Checking out composites

Dr Graham Sims from the NPL reviews the trends and issues in the manufacture, design and application of composite materials

The special feature of simultaneous product and material fabrication makes composites a unique subject. Composites are recognised as having immense flexibility in the choices of fibre type and grade (different grades of carbon, glass and aramid fibres), fibre length and format (continuous, fabric, discontinuous, aligned, random), the positioning of the reinforcement to match the applied stresses, the type of resin matrix (thermoplastic or thermoset) and the process route chosen.

Frequently, a fibre format will be principally associated with only one or two process routes. For example, chopped strand mat (CSM) is most commonly used in open mould processes and short fibre reinforced systems in injection-mouldings. Fabrics, however, can be used in hand lay-up as well as in resin injection, pultrusion and autoclave processes.

In addition it is possible to mix fibre types, as in hybrid carbon (for high stiffness) and kevlar fabrics (for high toughness), or to mix formats, such as in the combination of fabrics, rovings (ie continuous fibre tows) and mats in pultruded structures.

Even for fabrics alone, there can be a range of styles (plain, twill and satin) and weights. Recently, `non-crimp fabrics’ (NCF) have become available, where rovings can be laid at required angles to build an often heavyweight `fabric’ held by through-stitching. As well as allowing the properties to be tailored by the fibre angles employed, the lack of crimp can result in improved properties. The heavier weights are suitable for the rapid build-up of thicker sections.

The options regarding the through stitching parameters (eg density, spacing and so on) are still being investigated. Some of these composition options are illustrated in Figure 2 with the related process/reinforcement combinations given in Figure 3.

In the thermoset field, phenolics are being used for improved fire resistance. Indeed, glass-fibre mat/polypropylene (GRP) has changed from not being accepted on off-shore structures due to being combustible, to now being acceptable. GRP can reduce the spread of heat due to its low thermal conductivity coupled with the blocking effect of the high volume fraction of glass-fibres. Indeed, the UK offshore oil operators association has recently completed a code for the use of GRP offshore.

Thermoplastic matrices were initially restricted to short discontinuous fibre reinforced materials processed by injection moulding using high pressures and metal tools. Recently, interest in the aerospace market was stimulated by the availability of high temperature resins. These are reinforced by carbon-fibre with improved matrix toughness and moisture resistance. However, they were also accompanied by higher residual thermal stresses due to their high temperature processing, compared to the then current epoxy systems. Although these materials have stimulated the development of tougher epoxy systems, the same technology has also been applied to commercial low temperature, lower cost commodity plastics, such as polypropylene.

Considerable volumes of GMTs are used in automobiles in battery trays (Rover) and bulkheads (VW), alongside thermoset sheet moulding compounds (SMCs) that are more suitable for visible areas. SMCs are typically used in lorry cabs and grills where appearance is important. They provide a better surface finish, including Class A.

An interesting thermoplastic development recently released is the co-mingled fibre product. Here, polypropylene fibres have been included within the glass-fibre tow, giving good drapability for fabrics produced with these tows. The polypropylene itself can be melted out under thermal forming to become the thermoplastic matrix. This may have benefits for several health issues regarding styrene, factory wastage and equipment cleaning.

Currently, approximately 50% of composite fabrication uses open mould processing of predominately chopped strand mat glass-fibre/polyester. These processes are most affected by the gradual tightening of regulations regarding styrene in the work-place. Approaches to reduce the impact of styrene emissions include the use of low emission resins and the greater adoption of closed mould techniques, such as resin transfer moulding (RTM).

RTM uses either matched metal tools or low pressure/vacuum assisted, single face moulds. These techniques are being investigated by the aerospace industries, along with thermoplastic tape winding, where the main requirement is to lower the cost of manufacture, particularly for sections greater than 10mm thick.

An important area for composites, for both thermoset and thermoplastic composites, is the manufacture of pultruded sections. These can be stocked as standard sub-component sections. Thermoset pultrusions are formed continuously by drawing the reinforcement through resin baths followed by a heated shaping, die and curing sequence in an equivalent process to metal extrusion.

