When it comes to carbon fiber, there are many different fabric types to pick from. Their strength, weight, thickness, and conformability include just some of the properties to consider. With so many different variables to weigh out, how is the correct fabric determined? Before exploring the different fabric types it’s important to start by understanding the fundamentals of the fabric itself.
Fiber manufacturers combine thousands of individual fiber filaments into fiber bundles that are then combined once more by weavers to form tapes or fabrics. These fabrics are categorized by the way in which fiber bundles are woven or stitched together to form the fabric and classified by weight in terms of weight per unit area. A variety of different weave patterns and weights have been developed to achieve different properties for strength, stiffness, and manufacturing characteristics. Fiber reinforcements are best at carrying loads along the direction of the fiber orientation. The "strongest" weaves are those that have the most fibers aligned in the direction of the applied loads. Therefore, selecting the correct weave type and fiber orientation is critical for structural applications. Several common weave patterns are highlighted in this article.
Unidirectional
A unidirectional weave pattern contains fibers that all run in the lengthwise direction of the cloth. There are no fibers placed in the widthwise direction. Fiber bundles in unidirectional cloth are held together with fine string or glue filaments. Unidirectional weaves provide exceptional strength in the lengthwise (warp) direction but virtually no strength in the widthwise (fill) direction because there are no fibers in the width direction to support loads. Unidirectional cloth is commonly placed in a mold with multiple cloth layers oriented at different angles to create a laminate capable of carrying loads in multiple directions.
This weave type works well for adding localized reinforcement in parts requiring additional strength and stiffness while avoiding adding extra layers of fabric across the entire part. Wing skins and wing spars often incorporate layers of unidirectional cloth with the fibers aligned in the wingspan direction to support the bending loads generated from lift forces on the wing. Unidirectional weaves often do not conform well to compound curves or tight corners that run perpendicular to the axis of the fibers as they can wrinkle or bunch up around these types of geometries.
In the series of images below, unidirectional cloth can be seen by itself (left), in a cloth layup for a main landing gear strut on the DarkAero 1 (middle), and the final main landing gear strut assembly (right). Unidirectional cloth in this context is used to support the bending and axial loads that are generated in the landing gear strut during a landing.
Plain
Plain weaves are one of the most common weave patterns. A plain weave consists of a 1x1 pattern where each warp strand alternately passes over and then under each fill strand. Because there are fiber bundles running both lengthwise and widthwise, plain weaves can generate strength in two perpendicular directions, unlike unidirectional cloth, which is only strong in the length direction. The over and under path of the fiber bundles creates a stable weave pattern, but this can also slightly reduce the overall strength of the cloth because the fiber bundles deviate from running perfectly straight. This weave works well for flat or nearly flat parts with limited contours. Plain weaves do not drape well without distorting or wrinkling. This weave can be found in flat airframe ribs and bulkheads as well as airframe skins with limited curvature.
In the series of images below, plain weave cloth can be seen by itself (left), in the fuselage and wing skins of the DarkAero 1 (middle), and in the skins of the honeycomb sandwich panels that make up the ribs and shear webs in the aircraft (right). Where possible, spread-tow plain weave is used throughout the DarkAero 1 due to its low resin absorption. This improves the fiber-to-resin ratio of the parts where it is used, reducing their weight. As stated previously, plain weave does not conform well to parts with aggressive curves. For these parts, a different weave pattern is preferred.
Twill
A twill weave is similar to a plain weave in that each warp strand passes over and under the fill strands. However, each warp strand will pass over or under more than one fill strand at a time. The over and under pattern is staggered, which creates a series of distinct diagonal lines in the cloth. One of the most common twill weaves is the 2x2 twill where each warp strand passes under two fill strands and then back over two fill strands. Another common twill weave is the 4x4 twill where each warp strand passes over and under four fill strands. The main advantage of twill weaves over plain weaves is that they are easier to drape and conform to complex contours. They are less stable than plain weaves, so care must be taken when handling them to control the distortion of the weave pattern. Twill weaves work well for forming three-dimensional, contoured parts like cowlings, wheel fairings, and wing tips.
In the series of images below, twill weave cloth can be seen by itself (left), in the layup of the wing fillet of the DarkAero 1 (middle), and in the final, resin-infused wing fillet (right). Twill weave is a logical choice for the wing fillet as its geometry contains compound curves that blend the curve of the wing surface with the curve of the fuselage side.
Multiaxial Non-Crimp
A multiaxial non-crimp cloth or non-crimp fabric (NCF) consists of multiple layers of unidirectional fabric stitched together in different orientations. For example, a multiaxial NCF may consist of four combined unidirectional layers oriented at 0, +45, -45, and 90 degrees relative to the length of the cloth. Multiaxial fabrics provide strength and stiffness in multiple directions making them closer to a quasi-isotropic material. This cloth type works well for building up thickness quickly from fewer independent layers. Multiaxial fabrics can be used to create thick composite plates from which smaller, two-dimensional composite parts can be machined.
In the images below, multiaxial non-crimp cloth can be seen by itself (left) and in parts (right) machined from a solid infused billet made up of several layers of multiaxial non-crimp cloth. These parts are used in the control system of the DarkAero 1 and mainly experience in-plane loads rather than loads along their thickness direction, making multiaxial non-crimp cloth an ideal choice for this application.
Video: Why We Use Different Types of Carbon Fiber in the DarkAero 1
Summary
The weave types discussed in this article are summarized and compared in the graphic below:
Learn More
If you’d like to learn more about how to consistently make high-quality composite parts and get hands-on experience working with carbon fiber, consider enrolling in one of our courses:
The courses are structured as a mix of both classroom lessons and hands-on demonstrations in the shop. Composite material fundamentals, design methods, material and process selection, and design for manufacturability are covered in class. Examples specific to student applications are used to immediately apply what is learned. Online versions of both courses are available as well at the links below: