How Fiber Orientation Affects Composite Properties

The Three Most Common Fiber Orientations

Fibers can be oriented in any direction between 0° and 180°, although fiber orientations over 90° are often referred to as negative angle values, eg a 135° fiber angle equals a -45° angle. Most carbon fiber and fiberglass tubing on the market today uses a combination of two or more orientations:

0°—The zero fiber angle is the most commonly used orientation. Fibers are strongest and stiffest when they are in the same direction as the load. On pipe, the zero-degree direction runs along the length of the pipe and contributes to bending stiffness and strength.

90°—The 90° fiber angle is used when two-way bending is required. In the tube, the 90 degree fibers are oriented along the circumference of the tube. They help prevent the tube from crushing or bending under load.

±45°—45° angles are often used in combination with zero and 90° plies to create quasi-isotropic layups. A positive 45 degree layer is almost always adjacent to a negative 45 degree layer. The 45-degree plies contribute to torsional stiffness and strength when used on tubes.

Woven fibers are often said to have a fiber angle of 0/90 degrees because there are fibers in both directions. Some woven materials can contain more fiber directions, for example triaxial fabrics have fibers in three directions and are usually quasi-isotropic themselves.

Effect of Fiber Orientation on Properties

The fiber orientation of carbon or glass fiber layups affects their performance in everything from pipes to spacecraft. Processors must consider these characteristics during the design process. Below we explain how three common fiber orientations affect performance:

0° Orientation: If a part is only loaded in one direction, then ideally all fibers are oriented in that direction. Pultruded rods and tubes are examples of components that contain only 0° fibers. Since most parts are not loaded in only one direction, additional angles need to be added to maximize strength. A tube that only sees bends and no twists might still benefit from some extra fiber angles. Adding a 90° layer helps the tube better retain its shape so it doesn't buckle prematurely.

90° Orientation: As mentioned above, a 90° layer is often added to the pipe to make it more resistant to bending and crushing. A high proportion of 90° or "hoop" layers can also be found in pressure vessels. Since the force is trying to expand the pipe in the pressure vessel, the 90° ply is the most resistant. When the 90° layer is used in combination with the 0° layer in the board it is called bidirectional. Quickly build parts in 0° and 90° orientations is easy with woven cloth.

±45° Orientation: The 45° layer serves different purposes depending on the application. You will almost always see a +45° ply adjacent to a -45°, this is to keep the laminate "balanced" and avoid strong twisting when loaded. When a 45° layer is used on a plate that already contains an equal mix of 0° and 90° layers, the plate becomes quasi-isotropic. A bidirectional plate has properties equal in both directions, while a quasi-isotropic plate has properties quasi-equal in any direction. In the tube, the 45° layer acts to increase torsional strength and stiffness. This is because when the tube is twisted, the force acting on the laminate is effectively 45 degrees. Some laminates will use angles other than 45° as a compromise between bend, crush and torsional performance. Since 0° plies are not possible on filament wound tubing, 10° or 15° plies are often used instead.

You should now have a general understanding of how each fiber orientation affects performance so you can choose the correct ply. Bi-directional laminates are ideal if you need a tube that will work in all conditions.

If you need a tube that performs well in torsion, choose one with more 45° layers, while if you need to add thickness quickly, a braided material is a good choice.