What is Unidirectional Carbon Fiber?
In keeping with its name, in unidirectional (UD) carbon fibers all the fibers run in a single, parallel direction. The unidirectional ‘orientation’ of the fibers provides it the desired directional strength and stiffness.
Fibers are precisely arranged in a direction where maximum strength is needed, like in the direction of motion for propelling airplanes.
This uni-direction orientation of carbon fibers is typically found in non-woven fabrics, UD prepregs, or in unidirectional plies (or laminas) in the composites.
Unidirectional (UD) Fabrics and Composites:
Carbon fibers are processed to form manufacturing intermediates like dry carbon fiber fabrics, prepregs, etc. which are further used to make carbon fiber composite parts (refer to the image below) used in real-life applications.
Carbon fibers from tows (or spools) are used to make fabrics (woven or non-woven). And depending on how these fibers will be oriented — fabrics can be of different types, such as uni or bi-directional, multiaxial, random, etc.
A larger percentage of the fiber weight can be unidirectional carbon fibers (usually running in the ‘warp’ direction). While the rest can often be a resin binder or a secondary material like a weft stitch running in other directions, holding the fibers together.
For example, in prepreg unidirectional tapes, fibers are impregnated with resin (like epoxy or vinyl-ester resins) that locks the parallelly running fibers in one place. While UD carbon fiber fabrics can be used as reinforcements for larger structures, UD tapes can have a localized application to reinforce narrower components like skis.
Unidirectional composites as well use UD carbon fiber fabrics as reinforcement. These UD reinforcements can be overlapped at different angles to obtain unique properties. UD composites (with longitudinal, continuous fibers) have the greatest strength of all composites.
Moreover, advanced composites also owe their strength to unidirectional low-density plies – layers of UD fibers in a matrix.
The final characteristics of UD carbon fiber fabrics or composites — like the strength, elasticity, drapability, etc. are greatly influenced by the orientation and distribution of the fibers.
But before we discuss the unique applications of UD carbon fibers, it is worth taking a detour to understand — what makes them so unique.
Properties of Unidirectional Carbon Fibers:
- Strength and light-weighting:
- UD fibers constitute perfect light-weighing solutions with greater longitudinal tensile strength — making them ideal for isotropic applications. The more uniformity of fiber distribution, the higher the resulting strength.
- It can be customized flexibly for strength-related applications — adding a single layer of UD carbon fibers will add double the strength in the required direction compared to a woven fabric that has fibers running in two or more directions.
- They are fairly stiff (i.e., hard to deform) — both an advantage and a disadvantage for certain applications, as they don’t make for ideal contouring materials because of their rigidity.
- In her article, Anna Marie Hughes explained how UD carbon fiber could be used in carbon bike frames to strike the right balance between required stiffness and flexibility.
- Unidirectional carbon fibers aren’t suitable for draping. They reveal gaps, wrinkles, or creases when draped over complex surfaces. But their drapability can be changed or rather improved with resin infusion.
- Unidirectional fibers tend to fall apart during the layup process because they are non-woven. But their layering can be made creative — many UD fabrics can be layered in different directions for maximum strength without sacrificing their stiffness or lightweight.
- Non-woven fabrics cannot be as easily machined as woven ones, where the overlapping fibers rebound comparatively. And, unlike woven fabrics, a fiber pull-up in UD can go all the way across without interlaced fiber support.
- Crimp refers to the waves or undulations in fiber — the lesser the crimp, the better the composite mechanical properties.
- No cross-sectional weave means less or no crimping of the fibers — i.e., the fibers lay flat without any gaps or bumps.
- Relatively economical:
- Considering the exceptional performance of their applications, UD fibers remain rather inexpensive — not to forget all the savings due to their simple production.
- Apart from the many properties unique to the unidirectional orientation of the fibers, they also carry the properties of the carbon fibers themselves — like temperature and chemical resistance, durability, sustainability, and high electrical and thermal conductivity.
Applications of Unidirectional Carbon Fiber:
It is rare for UD carbon fibers to provide a complete solution for an application — but their fair share of advantages cannot be ignored.
Manufacturers may want to consider how they can leverage the UD properties (listed above) to produce ideal composite components. And end-users looking for strong and lightweight materials can always rely on unidirectional carbon fiber products for a pleasant surprise.
Most applications combine multiple layers of unidirectional fabrics (or plies) along with their woven counterparts in order to attain a balance among the spectrum of properties.
Undisputed use of UD carbon fibers can be found in applications relying on considerable front-to-back strength — ranging from aerospace, ship-building, and automobiles to sports equipment or travel gear.
UD carbon fiber shoe soles are a hit, and carbon fiber itself is now a label in the footwear industry (Check out UD carbon fiber cycling shoes.)
Unidirectional carbon fiber tubes used in bicycle frames are popular among riders for their flexibility and rigidity (UD carbon fiber bikes from road.cc.)
They also make for excellent reinforcements for structural applications and can replace metals in building repair or reinforcement. Other common applications of unidirectional carbon fibers are fishing rods, hockey sticks, prosthetic blades, boat hulls, etc.