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Composite Fabrication With Carbon Fiber Braided Sleeves

Introduction:

Filament wound tubing is more expensive, yet more brittle than sleeves. It is also not meant to be used in impacts, which excludes if from model rocketry and automotive applications, whereas the strength of braided sleeves is ideal for such situations.

Braided carbon fiber sleeves are commonly used in the composites industry simply because they enable lower finished composite costs. Braided reinforcements present composite fabricators with a variety of opportunities to be more cost effective because of a unique combination of attributes. Most fabricators realize substantial savings in lay-up labor costs compared to the alternative use of fabric which must be cut, wrapped, and overlapped. Because of its "Chinese finger trap" feature, biaxial braid easily and repeatably expands to fit over molding tools or cores, accommodating straight, uniform cross-section forms as well as non-linear, irregular crossection components. Braided sleeves are, in essence, near net-shape pre-forms.

Applicability:

Sleeves are most commonly used in any situation where strong, yet lightweight tubing is desired. Creating tubing is as simple as sliding a sleeve over a mandrel, stretching it longitudinally to tighten over the form, and then saturating it with an epoxy or vinylester resin.

Alternatively, sleeves can be slipped over a silicone rubber bladder, resin-saturated, inserted into an inexpensive mold cavity, and with the bladder inflated, allowed to cure.

Sleeves can be used to add strength to an existing tube (cardboard, for example), or a removable mandrel can be used to create a tube purely of carbon fiber, or carbon/Kevlar.

Sleeve Architecture:

Biaxial braid is the most common form of braid and is often chosen by composite manufacturers since it allows for predictable, consistent lay-up and conforms to any shape. This typical braid construction is most often a basket weave with two yarns crossing over and under each other.

Braids are commonly defined as a 45 orientation, but are often used at lower angles. When this sleeving is pulled over a mandrel with changing cross-sections the fiber orientation, the thickness, and the yield of the braid vary at each point along the mandrel. These variations are predictable and repeatable and, therefore lend themselves to easy and precise manufacture of composite parts.

Sleeve architecture resembles a combination of filament winding and wrapped woven cloth. As with filament winding, sleeves feature seamless fiber continuity along the entire length of a part. And, like woven materials, braided fibers are mechanically interlocked with one another. Taken together, as a composite reinforcement, braid exhibits remarkable properties because it is highly efficient in distributing loads. Because all the fibers within a braided structure are continuous and mechanically locked, braid has a natural mechanism that evenly distributes load throughout the structure. Since all of the fibers in the structure are involved in a loading event, braid absorbs a great deal of energy as it fails. This is why braid is used as fan blade containment in commercial aircraft and in energy absorbing crash structures in formula one racing cars.

Sleeves also perform well in shear and torsional loading. While interlaminate adhesion is no different from other reinforcement products, the layers move together. As a result, it is very rare for cracks to form and propagate between layers. Since braids are woven on the bias, they provide very efficient reinforcement for parts that are subjected to torsional loads. Braid is an ideal reinforcement for drive shafts and other torque transfer components.

Elevated Temperature:

The fiberglass and carbon sleeves have a 1000 F continuous operating temperature. However the limiting factor in using the sleeves will be the epoxy. Most general-purpose laminating epoxies have temperature limits of approximately 150 F to 200 F. Higher temperature epoxies are available, but are difficult to purchase, expensive and have strict handling and processing needs.


Application Notes:

Using a 2.5" sleeve on a 3" object will result in a change of length of -25%. This means you will need to order 25% more sleeve length at the 2.5" width to successfully cover your 3" tube. Likewise, using a 2.5" sleeve on a 2" object will result in a 17% increase in length. This means you will need to order 17% less sleeve length at the 2.5" width to successfully cover your 2" tube. As the angle of the braid increases (i.e. when it's stretched), the length increases and the width decreases. A decreasing angle results in increased width but a decrease in length.


 

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