Ah, carbon fiber, that silky black fabric. Rolling over complex and compound surfaces this seductive material is the object of many a gearhead’s race car fetish. Carbon fiber is not just bling to be simulated in vinyl wraps, or textured ABS plastic parts, it is an advanced material with specific properties, applications, drawbacks, and features. The rhetoric of carbon fiber is one of weight savings, strength, stiffness, and aesthetics. While all these factors have their place there are many misconceptions about this velvety, elemental material.
The engineering topic of composite materials is a huge field of ever-expanding research and breakthroughs. We will go over some fundamentals and hopefully expand your view of this amazing industrial textile.
What Is Carbon Fiber?
Carbon fiber is an industrial textile comprised of thousands of carbon filaments woven together to create cloth. It is produced in a variety of different weaves for different applications and is only one part of a multi-part structural material known as a composite. Composites are just things made from combining the qualities of different materials to complement weakness or strength. In the case of carbon fiber, fiberglass, Kevlar, and other such textiles, the composite in question is known as an FRP (Fiber Reinforced Polymer). In an FRP, cloth is used to bolster the structural rigidity of a resin substrate. Resin provides the strength in the composite and the carbon fiber provides structural integrity to the otherwise brittle plastic.
How Is Carbon Fiber Made?
Carbon fiber, as its name suggests, is an elementally pure fabric composed of carbon. Starting out with carbon and weaving cloth would be one difficult feat. Instead of using carbon as the raw material, textile refineries start with a more molecularly complex plastic polymer. The thread is finer than a human hair and undergoes several processes of heat and chemical treatment. The ultimate result of this complex process is that the polymer is reduced to its most empirical form of pure carbon.
Carbon fiber is often measured and sold by several criteria, the weave pattern, the modulus (measurement of individual filament strength), fabric weight in ounces/square yard, and tow thread count (usually in the 3,000-12,000 range).
What do Different Weave Patterns Mean?
Unidirectional carbon fiber is comprised of bundles (tows) of carbon filaments all oriented in the same direction, there is no apparent weave to this style of fabric. Because there is no weave, unidirectional carbon fiber needs to be held together, diagonally, or perpendicular thread may be used to keep the fabric neat and even in spacing (however it is not a structural element). Because of its single dimensional rigidity, unidirectional carbon fiber is not often found in motorsports where loads can come from all directions.
Bidirectional carbon fiber is the most basic fundamental weave. Tows are woven together at right angles to create a square checkerboard pattern propagating laterally and vertically across the cloth. With a woven pattern the tows are oriented such that loads can be applied in multiple axises and the composite will retain its stiffness.
Two-by-two twill is the most common weave of carbon fiber found anywhere that composites are used in motorsports. This weave is slightly more complex than bidirectional because two tows in each direction are woven in a two-over, one-over, one-under fashion. The resulting pattern creates a saw tooth diagonal pattern across the fabric. Because two-by-two twill creates a diagonal pattern with vertical and horizontal (warp and weft) tows, the resulting textile is extremely malleable and can be conformed to a variety of complex shapes. When faced with a compound curve twill weaves conform nicely without bunching, spreading, or requiring cutting darts.
Similar to two-by-two twill, four-by-four is a diagonally propagating weave pattern comprised of four tows in two directions. The resulting weave is more wide spread than two-by-two but offers even better coverage of curved surfaces, because the actual points of over under weaving are further apart four-by-four effectively has less hard seams, and drapes on or in compound curved molds with ease.
Spread tow carbon fiber is a very specific design of fabric, and the rarest seen of all we will discuss. Spread tow means that the filaments of carbon ranging from 3,000 to 12,000 per tow are laid out next to each other for the thinnest ribbon of carbon. Conventional tows are bunched up with many layers of carbon filaments. Spread tow fabrics are identified by large open patterns. The checkerboard pattern of a bidirectional carbon fiber, with spread tow construction, may have squares measuring one inch across.
Carbon fiber is also available in pre-cured, laminate form. If you need a block, plate, tube, or other shape you can purchase pre-fabricated laminates.
Composite laminates can be comprised of multiple materials other than carbon fiber; Kevlar, metals, metallic or polymer foams, honeycomb, etc.
Laminate blocks can be machined for solid composite parts with a layered sand-stone aesthetic.
As described by Easy Composites, a UK based supplier of materials and resins, “Spread tow fabrics are gaining popularity in advanced composites applications because of their incredibly flat profile which almost eliminates ‘print through’ or texturing of precise surfaces (like aircraft wings).”
Because the resulting plies of fabric are so much thinner, more layers must be employed to achieve the same strength. Often used in applications where aerodynamics are a priority over strength, spread tow carbon fiber has a distinct aesthetic that people either love, or hate.
Carbon fiber cloth is only a part of the composite material general referred to in motorsports circles. The other important ingredient is the resin that impregnates the cloth and offers the actual rigidity. Resins come in different polymer concoctions. The two most common are epoxy resin and polyester resin. Anyone who has worked with fiberglass to repair a surfboard or in automotive work knows that resin can be nasty stuff. Volatile Organic Compounds (VOCs) are the fumes that characterize many resins, although there are some available that avoid these brain-damaging chemicals. The most commonly know adverse effect of working with resin without proper Personal Protection Equipment (PPE) is the development of a hypersensitivity and allergy. The condition can become so severe we have heard anecdotes of people not able to be in the same room as the resin.
Epoxy resin is the most common general purpose structural resin. As with almost all resins it is generally a two-part solution of resin and catalyst. Reaction kick-off times vary but are variable depending on environmental conditions. Pot life (working time) is generally around 5 to 30 minutes. Generally speaking, heat always accelerates the curing process but full curing usually takes a good 24 hours if left to its own. Epoxy resin is the stronger resin compared to polyester but requires some patience.
