meet the requirements and are evenly distributed at 2000MPa, which is not a good fiber. So good fiber is uniform and standard. Japanese carbon fiber is doing better than us because their carbon fiber is closer to this standard. In the past few years, China's carbon fiber was poor in uniformity. Some of the carbon fibers of the nominal T700 were higher than the T1000, and some were not as good as the T300. In recent years, through unremitting efforts, the quality of carbon fiber products made in China has been greatly improved, and the performance uniformity has also been greatly improved.
When it comes to the uniformity of carbon fiber performance, we have to talk about the dispersion coefficient, which is the CV value we often propose. Usually, there is no such item in the product parameter table of carbon fiber manufacturers. This is just a parameter that is communicated within the industry. However, this parameter does affect the use of carbon fiber. We usually say that the quality of carbon fiber made in China is unstable, and it is also said that the dispersion coefficient is too large. So what is the discrete coefficient? The coefficient of dispersion, also known as the coefficient of variation, is a parameter that represents the degree of deviation of the two sets of data. For example, as mentioned earlier, measure the strength of the carbon fiber produced today and compare it with the strength measured by the carbon fiber produced yesterday. If the difference between the two sets of data is too large, the dispersion coefficient is too large, indicating that the product Unstable quality. Analogy to an image example: shooting a target. When shooting a target, our target is the bull's-eye, but in fact, the position of the shot will deviate. If the deviation from the bull's-eye is relatively large, the dispersion coefficient is large, and the deviation from the bull's-eye is small, that is, the dispersion coefficient is relatively small. But what use are these? One of the major features of composite materials is the designability of the structure. According to the different requirements of the workpiece, we can accurately design the fiber layer and other aspects to maximize the advantages of the material. The precise design here requires that we must accurately know the performance parameters of the material. However, if it is carbon fiber with a large dispersion coefficient, that is, carbon fiber with unstable quality, then precise design will be impossible. As difficult as threading a needle with trembling hands. It can be said that there is no high-performance composite material without carbon fiber with stable quality. CV is definitely as important as the strength and modulus we usually focus on. Therefore, the designer's point of view is that it is better to have low strength than unstable quality. As long as the quality is stable, even if the strength is not high, I can make up for it through structural design, which can reflect the advantage of the precise design of composite materials.
Of course, the biggest influence of the discrete coefficient is the application as a structural component. Those friends and product companies who like to develop appearance parts, do cladding, do non-structural parts, in fact, don't pay too much attention to the dispersion coefficient, you can choose carbon fiber made in China.