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How good is the impact resistance of carbon fiber?
Release date: [2017/8/25] Read total of [563] times

Carbon fiber is the most suitable material for racing cars at present, otherwise, so many formula one teams, without shortage of money and people, have already used better new materials. Compared with traditional metal materials, the strength, stiffness and impact resistance of carbon fiber are generally advantageous, especially the performance of unit weight is obviously superior to that of metal materials. And looking back on F1 history, we can see clearly that carbon fiber cars have a huge advantage in both performance and safety performance compared to past metal material cars.

For example that silverstone kimi raikkonen in 2014, when kimi raikkonen's ferrari car crashed into the wall at one end of the out of control after the final, with about 240 kilometers an hour straight hit the wall, the impact of the collision moment 47 g, then the car back track, on the track through a dense F1 car traffic, turn around and then hit on the other side of the fence, the massa's car during this fight. After such an impact, raikkonen just scraped his ankle and knee and could climb out of the car himself. If such safety features can't convince you, then you can think of what would happen if this was a heavy, closed-door car.

The design of the formula one car, or the design of all the performance cars, is a typical engineering problem, which is colloquially known as "dancing in chains". Faster cars, yes, but probably not safe; Safer cars, yes, but probably not. The so-called engineering design is to find the best balance in this. Simply put, it is to meet the minimum safety requirement, to achieve the highest car performance. And in terms of engineering materials, the best balance right now is carbon fiber.

Carbon fiber, as we usually call it, is a vague general term, and the performance of different kinds of carbon fiber varies widely. Note that what we call "carbon fiber" is actually a "carbon fiber reinforced composite" abbreviation and commonly known as "carbon fiber", which is different from real "carbon fiber". Simple to understand, the real "carbon fiber" as one of wool, and we say normally "carbon fiber" is the woven sweaters, scarves, gloves, etc. (and Yu Qian teacher wool underwear).

The so-called carbon fiber reinforced composite material, in fact, USES a lot of carbon fiber, which is arranged in a certain direction, and then is closely connected with resin or other adhesion materials. This one, for example, is a carbon fiber, and these cylinders are filled with resin filled in the middle. The distribution density of these fibers directly influences the final material performance. Because of this, we can control the ultimate performance of carbon fiber materials by adjusting the so-called fiber volume fraction, which is the fiber volume ratio. Simply put, the more dense the fiber, the more fiber in the unit volume, the higher the strength along the fiber direction; Conversely, the less fiber, the less fiber in the unit volume, and the lower the strength of the carbon fiber material.

For carbon fiber used in engineering, the fabric can be either single-directional or multi-directional. The most common of course is the multi-directional crossover, which is the appearance of the carbon fiber that we often see.

So let's say that this is a single direction.

So this is a multi - directional crossover, and the appearance of our usual carbon fiber is this bidirectional cross texture. The original carbon fiber material is like this, it is more like cloth, can bend, can be rolled into a roll.

This has led to a widespread misunderstanding of carbon fiber materials, which is to confuse the "fiber" and the ultimate "fiber composite". "Fiber" is a simple measurement of single cylinder, the performance of such tests of performance is very surprising, that is a lot of people and even some popular science readings often say several times or even ten times as much as steel. But really is not used in the engineering application of a single one of the fiber, but the fiber and resin of fiber composite material, the engineering performance depends not only on the performance of the single fiber, also the influence of the performance of resin and fiber density, more affected by fiber direction. In other words, the performance of carbon fiber material is actually a weighted average of fiber performance and filling resin performance. For most carbon-fiber composite materials, the test results may be stronger than steel, but the difference is not so different.

We can compare the average carbon fiber and general steel. For example, we can look at the contrast between strength and fracture energy. Simply put, we use different materials to make the same size of chopsticks. The so-called strength is the force needed to break this chopsticks, and a fracture energy is with a big hammer hit the energy needed to break this chopsticks, to some extent reflect the impact resistant ability of is material.

The vertical axis Strength in the above image is the tensile Strength, while the horizontal axis is the fracture energy, also called toughness. At the top left, for example, are the ones that are both in the upper left corner and in the area of the water, such as the diamond and the glass, which we know very well, but the strength is quite high, but the toughness is very low, and it can be broken when it is broken. For example, rubbers in the lower right corner, rubber materials, such as our usual tyres, are very good, and the deformation is very large and can be recovered by themselves. It will not break easily, but the intensity is not too high. The foams foam in the lower left corner, for example, is neither strong nor resilient. It is brittle and broken.

Obviously, for the car's chassis and body, we hope that we can have a kind of strength and toughness is very good material, durable and not easy to be broken, the upper right corner of the figure and lavender purple composites fiber composites of metals and alloys metal materials. From here we can see that the toughness of the FRP fiber composite and the traditional metal material, which is the impact performance, is basically similar.

Metals, for example, are more resilient than steel, but are significantly less powerful than steel. The low alloy steel has a much higher strength than steel.

