Technique

The carbon fiber frame: implementation, differences and problems of reliability

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Published on Monday, 26 January 2009 10:25

Written by Luca Salvatelli

What we commonly call carbon fiber frame should be more properly called fiber composite frame, we see why. The carbon fiber was discovered for the first time in 1958 by Dr. Roger Bacon, then the process of implementation was very expensive and complex, but the mechanical and physical characteristics of the new product pushed the industry to find immediately a process of implementation on a large scale in 1969 was the first fabric made of carbon in the world. The yarn of carbon fiber is achieved through a process of oxidation and thermal pyrolysis of polyacrylonitrile that is heated to 300 degrees in the presence of air. The product of oxidation is then heated to about 2000° C in an atmosphere of inert gas to arrive at the formation of graphite. From this process, a bit oversimplified, you get a filament of grafene with variable carbon content between 93-95%. The same product, but lower quality can be accomplished by other elements. The mechanical properties of carbon fiber that is obtained can be improved with ulterior subsequent heat treatments, warming the product at temperatures of 1500-2000° C (carbonization) gives a material with the highest tensile strength, with temperatures of 2500 -3000° C (graphitization) gives a higher elasticity. The single carbon has the smallest size (5-8 µm). In relation to their form of elasticity, there are different categories of carbon fibers with low modulus (up to 200 GPa), standard modulus (200-250 GPa), intermediate modulus (250-325 GPa), high modulus (325-440 GPa) and ultra high modulus (> 440Gpa). The yarn of carbon fiber is categorized according to its linear density (weight per unit length, with 1 g / 1000 m = 1tex) or the number of individual fibers that are intertwined to form the beam, the most commonly used in the frames are: 1k (1,000 fibers per bundle), 3k (3,000 fibers per bundle), 6k (6,000 fibers per bundle), 12k (12,000 fibers per bundle). The carbon fibers can be supplied in the form of continuous fibers, such unidirectional, with the vast majority of the fibers directed preferentially along one direction and only a small amount which flows across the other, with the sole aim of keep them together, or multiway, with fibers arranged in more than one direction (the most common type is the intersection 0 / 90). In the latter case the product is a tissue consisting warp and texture. The warp is formed by fibers parallel to each other, the plot is also made up of fibers parallel to each other but tilted a certain angle than warp which will be intertwined. Then differentiate the type of tissue in accordance with different types of carbon and materials that constitute the warp and weft, in addition to the type of straw, the number of fibers above and below that which runs every other fiber to it perpendicular. In a fabric of carbon 1K will, therefore, fewer fibers per bundle, which leads to greater compactness of the fabric that is obtained which is more lightweight, durable and flexible when working compared to tissues in carbon k more. In particular, the flexibility of the fabric in the processing can achieve the perfection complex structures, without empty air and with a minimum use of resins. Indeed so far we have spoken of carbon fiber instead of fiber composite, which is a material that does not exist in nature but which was born from the union of two or more different substances that together form a product with physical and chemical characteristics superior to sum of individual products. In carbon fiber substances are added to thermosetting resins that give the necessary firmness and solidity to the different layers of carbon. These resins can be incorporated into the fabric of carbon (woven prepregs) or added later. Obviously the more low k carbon is more compact tissue and less resin is used. With nano technology, then, have created new resins that contain "nano tubes" or "spheres" whose function is to "enter" in the plot of carbon and further compacting the surface, leading to a further improvement the mechanical characteristics (this is what happens explained in a very simple way). Now we see what are the problems of carbon fiber. We have seen that already choosing the raw material you can get a carbon better or worse quality and the same thing happens with the production process and temperature of cooking, the best is to obtain a constant and yarn up with a very high percentage of carbon. Then in the case of carbon fabric is important that warp and weft are made with absolutely perfect crossings and absence of empty air. Every small change compared to the optimal situation, leads to a decay of the fabric of benefits that spill, of course, the final product that you will realize, partly because the projects are implemented based on the specific basis of carbon fabric used. Another important factor is quality resins with is impregnated with carbon fabric and how this has been impregnated. All factors, unfortunately, impossible to assess, or almost, from a fan. Now we come to the realization of a carbon fiber chassis. Today there are two main processes for carbon frames: the monocoque and wrapped. The monocoque chassis is made from a mold of the main triangle where the skins are arranged carbon impregnated with resins, the series steering box and the bottom bracket. Once finished assembly (even here we have very simplified) piece is placed in an autoclave where the temperature is high dry resins and low pressure prevents the formation of air bubbles that are likely to arise, both for errors when relax the sheets of carbon, for the same drying process resins. The construction of a banded frame is more similar to that of a traditional frame, because the carbon tubes are cut and shaped to size and then placed on a specific template and united with each other through bandage with a tape of carbon. The subsequent cooking in an autoclave is the same monocoque chassis. The first production process is very economical if done on a large scale, but not suitable for small productions, as also fulfill the mold is very expensive, it is not appropriate to achieve a wide range of sizes and it is impossible to achieve sizes tailor. By contrast, the chassis is more rigid equal weight because you can optimize the use of sheets of carbon fiber and the same design is better because you can better calculate the distribution of carbon skin on every single point of frame. The frames wrapped instead, have the advantage of achieving a tailor, but are more expensive to make the manual more necessary for their implementation, the bandage also in areas with connectors affects weight, which becomes greater, and rigidity of the area, which becomes less, compared to a monocoque chassis of equal weight. Now it's useless debating what the best solution, because they simply do not hesitate. Both options have their pros and cons and it is for the designer, determined by the chassis design and dynamic performance that should have, in addition to its target price, choose the most appropriate solution. The problems and disadvantages of carbon frames are born all, or almost under way. Indeed, first there is the raw material, namely carbon fiber, which as we have seen can be obtained from different raw materials and in different production processes, changing the characteristics. Then the same sheets of carbon can be different not only for K, taken too simply a metro quality of the product, but also and above all, quality and compactness of its yarn weaving resins and the presence or absence of bubbles d 'air. So far, the raw material, then there is the realization of the frame. This is more skins of carbon arranged to each other, is not that they should all be the same type and quality also is important that the provision of the start of carbon are carried out with the crossings planned in the project, which there are no empty air, and that the carbon panels are well drawn. The process of cooking in an autoclave is crucial to the success of the frame. In each stage, therefore, mistakes and superficiality can determine the success or otherwise of an excellent frame regardless of its design. Here lies the problems. Indeed, the designer can design the best chassis of the computer world, but then it should be realized in practice and this is what's much more difficult. It is important that the skins of carbon in the real have the same specifications as those provided for in the design and are arranged according to the project, it is not easy. More still is easy that there may be errors that can be taken to save or not. For example temperatures in the lower oxidative process leading to significant energy savings, as well as use different raw materials from polyacrylonitrile. Even in the later stage there may be substantial savings, minimizing waste production, through good those carbon panels that have a texture not perfect or empty air. Then we have the material realization of the chassis, which reduced the time of implementation left to manual, can lead to the final cost savings, but in contrast to a not perfect realization, with a draft summary of the skins of carbon, for a cut no precise tubes for chassis and wrapped in a last time pressures and temperatures reached optimal autoclave. Let's leave out, then, cases in which a surface layer of a type of carbon if they submit other carbon very poor and even fiberglass! Or pipe badly cut and shaped but are also used since then bandage hides them. In all these cases, unfortunately, the chances of defense for the customer are virtually nil, if not rely on the seriousness of the manufacturer. In fact, the result of the savings listed above tarder not to occur. Here is a list of the main points of a frame in carbon badly done:
1 serial break glue of steering and bottom bracket
2 break glue of dropouts
3 rupture of seat post dinghy near the neck of the closure of seat post and neck derailleur
4 crack near the conjunctions
5 peeled the paint

