The combination of matrix and reinforcement materials can lead to a composite material with properties that are distinct from its constituents. For example Kevlar® fibres are very flexible, and epoxy resins neither tough nor stiff. Yet a Kevlar®/epoxy composite is stiff, strong and tough, hence why they are used to protect against ballistic and blast threats. A fibre reinforced composite is usually made from individual layers (with each layer called a lamina or ply) which are stacked upon one another to form a laminate.
This allows the fabrication of structures with reduced weight, which is particularly valuable in applications that involve movement (e.g. aerospace and automotive) as operational energy/fuel costs are reduced.
In addition to excellent material properties, composite structures can often be formed in a single shot where conventional materials would be assembled from multiple parts. This can have both structural and processing benefits.
As the range of challenges faced by the composites industry is large it is not possible to cover them all here but rather outline some particular considerations.
1. Determining the breadth of composite material properties is notoriously difficult and still at the forefront of academic research. Indeed, such is the challenge of the task that a number of material properties cannot be obtained directly. Further, the measured properties of a composite are heavily influenced by the equipment and methods used to determine them. Even then, there are still failure mechanisms that are not replicable in a laboratory. Whilst FAC Technology have developed a range of methods to better predict composite material properties (see multiscale mechanics of composites) and thus reduce the amount of experimental work required, they are not a substitute for experimental testing.
2. Beyond determining material properties, there lies the challenge of how to make use of them. In parallel to research on experimental methods for composites is research on theories to predict the behaviour of composites. This area is also the subject of significant attention in academia. This attention is motivated by an absence of consistent, reliable theories for predicting the behaviour of composite materials. This fact cannot be understated and on this matter we’d refer to a quote from one of the World’s leading experts on the behaviour of composite materials:
3. Manufacturing methods also give rise to various difficulties. For example, even if one took the same type of carbon fibre, woven into the same type of fabric, using the same machine, combined it with the same resin and then cured it in the same manner, you’d still measure different properties using different manufacturing techniques. Further the measured properties of one laminate do not necessarily translate when they are used within another. If one took, for example, a laminate for determining transverse tensile strength from an ASTM standard test and loaded it, one would observe catastrophic failure at a given strain. But if you then took that same material and combined it with laminae with different orientations then one would often find that not only would the strain at which damage starts to occur to be higher, but also that this damage would not necessarily lead to the part breaking.
4. A final consideration is the effect of part geometry on the orientation of fibres within a composite component. Geometry has a large effect on the alignment of fibres and thus the behaviour of composite materials. In addition, the geometry of a design determines how, and even if, it can be manufactured. In order to investigate this it is necessary to carry out computer simulations to assist in the design of suitable flat patterns.
At the other end of the performance spectrum are dry fibres coated with liquid resin on an open mould. The performance of composites made in such a manner is vastly inferior to composites made using closed mould processes and should not be considered comparable.
Between the above two approaches sit various methods based on combining liquid resin with dry reinforcement in closed moulds. The dry reinforcement typically takes the form of woven or stitched textiles. Although these fabric based composites do not quite match the stiffness of UD prepregs, they can be tougher and more damage tolerant. In addition, it is possible to form composite components with more complex geometries using textiles than using UD fabrics. By exploiting the flexibility of dry textiles, it is often possible to produce a more optimised structure that would be stiffer, stronger, and more damage resistant than one made of UD prepreg.