In this post I discuss the basics of composite materials. A composite material is a composition of two or more materials, combined to have “best of both worlds”. The bulk material is called “the matrix” and is somehow reinforced, e.g. by fibers or small particles. They increase the strength and stiffness of the matrix.

Introduction to composite materials

Three groups of composites can be distinguished: Polymer Matrix Composites (PMCs), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs). The PMCs are most used although MMCs are getting more and more popular, especially in the automotive industry. For wind turbine blades, PMCs are the way to go.

In a Polymer Matrix Composite, the matrix is a polymer, a resin system which is easily formed into complex shapes. The  combination with strong and stiff fiber reinforcements gives a resulting composite superior to metals in many aspects. The properties of the composite are a result of the properties of the materials it’s build with, the ratio of the different materials and also the geometry and orientation of the fibers in the composite. Because the fibers have their highest mechanical properties in the length direction, composite materials are highly anisotropic, although a specific reinforcement scheme can give a more or less isotropic composite if wanted.

As for all materials, we are interested in the performance of the composite. The ultimate tensile strength indicates the maximum stress which can be applied before the material completely fails.  In composites however, small cracks can be formed and grow away from the reinforcement fibers who are not aligned with the applied load long before the ultimate tensile strength is reached. This is called micro-cracking. The micro-cracked laminate will also absorb more water, leading to a higher degradation of the fibers, a lower stiffness and a higher weight. Overall, it’s clear that micro-cracking should be avoided. It’s also important to keep in mind that the resin must be able to deform as much as the the fibre to have full benefit of the reinforcement’s properties. Misalignments of the fibres results in significant losses in mechanical properties. In comparison to metals, composites have very good fatigue resistance.

To estimate simple properties, it’s enough to add the properties of each material weighed with their volume fraction (the so-called rule of mixtures).  The laminate theory is able to calculate properties of multi-layered composites (laminates).

Possible polymer matrix materials

A polymer is a large molecule build up by connecting small building blocks into large chains. One can distinguish three categories: thermoplastics, thermosets and elastomers. Thermosplastics have the unique property they can be heated to melt and formed into shape not only once, but theoretically infinitely. Thermosets however, can only be melt and formed once. When a thermoset is heated a second time it will not melt again and will eventually carbonize. Elastomers are highly elastic polymers such as rubbers. In wind turbine blade manufacturing (and other high-tech applications as well), three thermosets are very popular as resin material: polyester, vinylester and epoxy.

Polyester manufacturing is mature and gives reliable and relatively cheap results. Polyesters are formed by a reaction of glycol with di-basic acids. Adding styrene makes it easier to handle because it lowers the viscosity. It’s also a crucial element to form the cross-links between the polyester chains, which turn it into a very strong three dimensional network (this is called polymerisation). Catalysts are added to speed up the this linking process. An important result of this closely linked structture is a limited shock loading resistance, because the chains can’t move alongside each other.

Vynilesters’ properties differ from polyester’s because their reactive groups are positioned at the end of its chains. Now the chock loading can be absorbed by the whole chain and thus vynilester resins are tougher and more resilient. They also have a better water resistance because of a smaller amount of ester groups, which are prone to water degradation.

Epoxy resins are superior to polyesters and vynilesters: they have better mechanical properties because of two ring groups in the centre of the chain which are able to absorb mechanical an thermal stresses better than linear groups. Their resistance to degradation by water and chemicals is better. They show less shrinking during the curing process (see last post).

Possible fibre materials

There’s a broad range of materials which can be used for the reinforcement fibres. I will give a short overview of the most used ones an their particularities.

  • One of the most common fibers are the glass fibers, which are found in different types: Electrical glass (E-glass), Chemical glass (C-glass) and R,S or T-glass (manufacturers’ names for specific glass types). Each glass type is be found in different forms, such as rovings, strands or yarns.
  • Aramid fibres have a very high specific strength, good impact resistance. Compressive strength is similar to E glass. It has a good resistance against abrasion and degradation by chemicals or thermal fluctuations. It is commonly known as Kevlar, the trade name used by Dupont.
  • Carbon fibres, produced by oxidizing, carbonaistion and graphitisation of polyacrylonitril fibres which are rich in carbon. By altering the graphitisation temperature, carbon fibers with high strength or high moduli can be obtained.

Besides the well-known fiber materials listed above also polyester, polyethylene, quartz and ceramics are used.

Up to present, natural fibers have not been used in high-tech applications, although they can have high specific strengths. It’s exactly this group of fibres which are the subject of my research and I will try to find out if they can be used to replace the glass and carbon fibres in the wind turbine blades.

Possible sandwich core materials

Most sandwich structures use a kind of foam as a core for the obvious reason of their low density. There are plenty of options, but PVC is probably most used. It has fair or good properties and can be used in combination with the popular polyester resin we discussed above. Other foams like polymethyl methacrylamide offer better strength and stiffness, but are much more expensive.

Instead of foams, a honeycomb structure is a widespread core technique. Again, a range of materials can be used: from paper to Balsa to aluminium. It is clear the properties of the honeycomb vary greatly according to the material they’re made out of.

There are a few things we have to keep in mind when choosing a core material. The foams have a low density which is great, because it reduces the overall weight of the sandwich structure. But, this lower density also results in a larger resin absorption. Weight savings can also be undone when the core doesn’t fit nicely with the skin layers and require a lot of adhesive.