Eckold Design and Manufacture of Composite Structures

1. Auflage 1994
ISBN: 978-1-84569-856-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 400 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

A practical book of value to those in the automotive, chemical, aerospace and offshore industries. Case studies are included and as well as covering flexible manufacturing systems and non-destructive evaluation, the author looks ahead to metal matrix composites and ceramic matrix composites.

Autoren/Hrsg.

Eckold, G C

Inhaltsverzeichnis

1;Front Cover;1
2;Design and Manufacture of Composite Structures;4
3;Copyright Page;5
4;Table of Contents;6
5;PREFACE;8
6;Chapter 1. INTRODUCTION;10
6.1;Historical perspective;11
6.2;Composites: advantages and disadvantages;15
6.3;Design methods;19
6.4;Nomenclature;24
6.5;References;28
7;Chapter 2. MATERIALS AND PROPERTIES;29
7.1;Matrices;30
7.2;Property measurement;49
7.3;References;57
8;Chaper 3. MECHANICS OF MATERIAL BEHAVIOUR;58
8.1;Stiffness properties;58
8.2;References;106
9;Chapter 4. BEAMS, PLATES A N D SHELLS;109
9.1;Beams;109
9.2;Plates;122
9.3;Shells;138
9.4;References;170
10;Chapter 5. ASPECTS OF DESIGN;172
10.1;Jointing;173
10.2;Impact;186
10.3;Free edge effects;203
10.4;Hybrid composites;212
10.5;Flexible composites;221
10.6;Environmental effects;230
10.7;Residual stress;242
10.8;Statistical approach to design;251
10.9;References;255
11;Chapter 6. MANUFACTURE;260
11.1;Continuous reinforcement processes;261
11.2;Hand layup processes;277
11.3;Moulding processes;280
11.4;Resin injection processes;286
11.5;Integrated manufacturing systems;289
11.6;Processing of thermoplastic composites;290
11.7;Manufacturing defects;293
11.8;Machining;296
11.9;Non-destructive testing;302
11.10;References;310
12;Chapter 7. METAL AND CERAMIC MATRIX COMPOSITES;314
12.1;Metal matrix composites;315
12.2;Ceramic matrix composites;325
12.3;References;334
13;Chapter 8. EXAMPLES OF COMPOSITE APPLICATIONS;337
13.1;Aircraft;337
13.2;Space;344
13.3;Process plant;351
13.4;Medical;363
13.5;Automotive;368
13.6;Offshore;378
13.7;Wind turbines;390
13.8;Other applications;394
13.9;References;399
14;INDEX;403