See causes:
Steps 1 and 2 are caused by a high quality resins used as well as any possible design is not optimal, not a high quality of carbon used and a drawing is not optimal in those areas subject to vibration and forces surrender soon. Even temperature and duration of pressure cooking in an autoclave affect the final yield.
Point three, if you exclude the mistake of having too close to collars, testifies to a pipe too thin or made with carbon poor who do not bear the workload.
Point four is a clear example of when you realized a frame with banded pipes badly cut, badly shaped with a bandage and not made in accordance with best practice and badly designed.
Section 5, however, is not just a lack of realization of the frame, but a symptom, the first thing is for sure that the painting was not successful, then maybe the chassis flexes and vibrates too and that is bad and realized designed, in all cases we are faced with a poor product.
Having said that the mistakes of production and there are still defects and a 5% is widely tolerated, then if this is greater then it is a symptom of poor quality. But a carbon fiber chassis is very nice made long and stable performance. We just need to take care of submitting them to avoid accidental shock, to forge the collars over the couple prescribed by the manufacturer and not aggressive cleaning solvent for carbon, ordinary care that should be used with all products. In conclusion, therefore, one can not deny that carbon composites have led to a significant evolution of bicycle frames, with undeniable advantages in terms of weight, rigidity and comfort. On the other hand is more difficult to recognize the real quality and require more attention from the end.