Composite materials “benefit” a structure. Composite theory, properties of materials, various and varied processing techniques have been discovered through research to bring strength with low weight to composite bonded structures.
Basically, a composite material is one which is composed of at least two elements which, when put together, produce material properties that are different to the properties of those elements on their own. In practice, most composites consist of the matrix, and a reinforcement of some kind, added primarily to increase the strength and stiffness of the matrix. Reinforcement is usually in fibre form. There are 3 basic types of man-made composites:
Polymer Matrix Composites PMCs… Also known as Fibre Reinforced Polymers FRP (or Plastics), these materials use a polymer-based resin as the matrix, and a variety of fibres such as glass, carbon and aramid as the reinforcement.
Metal Matrix Composites MMCs… mostly used in the automotive industry, these materials have a metal such as aluminium as the matrix, and reinforce it with fibres such as silicon carbide.
Ceramic Matrix Composites CMCs… Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibres such as those made from silicon carbide and boron nitride.
Composite Resin Systems
Any resin system for use in a composite material will need good mechanical properties, adhesive properties, toughness and resistance to environmental degradation. The resin must be able to deform to at least the same extent as the fibre. High adhesion between resin and reinforcement fibres is necessary for any resin system. This will ensure that the loads are transferred efficiently and will prevent cracking or debonding when stressed.
Toughness is a measure of a material’s resistance to cracking. Generally the more deformation the resin will accept before failure, the tougher and more crack-resistant the resulting composite material will be. Good resistance to the environment, water and other aggressive substances, together with an ability to withstand constant stress cycling, are properties essential to composite resin systems.
Plasma Spray Coatings
Protective coatings and barrier layers include gelcoats, which are used as coatings in the mould. They involve colour technology, air release, thick film build-up and rapid cure times to produce finished surfaces with excellent gloss, colour and surface integrity retention after years of environmental exposure. Gelcoats provide both excellent protection for structural laminates as well as the levels of gloss and colour retention. Thermal sprayed aluminium coatings provide wear and corrosion resistant coatings.
The role of the reinforcement in a composite material is fundamentally one of increasing the mechanical properties of the neat resin system. All of the different fibres used in composites have different properties and so affect the properties of the composite in different ways. Individual fibres or fibre bundles can only be used on their own in a few processes such as filament winding. For most applications, the fibres need to be arranged into some form of sheet, known as a fabric, to make handling possible. Different ways for assembling fibres into sheets and the variety of fibre orientations possible lead to there being many different types of fabrics, each of which has its own characteristics.
Engineering theory shows that the flexural stiffness of composite panels is proportional to the cube of its thickness. The purpose of a core in composite laminates is therefore to increase the laminate’s stiffness by thickening it with a low-density core material. This can provide a dramatic increase in stiffness for very little additional weight. In addition, particularly when using lightweight, thin laminate skins, the core must be capable of taking a compressive loading without premature failure. This helps to prevent the thin skins from failing when buckling.
In summary, there are many different composite materials, all with their own strength, stiffness, toughness, heat resistance, cost, production rate etc. Composite design includes composite materials selection for bonded structures.
Damaged lightweight composite structures are repairable, but sometimes unpredictable due to variable adhesion during manufacture. They can be damaged unwittingly and no two composites are identical.
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