Leseproben

2 Materials and Properties
One of the first steps in any design exercise is the consideration of candidate materials. For fibre composites this requires an assessment of each constituent phase and usually focuses on the selection of matrix and reinforcement. In addition to characteristic mechanical.properties, equal attention must be given to compatibility and processability. Clearly these are interrelated as the processes for forming the material and fabricating the structure occur concurrently. Other materials may also feature at this stage depending on the application. These may include fillers, stabilizers, pigments, fire retardants, etc. In almost all cases of structural application the fibre acts as the primary load bearing constituent and the matrix serves as a medium of load transfer on to those fibres. These are the basic mechanics of a composite material. The interface between fibre and matrix is therefore vitally important as this transfer occurs through shear at this connection. A further function of the matrix is to protect the interface and the fibres from the action of any environmental effects. For many components the role of the matrix may not be entirely non-structural, however, as it cannot always be assumed that all loads will act in the direction of the fibres. In the non-fibre directions properties become strongly influenced by matrix characteristics. During fabrication the materials can be handled in one of two ways: on-line impregnation or use of preimpregnated tows. In the former case the matrix is used in a liquid form, or converted into such by the use of a solvent or by melting, and the fibres are wetted out during component manufacture. Preimpregnated tows (prepregs) on the other hand are fibres that have been combined with matrix in a preliminary processing operation which can then be fabricated into a final component form. This has certain advantages as it eliminates much of the chemistry from the component fabrication, but can limit flexibility and is usually more expensive. Whatever choice of matrix and reinforcement is made, the key point is that they are selected as a system. It is only by considering constituents in this way that the full design potential can be realised. Matrices
In general terms a matrix can take the form of almost any material. However, those that have attracted most interest are those based on polymeric systems. New materials based on metals and ceramics are becoming available, but are still in their formative stages of development (see Chapter 7). Polymers used as matrices can be one of two types. The first, and most common, are of thermosetting character where solidification from the liquid phase takes place by the action of an irreversible chemical cross-linking reaction. This usually occurs as a result of the addition of other chemicals to initiate and accelerate the reaction and may involve the application of heat and pressure. The second type of polymers are thermoplastic in nature and forming can be carried out as a result of the physical processes of melting and freezing. Generally speaking, these reactions are reversible. The type of resin matrix will govern the details of the manufacture technique employed. For example, some thermosets are sufficiently fluid to allow processing without further modification, whilst others require the application of heat or the use of diluents to lower viscosity levels. Prepreg materials, where the resin which is already incorporated and has been allowed to react to an initial stage, can be handled as a solid feedstock which is then consolidated through the action of pressure and temperature. The fabrication of thermoplastics is carried out primarily through the action of heating and cooling. Whatever the application, the selection of matrix cannot be divorced from either design or processing. Unsaturated polyester resins
Polyesters are, certainly in tonnage terms, probably the most commonly used of polymeric resin materials. One of their major advantages is the ability for cure at room temperature. This allows large and complex structures to be fabricated where an oven cure would not be practical. Essentially they consist of a relatively low molecular weight unsaturated polyester dissolved in styrene (Table 2.1). Curing occurs by the polymerization of the styrene which forms cross-links across unsaturated sites in the polyester. A good degree of chemical resistance gives them wide application. A point to note is that the curing reaction is strongly exothermic (Fig. 2.1), and this can affect processing rates as excessive heat can be generated which can damage the final laminate.1 Shrinkage on cure (approx 7–8%) can also be a problem. Table 2.1 Structure of common thermosetting resin systems 2.1 Typical exotherm cure for a polyester resin. Because of the popularity of these systems, a family of resins has been developed to offer specific properties. Amongst the most important of these are those tailored for chemical resistance. For example, alkali resistance can be enhanced through the use of the so-called bisphenol modified resins where the number of sites for alkali hydrolysis is reduced to a minimum. A further system related to the polyesters, in that diluents such as styrene are used, is the vinyl ester family. Here the unsaturation occurs at the ends of the polymer chain, giving good chemical resistance and comparatively large strains to failure. In all these cases additives may be used to impart certain characteristics, The most notable are those for fire and flame retardance, and ultraviolet absorbers to improve weathering resistance. Epoxy resins
These resins are those most often used for advanced structural applications, They are generally two-part systems consisting of an epoxy resin and a hardener which is either an amine or anhydride. A wide variety of formulations is available to give a broad spectrum of properties after cure and to meet a diverse range of processing conditions. The higher performance epoxies require the application of heat during a controlled curing cycle to achieve the best properties. Prepregs can be made with epoxies where the fibres are impregnated with resin which is then partially cured. The objective of prepreg manufacture is to derive a material that can then be used in a fabrication environment, has an acceptable shelf life and good tack. This last property is important as it permits good adhesion between adjacent layers during assembly operations. Compared with polyesters, epoxies tend to demonstrate better mechanical properties, better performance at elevated temperature (depending on cure cycle) and a lower degree of shrinkage (typically 2–3%); Table 2.2.1 Table 2.2 Typical properties of cast resin systems Property Polyester Epoxy Specific gravity 1.1–1.5 1.2–1.3 Rockwell hardness M70–M115 M100–M110 Impact strength (Izod) (J/m) 16–32 8–80 Thermal conductivity (W/m/°C) – 0.17–0.21 Thermal expansivity (°C_1 × 105) – 5–8 Specific heat (kJ/kg/°C) – 1.25–1.80 Volume resistivity (O cm) 1015 1017 Dielectric constant (at 60 Hz) 3.0–4.4 2.5–4.5 Tensile strength (MPa) 40–90 55–130 Flexural strength (MPa) 60–160 125 Tensile modulus (GPa) 20–44 28–42 Phenolic resins
Phenolics are of particular interest in structural applications owing to their inherent fire resistance properties. This is accomplished without the use of fillers which, although effective in inhibiting flame spread, tend to increase smoke generation. The relatively recent development of ambient temperature cure systems has provided a stimulus to their use in a range of applications. They possess two significant disadvantages, however: low toughness and a curing reaction that involves the generation of water. This latter effect can cause problems since if it remains trapped within the composite, steam can be generated during a fire which can then damage the structure of the material. High temperature thermosetting resins
Because of the increasing level of interest for the use of composites at ever higher operating temperatures, there has been a continuing programme to develop organic matrices with good performance in this respect. There are now a number of options with reported survivable temperature capability in the region of 200 °C.2 Examples include: • Multifunctional epoxies. • Polyimides. • Bismaleimides. • Polystyryl pyridenes. • PMR (in situ polymerization with monomeric reactants). All of these systems have glass transition temperatures in the range 180– 400 °C. Most epoxy resin systems in...