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post #21 of 29 Old 03-25-2009, 07:24 AM Thread Starter
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Disadvantages of Carbon Fiber

This white cloth is a flow medium sandwiched between a pair of carbon fiber weaves, and the package is then sealed. Resin is vacuum-pumped into the combination and the diamond-shaped grooves in the flow medium actually suck the resin into the carbon fiber, allowing the resin to spread to all sections of the component.


Carbon fiber composites are not without drawbacks. One of the biggest and most misunderstood is material compatibility. According to Vermillion, many enthusiasts put carbon fiber and aluminum together. This process sets up a potential galvanic reaction, Since the carbon fiber is the cathode (positive charge) and aluminum is the anode (negative charge), in the presence of heat and/or moisture, the aluminum becomes the sacrificial material and it will begin to corrode immediately. Vermillion, who has a wealth of high-tech aircraft engineering experience, has seen cases in aircraft where the aluminum has been completely eaten away. The only evidence that aluminum was there in the first place was an imprinted outline of the aluminum structure and a plethora of loose stainless steel rivets!

He also points out that moisture and temperature are key environmental concerns when dealing with carbon fiber. In Kevlar, the moisture actually "wicks" or moves through the fabric through capillary action. The result is an interdelaminating of the individual plies in the laminate structure and a gradual softening of the structure. High temperatures can also soften and weaken a composite structure. The worse case is high temperature with high humidity.

So what are the solutions? The designer must choose the matrix (polyester, epoxy, or polyimide) based on the highest anticipated temperature. The design must also be based on the ultimate material strength measured at the anticipated service temperature, not room termperature.

Evolution of Composites

There are several different types of composite materials available. The most common is fiberglass. The very first fiberglass composites were manufactured from "E-glass" or "electrical glass" fibers. E-glass had a number of desirable characteristics, but it is far less stiff than metal. While composites manufactured from E-glass can withstand considerable forces before they break, they have a tendency to deform when placed under a load. In the end, a component manufuctured from E-glass can be expected to have a stiffness-vs-strength ratio of approximately 40% that of metal.

The next evolution came from S-2 glass and R-glass. They offer slightly better mechanical properties, but their prices are significantly higher than many other (often superior composites). Components manufactured from S-2 can be as much as 1/3 stronger and 20% stiffer than more conventional E-glass composites. Although more expensive to produce, S-2 glass has found a number of uses in the aviation industry, especially with regard to areas that must have impact and abrasion resistance.

Aramid fiber is a generic name for a class of synthetic fibers called "aromatic polyamide fibers." These are manufactured fibers in which the fiber-forming substance is a long-chain synthetic polyamide with at least 85% of the amide linkages attached directly to two aromatic rings.

Researchers at the Monsanto and DuPont companies were independantly able to produce high-modulus aromatic fibers. Only DuPont, however, has produced them commercially under the trade name Kevlar, and it has been doing so since 1971.

Components manufactured from aramid laminates tend to be less stiff than those fabricated from carbon fiber. But they do have a redeeming quality: whereas structures built purely from carbon fiber can virtually erupt into a pile of dust when struck with sufficient force, those same carbon fiber pieces can have incredible impact resistance when Kevlar strands are introduced into the composite.

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post #22 of 29 Old 03-26-2009, 09:37 AM Thread Starter
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Carbon fiber came into use in the late 1960s. The actual fibers prove far stronger and far stiffer than any fiberglass mix. Carbon fibers are built by long carbon-carbon molecular chains that yield extremely stiff fibers. Trends in the aerospace industry have driven developement of carbon fibers in two directions: high-strength (HS) fibers with very high tensile strength and a fairly high strain to failure, and high-modulus (HM) fibers with very high stiffness. HM fibers have found use in advanced aerospace applications where lightweight materials with high stiffness are essential. Carbon fibers have a low coeffecient of thermal expansion, good friction properties, good X-ray penetration and are non-magnetic. The main drawbacks are the cost and the fact that all "pure" carbon composites are relatively brittle.