Polyester resin is the quick-curing, cheaper alternative to epoxy. It is generally used in situations were structural integrity takes a back seat to aesthetics, according to easycomposites.co.uk, “There are however circumstances where the structural performance of a laminate is less important, and properties such as appearance, UV stability and cost are higher priorities.”
Certain carbon fiber cloths may come pre-impregnated with a resin solution and rely on heat as a catalyst. Prepreg cloth is used in most industrial composite environments because of the ease of application (often sticky backed) and the minimal mess that is created by mixing resins and applying wet fabric layer after layer.
Prepreg cloth is also a preferred material of weight conscious industries such as the aviation market because the majority of a composites weight is in the resin, not the cloth. With the minimum necessary resin carefully and evenly impregnated into the cloth, it can be used to create the strongest and lightest combination.
Traditionally small parts are laid up wet, with the female mold, and a plug created (that’s another story). Then dry cloth is placed in the mold. Resin is applied with a paint brush until the cloth is thoroughly “wetted-out” or saturated. Subsequent layers of cloth are placed on the first layer, taking care to alternate the weave direction, 45 degrees for bidirectional, and 90 degrees for twill fabrics. If the layers of fabric are not alternated in weave direction, the final part will suffer stiffness along one axis while the other is excessively reinforced.
With as many layers as desired for the thickness and strength of the part saturated, excess resin is squeegeed out much like the way you would remove water from your windshield. The part is then vacuum-bagged and a low pressure is pulled on the contents drawing all the resin into any existing air voids, removing tiny bubbles, and any excess resin.
In certain instances these processes will be reversed. Dry fabric is vacuum-bagged in the mold and then resin is applied. This method cuts down on waste and mess. The final stage is to apply heat. A pressurized oven known as an autoclave is used to bake the parts and fully cure the resin.
While most of us do not have access to specialty equipment, vacuum-bagging and autoclave baking are optional for casual parts that do not need to meet specific structural requirements.
Carbon fiber has gained momentum in the fabrication of a variety of automotive products. In the aftermarket niche, carbon fiber is most commonly used to dress-up parts. Carbon fiber body pieces, trim, etc., lend a high-performance look. Functional applications for carbon fiber parts range from automotive to marine and aviations applications.
Carbon fiber is used to construct racing seats, driveshafts, safety equipment like helmets and Head and Neck Restraints (HNRs), even composite spring technology is making its way into suspension systems.
Carbon Fiber is Not the Solution For Everything
The allure of carbon fiber is so strong to many that it has a tendency to be a misused material in applications where a metal alloy is still the best solution. Carbon fiber, resin specifically, is not tolerant of high heat-situations, heat shielding, exhaust components, or other engine pieces must be carefully evaluated before fabrication when carbon is elected as the material. High-heat resins exist, but applications are still limited.
Carbon fiber boasts the catch phrase ‘lighter than aluminum, stronger than steel.’ While this rhetoric has its place, it is important to understand this is a reference to tensile strength, not toughness or hardness. In engineering terms “toughness” is a technical term referencing abrasion resistance, because this composite is a layered FRP impact resistance is low. Even the slightest pin point impact can cause delimitation and ultimately failure of the material. For these reasons carbon fiber does not make long-lasting, or multi-strike tolerable skid plates, suspension components, or other exposed high-stress parts.
Carbon is conductive! Pure carbon transmits heat through itself extremely efficiently. For example, a car hood in raw carbon fiber can heat up to several hundred degrees in the sun very quickly. The UV light can have damaging effects on the composite—yellowing or resin cracking, and warping are common. Many aviation applications of carbon fiber are painted gloss white because the heat generated by UV would compromise the airframe, alter aerodynamics, and have other negative impacts on the structure of the airplane.
Carbon fiber is electrically conductive. It may be perplexing how a plastic-based composite would be electrically conductive but the pure carbon cloth running through it provides a path for electricity even though it is impregnated with the insulating polymer. When using carbon as a surface for electronics, or mounting shroud for a cooling fan, be sure grounds do not run through the carbon. A personal anecdote… we have witnessed an owner of a Geiser Trophy Truck pre-runner nearly start an engine fire because he didn’t believe that carbon fiber was conductive, resin fires are nothing to trifle with.
If you’ve ever had fiberglass filaments in your skin you know how irritating the invisible splinters can be, but carbon is much worse! Avoid ragged edges and chopped fiber with bare hands.
When ordering carbon fiber fabric it is important to make sure it is shipped on a roll, like wrapping paper. Flat-folded carbon fiber will crease and destroy the structural integrity of creased filaments. Care in handling is advised, as is keeping the the fabric clean and free of any dust or oily fingerprints ensure the best possible lay up. Mixing resin in small paper cups is the norm, but take care not to mix resin in a wax coated cup. Wax will interfere with the resins adhesion and curing of resins. Resin curing is an exothermic reaction, meaning a lot of heat is generated as a by-product of the chemical reaction. If mixing a larger quantity of resin, be sure to keep an excess resin away from flammables while it kicks off, or a fire can result.
There is a huge base of knowledge we didn’t even touch on in this article, but we hope this general overview has helped broaden your understanding of carbon fiber. It is an incredibly versatile and strong material, if treated with intelligence, but can be a misplaced eyesore when used improperly. Home fabrication of simple parts is not difficult, but expect to pay several times more in raw materials than you would in fiberglass. Consider your project, goals, and budget then decide if carbon fiber construction is the right choice or are you just looking to improve aesthetics?