To fully appreciate the role and application of composite materials to a structure, an understanding is required of the component materials themselves and of the ways in which they can be processed. This guide looks at basic composite theory, properties of materials used, various processing techniques commonly found and applications of composite products.
In its most basic form a composite material is one which is composed of at least two elements working together to produce material properties that are different to the properties of those elements on their own. In practice, most composites consist of a bulk material (the 'matrix'), and a reinforcement of some kind, added primarily to increase the strength and stiffness of the matrix. This reinforcement is usually in fibre form. Today, the most common man-made composites can be divided into three main groups:
Polymer Matrix Composites (PMC's) These are the most common and will the main area of discussion in this guide. Also known as FRP - Fibre Reinforced Polymers (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 (MMC's) - Increasingly found in the automotive industry, these materials use a metal such as aluminium as the matrix, and reinforce it with fibres such as silicon carbide.
Ceramic Matrix Composites (CMC's) - Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibres, or whiskers such as those made from silicon carbide and boron nitride.
Any resin system for use in a composite material will require the following properties:
 Good mechanical properties.
 Good adhesive properties.
 Good toughness properties .
 Good resistance to environmental degradation.
MECHANICAL PROPERTIES OF RESIN SYSTEM.
The figure below shows the stress / strain curve for an 'ideal' resin system. The curve for this resin shows high ultimate strength, high stiffness (indicated by the initial gradient) and a high strain to failure. This means that the resin is initially stiff but at the same time will not suffer from brittle failure.
It should also be noted that when a composite is loaded in tension, for the full mechanical properties of the fibre component to be achieved, the resin must be able to deform to at least the same extent as the fibre. The figure below gives the strain to failure for E-glass, S-glass, aramid and high-strength grade carbon fibres on their own (i.e. not in a composite form). Here it can be seen that, for example, the S-glass fibre, with an elongation to break of 5.3%, will require a resin with an elongation to break of at least this value to achieve maximum tensile properties.
Adhesive Properties of the Resin System.
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 fibre / resin debonding when stressed.
TOUGHNESS PROPERTIES OF THE RESIN SYSTEM.
Toughness is a measure of a material's resistance to crack propagation, but in a composite this can be hard to measure accurately. However, the stress / strain curve of the resin system on its own provides some indication of the material's toughness. Generally the more deformation the resin will accept before failure the tougher and more crack-resistant the material will be. Conversely, a resin system with a low strain to failure will tend to create a brittle composite, which cracks easily. It is important to match this property to the elongation of the fibre reinforcement.
Environmental Properties of the resin System.
Good resistance to the environment, water and other aggressive substances, together with an ability to withstand constant stress cycling, are properties essential to any resin system. These properties are particularly important for use in a marine environment.
Published courtesy of David Cripps, Gurit.
As the fibre reinforced plastics industry has grown and become more sophisticated, so has the demand for protective coatings and barrier layers. The first materials marketed as protective coatings for composites were pre-formulated, compounded products called gelcoats, which are used as ‘in-mould’ coatings. Today, this has become a highly specialised business involving colour technology, air release requirements, thick film build-up and rapid cure times to produce in-mould finished surfaces with excellent gloss, colour and surface integrity retention after years of environmental exposure. The new gelcoats provide both excellent protection for structural laminates as well as the levels of gloss and colour retention demanded by the motor industry.
In the marine industry problems resulting from osmosis, in the form of surface blistering, prompted the development of fibre reinforced barrier or skin coats to be used immediately behind gelcoats in the form of match performance systems designed to minimise water pick-up and the possibilities of blister formation.
Generally, in pipe, tank and chemical plant component manufacture it is essential to protect the structural laminate from the environment to be contained. Often this can be achieved with a fibre reinforced barrier coat 2 to 3mm thick manufactured using surface tissue, light weight fibre mats and cloths using a suitable chemically resistant resin. In such cases gelcoats are not used because a pure resin without additives provides a greater level of chemical resistance. Hence, the resin-rich surface tissue provides the initial chemical resistant surface and will contain around 95% resin by weight, which is further supported by a resin-rich, structural laminate barrier layer before the final GRP structure is manufactured.
In this Section the need and performance of specialised protection systems for composite materials will be discussed with reference to the various market requirements.
Published courtesy of Dr L S Norwood, Scott Bader Company Ltd.http://www.scottbader.com
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. The properties and characteristics of common fibres are explained below.
However, individual fibres or fibre bundles can only be used on their own in a few processes such as filament winding (described later). For most other 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. These different fabric types and constructions are explained later.
Published courtesy of David Cripps, Gurit
Engineering theory shows that the flexural stiffness of any panel is proportional to the cube of its thickness. The purpose of a core in a composite laminate is therefore to increase the laminate's stiffness by effectively 'thickening' it with a low-density core material. This can provide a dramatic increase in stiffness for very little additional weight.
The figure below shows a cored laminate under a bending load. Here, the sandwich laminate can be likened to an I-beam, in which the laminate skins act as the I-beam flange, and the core materials act as the beam's shear web. In this mode of loading it can be seen that the upper skin is put into compression, the lower skin into tension and the core into shear. It therefore follows that one of the most important properties of a core is its shear strength and stiffness.
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 wrinkling, and failing in a buckling mode.
Published courtesy of David Cripps, Gurit http://www.gurit.com
Taking composite materials as a whole, there are many different material options to choose from in the areas of resins, fibres and cores, all with their own unique set of properties such as strength, stiffness, toughness, heat resistance, cost, production rate etc.. However, the end properties of a composite part produced from these different materials is not only a function of the individual properties of the resin matrix and fibre (and in sandwich structures, the core as well), but is also a function of the way in which the materials themselves are designed into the part and also the way in which they are processed. This section compares a few of the commonly used composite production methods and presents some of the factors to be borne in mind with each different process, including the influence of each process on materials selection.
Published courtesy of David Cripps, Gurit.http://www.gurit.com
Damaged composite structures are definitely repairable, shown clearly in the before and after images below.
However, there are challenges:.
Hidden damage issues, including manufacturing defects. (for example, a low velocity impact, which normally wouldn’t cause much damage may cause a sandwich structure to disbond between the skin and core due to poor adhesion during manufacture. If this disbond is the only damage, there may be no visible trace of it from the surface.)
Unexpected damage sources. (for example, an aircraft vertical tail part may be designed to withstand hailstone impact but not able to resist damage from being dropped during shipping or removal for inspection)
"Best" repair techniques are heavily dependent on details of the structure. In other words, because composites excel at being tailored to meet very specific needs, there are few “universal” materials and methods that can be used to achieve successful results. Composite repair specifics really have to be determined on a case-by-case basis.
BASIC REPAIR PROCESS.
The very basic fundamentals of composite repair include the following steps:
Inspect to assess damage (extent and degree)
Remove damaged material.
Treat contaminated material.
Prepare repair area.
Complete composite repair.
Inspect repair for quality assurance (e.g. delaminations, inclusions, proper cure, etc.)
Restore surface finish.
Published courtesy of Abaris Training Resources, Inchttp://www.abaris.com
Fibre Volume Fraction from Fibre Weight Fraction.
Fibre Weight Fraction from Fibre Volume Fraction.
FVF = Fibre Volume Fraction.
FWF = Fibre Weight Fraction
ñc = Density of Composite (g/cm3)
ñm = Density of Cured Resin/ Hardener Matrix (g/cm3)
ñF = Density of Fibres ( g/cm3)
WF = Fibre Area Weight of each Ply (g/sqm)M
Published courtesy of David Cripps, Gurithttp://www.gurit.com