Material Strength vs Weight

Composites can be somewhat easily molded into structural members with complex cross sections. They also have some very impressive mechanical properties. As an example, T6061 aluminum is roughly 1/3 as heavy, 1/3 as stiff, and at best, is about 80% as strong as 4130 chrome-moly steel. Titanium is roughly 2/3s the weight of steel, 1/2 as stiff, and about 60% as strong as steel (because of this, titanium may not be a magical material for certain ultimate-performance pieces). If an identical component is manufactured from a carbon fiber composite, it will typically be less than 1/4 the weight of steel, but it is about as stiff (which makes is almost 4 times as stiff on a weight-to-weight basis), and it is roughly roughly 4 times as strong in tension. Carbon fiber also has a better fatigue life than steel, titanium, or aluminum, and the resins typically used to bond the fibers offer extremely good vibration dampning.

Vibration and shock damping are two important factors that affect components manufactured from composites. Unfortunately, they are two of the least understood subjects in materials science. According to the experts, there are so many variables involved (such as how atoms in a material absorb and dissipate vibrational energy, how the structure is built, what type of paint is applied), that it is difficult to predict how a structure will react to vibrational input. Suffice it to say that a properly prepared piece of carbon fiber will actually be superior to metal when it comes to vibration and shock damping, provided the engineering of the part is sound.

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post #23 of 29 Old 03-26-2009, 01:58 PM Thread Starter
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Composite Q&A



The world of composites can be confusing, especially when you're dealing with new terms, new materials and countless different construction techniques. The difficulty that arrises is the fact that composites are used for many different applications in both the racing and non-racing worlds.

For example, the technilogy to build something like a tub for a Fomula 1 car is far different than what's required to build a lightweight, lift-off hood for a warmed-over Honda street car. On a similar note, the level of sophisitication & engineering required to fabricate something as simple as a rowing shell (which is basically an Olympic-style raciing rowboat) is again far different from the needs of the average tuner car enthusiast.

In order to cut through this confusion, we spoke with Joe Van Overbeek Race Car Bodies in Ontario, Canada (519-264-9899), in regard to composite body panels. Van Overbeek proved candid in his responses.

HCI: How do carbon fiber composites compare to traditional fiberglass in terms of weight?

Van Overbeek: Depending on the weight, a typical fiberglass hood will weight in the range of 16-22 pounds. That same hood in carbon fiber weighs between 10-16 pounds. A raw, unmounted fiberglass pro nose can weight between 15-35 pounds. That same nose in carbon fiber weights 5-10 pounds less.

HCI: Why are some fiberglass parts sold in the aftermarket heavier than others?

VO: In many cases, the ultimate weight of the fiberglass is offset by the strength requirements. As an example, we can use a high-strength woven fabric instead of our conventional fabrics. This typically adds 10% to the overall component weight, but increases the strength considerably. Keep in mind that some fiberglass components are manufactured with a chopper gun. These pieces are always heavier than a hand lay-up; but even though they are heavier, they're not as strong.

HCI: We notice on some cars where the carbon fiber literally caves in at high speed. What is the cause of this?

VO: The primary reason is the carbon fiber component in question was constructed too light. This is very easy to do. I have a theory about this, and it stem from my background as a chassis builder. Virtually all pieces, such as a Pro nose, require some form of aluminum monocoque and chrome-moly bracing to support the structure. Why make the carbon fiber piece one pound lighter than it needs to be, only to have to add another 2 pounds of bracing so it doesn't collapse? When I build a specific component, I always try to visualize how the part will react to the forces placed upon it on the car. These forces can be aerodynamic or they can simply be created by things like pit maintainance.

HCI: How does carbon fiber compare to fiberglass when it comes to price?

VO: It depends on the component. For instance, we charge $235 for a typical lift-off fiberglass hood. That same hood in carbon fiber costs $415. A typical fiberglass Pro nose costs $630, while the same nosepiece in carbon fiber costs $1250. An entusiast can save money by mixing and matching fiberglass and carbon fiber. For exampe, we sell a complete body package for certain applications. In carbon fiber, the package sales for $5,350. In fiberglass, that same package sells for $2,950. We also offer the same package with a fiberglass main body along with carbon fiber hood, nose and doors. We charge $4,075 for this package. Mixing carbon fiber pieces with the main fiberglass body can reduce the weight by 10-20 pounds.

HCI: What precautions should racers take when handling carbon fiber components?