Standard sections stocked by retailers allow designers to familiarise themselves with composite materials more easily. The European Pultrudrers Group of the European Organisation of Reinforced Plastics/Composite Material (GPRMC) has recently approached the Commission European Normalisation (CEN) requesting standards for their products to further increase their commercial potential. This new working group will be convened in the UK, reporting to CEN/TC249/SC2. CEN TC249/SC2 deals principally with composite materials specifications and looks towards ISO for test method standards.

When selecting a material, designers should also consider its recyclability. Thermoplastic composites can be recycled through re-grinding as a lower quality product, or included as a diluent with virgin material in some sheet products (eg GMT). For thermoset materials, the preferred route for re-use or disposal (eg incineration) is being assessed in different programmes, including one involving the BPF. The BPF programme is funded under the LINK Structural Composites Initiative and is lead by Brunel and Nottingham Universities in conjunction with several industrial companies, including a group of BPF members.

DESIGN and PRODUCT STANDARDS

The ability to `design’ or construct different composite materials, together with a multitude of test methods, has been partly responsible for the absence of good quality, validated databases. The anisotropy of composites, possessing different properties in orthogonal directions, varies with the system being considered. This complicates the generation of databases. In many cases, there are more standardised products available, such as aerospace prepregs. These are normally supplied at 60% fibre volume fraction, although a range of resins and fibres will be offered by different suppliers.

Increasingly, there is interest in ensuring the quality of composite manufacture for the general GRP market through the BSI standard (BS 4549) on quality assessment, which (following revision) will be suggested as an ISO standard. At the advanced composite level, the extensive use of ultrasonic C-scan techniques is being standardised with operational, calibration and reference documentation.

It is unclear whether the request from some industry quarters for a more `alloy’ type for composites (competing metal systems) can be achieved, but with final material properties relying on the processing skill of the fabricator, the material supplier may not support this end-user ideal. Certainly, it would aid the design process if materials produced to a standard specification were available, and some work towards this end will be started shortly on pultruded structures. Ideally, in the repair of structures, similar materials would be permitted rather than the requirement of using the identical material, as is currently the case for aerospace repairs.

Traditionally, qualification and design data have been collected and `owned’ by a end-user company using their preferred test methods. These have not been made publicly available for many years. However, as tests methods are harmonised and the responsibility for product data is directed more at the supplier, who has paid to have his material qualified, they are prepared to release the data.

Available data are beginning to be held on reference databases, including that collected by NPL from their DTI funded test method development and validation exercises, which will be made available nationally. The International Standards Organisation (ISO) committee, TC61/SC13, for composite materials is preparing an international standard (ISO 10350-2) detailing the data, and related test methods, required for a `standardised’ data sheet. These standards already exist for polymers, for both single (ISO 10350-1) and multipoint data (ISO 11403 Parts 1 to 3)

Significant progress has been made in developing the basic set of test methods for tensile, compressive, shear and flexure properties required for materials qualification and design. A new set of test methods, partly drawn from existing ISO, EN Aerospace, ASTM and the UK CRAG recommendations, will shortly be published as dual numbered ISO/CEN test methods, having achieved a higher degree of harmonisation with ASTM standards. These standards have been prepared by NPL with UK Department of Industry and British Plastics Federation-Composites Group support. It is mandatory to publish these standards as national standards (eg BS, DIN, UNI, NF) in EU and remaining EFTA countries and withdraw other standards of the same scope. Although using these EN standards is not the only method of showing compliance with EU directives, it will be the most cost effective for trade and product liability issues.

These new standards for composite materials cover tensile properties in EN ISO 527 Part 4 and 5, Flexure in EN ISO 14,125, compressive properties in EN ISO 14,126, shear by +/-45 degrees tension in EN ISO 14,129, and shear by short beam in EN ISO 14,130.