In the same fiber composite, carbon fiber CFRP is significantly stronger than GFRP, but carbon fiber is less resilient.

Take the carbon fiber and our common steel, compare the two pictures, the intensity of carbon fiber in about 400 to 800 million mpa, and the intensity of ordinary steel from 200 to 500 million mpa, didn't reach several times and even ten times. The toughness, carbon fiber and steel basically similar, there is no obvious difference.

For shock, of course, is very complex, to evaluate the performance of the test method also has a lot of kinds, such as the traditional measurement of fracture toughness, and low speed of Charpy or the Izod impact test, such as high-speed bullet impact test again, or is specialized for the material of FRP flat drop weight impact and so on. The impact performance of carbon fiber materials is also affected by temperature and loading speed. Different application areas are concerned with different test conditions, and the corresponding destruction patterns are different. Here we are just using the concept of toughness in general, just to show that the toughness of carbon fiber is on the same order of magnitude as steel.

The question is, since there is no obvious difference, why do we use carbon fiber? Because we haven't considered yet another important parameter, the density. Carbon steel, aluminum alloy is much higher than the density of the metal material, that is to say, to do the same parts, almost volume, meet the similar mechanical properties, carbon fiber parts is much lighter than metal parts, and it is vital for motor racing.

We all know that the concept of weight ratio is very important for a car. For example, the famous civilian car, subaru WRX STI, has 310 horsepower, but as a four-door sedan, it weighs nearly 1.5 tons, so it has 0.2 horsepower per kilogram. The kawasaki H2R also has 310 horsepower, but as a motorbike, it weighs only 215 kilograms, averaging nearly 1.5 horsepower per kilogram. In this contrast, it is obvious who will eat the ash on the straight. This is especially true of formula one cars, which can be marked by a small loss. So the formula one car design is very sensitive to the weight of the car. In other words, we want to be the least weight, and hope that the strength and rigidity are highest, and carbon fiber is almost the only option under such design requirements.

This led to many another misunderstanding of carbon fiber, also is that the carbon fiber is a kind of super material, so the things made of carbon fiber has certain aspects are far stronger than the things made of metal. In fact, engineering design is a combination of materials and sizes, not just material. It's like saying, we all know that steel is obviously stronger than wood, and it's easier to say that the steel is stronger, but the steel is definitely stronger than the stuff made of wood. For example, a steel bar with a diameter of 1cm and a piece of wood 10cm in diameter, which is more important?

For an example of the simplest, such as a part of formula one racing, in the game when the stress of the need to meet certain requirements, such as 100000 oxen, and if I use the strength of 400 million mpa, so this part of the area to 2.5 square centimeters; As a comparison, if I use the strength of the carbon fiber of 800 mpa, then the sectional area of this part needs only 1.25 square centimetres. That is, because carbon fiber is twice as strong as steel, it can be half as large as steel. Combined with the density of carbon fiber is only about one 5 of the steel, so the weight is only one over ten of the steel parts, carbon fiber parts force performance is exactly the same, but can carry 100000 cattle.

However, we also see that the carbon fiber and steel of the unit area have similar toughness, like 20 kilojoules per square meter. For these two parts, to meet the same intensity requirements, the area of the carbon fiber parts is only half the size of the steel parts, so the toughness naturally is only half of the steel parts. That is to say, the same design and the mechanical properties meet the same requirements, if you don't do any additional remedies, the carbon fiber parts impact fracture energy is only half of the steel parts. Obviously, the lower the fracture toughness when subjected to the impact load, the worse the safety performance.

So what happens? How can you improve the safety of a car in an accident? On the one hand, the engineer will enlarge the appropriate thickness of carbon fiber components, etc., in fact, now of the carbon fiber weight of the car is less than the minimum requirements of the FIA, before the game F1 will be placed in the car tungsten as the ballast weight to satisfy this requirement. On the other hand, engineers will improve the design of carbon fiber racing cars to improve the anti-impact performance of the vehicle, thus protecting the safety of the drivers.

For example, today's F1 car chassis and body, and is not a simple single carbon fiber, but a sandwich structure, upper and lower two layers of carbon fiber materials, aluminium alloy or other fiber composite materials is among the cellular structure. The carbon fiber on the upper and lower surfaces is usually composed of layers of carbon fiber, and each layer is the kind of carbon fiber fabric that we refer to when we talk about multiple directions. The stiffness of the entire system can be adjusted by changing the height of the middle cell. And these honeycombs, because they are hexagonal hollow structures, are actually very light weight. Honda tests show that compared with no middle of the aluminum alloy honeycomb pure carbon fiber, joined the thickness after 3 times the thickness of carbon fiber is made of aluminum alloy honeycomb, weight increased by six percent, but stiffness into the original 37 times. At the same time, in the event of the accident, these aluminum alloy honeycomb deformation and fracture can absorb a lot of impact energy. It's like putting a lot of cans in the air, and you have to crush the cans before you can reach the inner carbon fiber.