VO: Carbon fiber is more brittle than fiberglass. Think of carbon fiber as a pane of window glass in your house and think of conventional fiberglass as a sheet of Plexiglas in the same house. During a severe hailstorm, the Plexiglass won't shatter. The sheet of "real" glass will. Typically, carbon fiber racecar parts have thinner edges and as a result, are a bit on the dainty side. Pnce carbon fiber is locked into place Dzus fasteners, it keeps its shape and is very strong. On the other hand, fiberglass is very resilient and more forgiving.

HCI: Are there appearance considerations with carbon fiber when compared to fiberglass?

VO: Yes carbon fiber has a characteristic called "print-through." You can see the carbon fiber check pattern coming through the finish. This isn't common in a quality fiberglass part, and to be honest, it is extremely difficult to get as nice a job in carbon fiber because of it.

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post #24 of 29 Old 03-30-2009, 03:56 AM Thread Starter
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Material Strength vs Weight Cont.



Sophisitcated, finite element analysis programs and laminated-plate theory help define the properties of a composite structure. An inherent difference between composites and metals is that composite products are constructed in layers of unidirectional material. The potential for seperation under shear or compressive loads must be taken into account when analyzing an advanced composite design. While this is isn't really critical for body panels, it is absolutely mandatory if a structural member is crafted from carbon fiber.

For example, Vermiliion states that the loads placed upon something such as a three-element Top Fuel car rear wing can exceed 8,000 pounds of downforce - that's 4 tons - during a typical 300mph pass. These loads generate enormous wing stresses and deflections that are difficult to measure when the wing is mounted on the car. To resolve this issue, Vermillion's company, Automotive Racing Concepts (ARC) designed a unique testing program that accurately reproduces these aerodynamic loads.

The static testing program ARC came up with simulates various racing load conditions. Its test apparatus produces a predetermined static load on the upper surface of the wing that simulates the true wing air load at quarter cord. ARC's testing program alos measures wing tip deflections and compares those results with a computer-calculated deflection. In addition, visual and sonic observations are made at each load stage. The test program criteria begin with a search for "micro cracking" of the surface resin and progresses to total wing failure. Obviously, by using the destructive testing program, Vermillion has a very good grasp of wing loads and their effects upon carbon fiber structures.

Composites differ from metals in that they don't carry loads equally in all directions, but accept loads best in tension. An engineer we spoke with described it best this way: "A composite is similar to a bundle of strings soaked inside a layer of glue. The bundle can accept more weight and flex less if pulled from end to end, or flexed like a diving board, than if compressed or loaded transversely. The real strength of the bundle comes from the string, not from the resin. The primary function of the resin is to lock the fibers in place, transfer loads among fibers, protect the fibers from environmental damage and provide the structure with impact strength.

Composites have made many advances in the last decade. Resins, fibers and epoxies are a lot stronger today than they were yesterday. A well designed composite body panel or wing performs betten than a metal one. After taking some tentative steps, composites have become a viable material in the high-performance industry. A select few manuctacturers have taken the necessary steps to learn the capabilities and limitations of composites, which may be the key to a more widespread use of carbon fiber composites in the motorsports world.

Different Methodologies
There are a different numer of methods used to fabricate components constructed from carbon fiber. Some are far more costly than others, and some of the processes can be considered overkill for certain components. Typically replacement body panels are almost always fabricated using a conventional wet lay-up process. Structural components are almost always built using a vaccuum bag and elevated temperature cure process. In Formula 1 racing, the tub of the car is built in an "autoclave" oven. The auotclave approach costs thousands, even millions, of dollars, and involves the use of a massive pressure vessel with varying temperature and climate control systems. For the purposes of this article, we'll take a quick look at the wet lay-up method process, as well as a brief look at the "prepreg" system.

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post #25 of 29 Old 03-31-2009, 03:22 AM Thread Starter
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Wet Lay-up



This is one of the oldest and commonly used methods to manufacture sandwich components with composite faces. This manufacturing method is very flexible, yet very labor-intensive. Because of this, it's best suited for short production runs. The wet lay-up can be performed by hand lay-up or spray lay-up (although hand lay-up is by far the preferred method). The process uses a single-sided mold, male or female, which is treated with the mold release agent. Normally, a neat resin layer (gelcoat) is deposited directly onto the mold, which is allowed to gel before lamination starts. The gelcoat resin is usually a high-quality material with good environmental-resistant properties (in simple terms, it will "live" when eposed to ultraviolet rays and the atmosphere). The gelcoat also produces a smooth, cosmetically appealing surface that hides the uneven texture of the reinforcement structure, which otherwise would be visable on the compostie surface.