ISO 1268 is being revised as a multipoint standard for the preparation of panels, to be used for the cutting of test specimens by all process routes including press moulding, filament winding and resin injection moulding.

The alternative approach using the large number of micromechanics equations based on fibre and matrix properties to predict composite material properties has not been extensively used as yet. That is because the agreement between the available theories and experimental data is still not perfect.

New software prepared by NPL dubbed CoDA (Composites Design Analysis) includes a correlation or `reality’ factor which gives a measure of the agreement between the predicted and experimental values for each property and for each class of composite. In common with other prediction methods, there is a need for good quality input data on the constituents.

Other predictive tools are being developed using virtual modelling approaches. The interface, which is equally important to the composite response, is particularly short of validated characterisation procedures. But a new international pre-standards programme under the VAMAS initiative aims to rectify this weakness. Synthesised composite materials properties, experimental data or combinations of data can then be used with a variety of design tools.

A range of `design aids’ exist that range from engineering text books to finite element analysis (FEA) packages, with manufactures’ loading tables and PC windows software such as CoDA (covering beam and panel design) at an intermediate level. However, there are few design procedures or standards available.

The design rules for GRP pressure vessels and pipes have been available for several years in two British Standards, BS 4994 and 7159. These are now being produced as equivalent European standards in CEN TC210 and TC155. Basic questions still exist on the failure criteria to be applied under multi-axial stress situations and on whether composites products are designed most effectively using a prescriptive or performance approach. Presently, both approaches exist. Unfortunately, this does not aid the designer planning to use composites for the first time.

Other areas with a degree of standardisation include the rules for small boat design (Lloyds) and filament-wound gas cylinders (CEN/TC23). Progress can be expected for structural elements in civil engineering structures resulting from work in the `Eurocomp’ Eureka project and from the UKOA work on GRP off-shore. Some 31 CEN and ISO committees could relate to composites, frequently in competition with other materials. It requires careful monitoring to ensure that all these product standards consider the merits of composites so that they are not unnecessarily eliminated.

These design rules indicate some of the growth areas for composites. The replacement and reinforcement of the infrastructure will particularly demand new data and predictive techniques to ensure satisfactory service lives of up to 120 years. Off-shore also offers many opportunities for composites since they are now considered suitable for fire risk situations due to their low thermal conductivity and the thermal barrier that is produced from charred resin and glass-fibre.

Air, train or automobile applications continue to attract extensive effort from research and development teams. A particular area expected to be dominated by composites in the next few years is engine inlet manifolds.

The marine sector, having been virtually saturated at the leisure boat level, continues to offer possibilities for high performance craft using composite sandwich structures. It is interesting to note that HMS Wilton, a UK minehunter has been re-lifed for a further 15 years bringing its total life to 40 years. In the process, it was noted to still be in good condition.

The next few years will see a consolidation in thermal, environmental and electrical properties of composites. New test methods will cover through-thickness properties and structural elements, leading to an improved ability to design structural elements containing stress concentrations due to holes, bolted connections and flanges. Ultimately, product guides will be developed into standards and design codes to increase the chances of composites realising their full potential.

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{{TABLE 1: Design options for composite material formulations

Fibres Formats Resins

Continuous: Milled Thermosets:GLASS Chopped POLYESTERSCARBON Tapes VINYL-ESTERSARAMID Fabrics EPOXIESPOLYETHYLENE Knits, 3-D PHENOLICSBORON Non-crimp fabrics Thermoplastics:SILICON CARBIDE Continuous POLYPROPYLENEWhiskers: Hybrids NYLONGraphite PEEK

Fillers: Cores: Additives:Microspheres Foams, balsa, Inhibitors, fire fibre/spheres retardants,Kaolin, Calcium Honeycombs, UV stabilisers,carbonate, talc + filled plasticisers, pigments, catalysis}}

{{TABLE 2: Material and process relationships

Material type Process route Reinforcement format