Carbon Fiber Composite Laminate Structure

There are many different types of composite materials, but the one we focus on at Kumair is laminated composites, which comprise layers of material built-up to create a finished part. The simplest laminated composite is a flat sheet, whereby the individual layers, called plies, are stacked on top of one another, then pressed together during cure. The pressure can be from a variety of sources, but is usually from either vacuum or a platen press. In the case of more complex parts, instead of a press creating a linear force to crush the plies together, more complex tooling can be used to apply the pressure along curved surfaces and even from multiple directions.

Individual plies of carbon fiber must have the fiber reinforcement and resin (for example, epoxy) in the correct ratio. There are multiple ways this is often stated, but a common value used is called resin ratio, which is the percent of the total weight that is resin. Resin ratio for wet layup and vacuum bagged parts is usually in the 45% to 55% range, whereas prepreg is typically lower in the 35% to 42% range. There are often clear advantages to using a prepreg materials, especially for carbon fiber, since the final part can be made with more reliability of resin uniformity and a lower resin ratio (i.e. greater number of fibers, and thus higher stiffness and strength).

Carbon fiber plies range in thickness from about 0.003” (for 1k woven or spread tow carbon fiber) to over 0.030” thick (for heavy 12k woven or stitched uni-directional). By utilizing a combination of different weight materials, essentially any thickness part can be built up. For example, if the part required a final thickness of 0.067”, then a layup schedule (the recipe for the laminate structure) might be 3k/12k/12k/3k, with individual ply thickness of 0.0085” for the 3k plies and 0.25” for the 12k plies.

One of the benefits to carbon fiber composites is the ability to tailor the strength and stiffness along different axes. For example, if your part requires additional stiffness along one direction or bending axis, this can be achieved by orienting more fibers along this axis at the expense of the orthogonal direction. The most common way to do this is by using plies where all of the fibers go in a single direction, called uni-directional materials, or uni. By incorporating uni into the laminate structure, one achieves tremendous stiffness along a single axis, and in the case of carbon fiber, modulus values higher than steel are possible (yet still at half the weight of aluminum).

Other materials can also be incorporated into a composite laminate to yield a hybrid part that makes use of the benefits of each type of material. For example, a metal core of aluminum or titanium can be sandwiched in between carbon fiber layers to make a part that can be drilled and tapped for threaded holes. It will also greatly increase the impact toughness of the part. Fiberglass can be added to the core of a panel for cost reduction or to the surface of a part to increase electrical resistivity, which can help eliminate galvanic corrosion issues. If an ultra lightweight part is required, foam or honeycomb cores can be used with thin carbon fiber skins, producing a composite with stiffness to weight ratio orders of magnitude higher than any solid homogeneous material could achieve.