The wet lay-up methods typically incorporate resins that "cross-link" or cure at room temperature with little or no applied pressure. They are relatively tolerant to variations in processing temperature and can used uncomplicated tooling because of the modest cross-linking requirements. As mentioned above, wet lay-up techniques are labor intensive, but relatively cost effective for short production runs. Because of this, wet lay-up is well suited in the manufacture of body panels. It is not well suited for the construction of structural members.

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post #26 of 29 Old 04-01-2009, 02:57 AM Thread Starter
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More Sophisticated Lay-Ups



As popular as wet lay-up is, it is usually best suited for use with moderately loaded structures. Both single-skin laminates and sandwich structures for more advanced components, such as those used in our drag racing wing example or some aerospace application, tend to be laid up by way of "prepegs." A prepeg will ensure that the reinforcement is well impregnated with resin. The resins used in prepegs also tend to have better properties than the ones available for wet lay-up.

Prepeg is a composite material in which a specific amount of resin is pre-impregnated into the fabric. The fabric is partially cured, then frozen to retard the curing of the resin. When ready for use, it is thawed and placed in the mold. The advantages of a prepeg include an exact resin content for large quanities of materials, labor savings, lighter finished products and no mess.

Damage Control
Impact resistance isn't found in every composite, but it can be designed in. Some body panels are historically subject to stress from aerodynamic forces, minor impacts and simply abusive storage and transportation. Such things as the cumulative effects of numerous abrasions or a single casatrophic event are concerns for carbon fiber component manufacturers. Some manufacturers have used a core produced with a mix of carbon fiber and Kevlar, providing insurance in case of severe abrasion and eliminating problems associated with bonding non-similar materials. Sandwich panel or "cored" structures can result in a coponent that has an incredible stiffness-to-weight ratio.

As far as body parts are concerned, fabricating panels with a margin of strength that will tolerate some minor abrasion, despite the loss of a number of fibers, is relatively easy for an experienced shop. In order to prevent minor abrasion, the choices include the use of a sacrificial outer layer or Kevlar outer layer in the component structure. In the end, any body panel must have an appropriate minimum skin thickness to ensure it has optimum damage resistance.

The Bottom Line
Carbon fiber exhibits the most desirable performance characteristics of any of the lightweight materials explored to this date. It can be designed to be stiff under heavy aerodynamic forces and still be light. It can absorb road shocks satisfactorily. It can be durable and not subject to fatigue failures, while remaining strong enough to stand up to unexpected impacts and torsion forces. It can lend itself to attractive finishing and resist corrosion or attack by the elements. It can be formed in an attractive, functional way, and in the case of a hood scoop or a wing, it can be aerodynamically effecient.

The big question is which designs best maximize these benefits and minimize any drawbacks. Certain designs and their manufacturing methodologies exhibit strengths in some areas while making compromises in others. The best carbon fiber component is one that offers the most performance benefits while minimizing the drawbacks. Also, remember that the composite manufacturing business is unique in that the person doing the works physically builds not only the component, but also the parent material. That's the true magic of carbon fiber.

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post #27 of 29 Old 04-01-2009, 03:48 AM Thread Starter
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OK, That's it for the story behind the parts.....

I've competed one topic & I believe the most important one detailing carbon fiber manufacturing. If you read this, it should give you an idea of what you want out of your carbon fiber pieces & the method you would prefer for these pieces to be developed.

Next, I'll give more details on carbon fiber and when/where it is normally used. How to choose the correct parts for you. A carbon fiber buyers guide listing some reputable companies to purchase from. How to install a carbon fiber wing. And, finally a top panel skinning of a 240SX. I will follow it up with a lot of cool pics from other pages in the magazine.

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post #28 of 29 Old 10-27-2009, 08:10 PM
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WOW!!! sooo much info so by reading this i think i'm understanding that using carbon fiber mixed with kevlar will be the best combination for lets say a daily drivers gen coupe? for the hood, roof and doors?
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post #29 of 29 Old 05-14-2011, 04:00 AM
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Combining these woven fabrics with a thermosetting adhesive (using the hair-like fibers of carbon, glass, & boron) creates a material with amazing strength & stiffness. They make structures that are as strong & rigid as a metal one of equal size, but with considerably less weight.
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