Design and Manufacture of Fibre-Reinforced Composites

The concept of fibre-reinforced polymer composites is introduced in this chapter. Most fibre-reinforced composites (FRCs) comprise two constituent materials (known as phases): a fibre phase and a matrix phase. It is noted that thermosetting matrices are far more frequently used for FRCs than their thermoplastic counterparts. An overview of the three most commonly used thermosetting matrices (i.e. unsaturated polyester, vinylester and epoxy) is provided. Available fibre types and forms are considered. The representative mechanical properties for the most common fibre types (glass, carbon and aramid) are presented and compared to traditional materials.

This chapter addresses the mechanical properties of a fibre-reinforced composite (FRC). The focus is on calculation of the elastic modulus and strength for unidirectional FRCs using the rule of mixtures expressions, but woven and random fibres are also considered. A unidirectional FRC exhibits anisotropic behaviour. It is stiffest and strongest in the fibre direction but is relatively compliant and weak in the transverse orientation. Woven structures provide similar mechanical properties in their axial (longitudinal) and transverse orientations, whilst random fibre composites simulate in-plane isotropic behaviour. The effect of the fibre and matrix properties on the structural behaviour of a composite is investigated in the context of fibre volume fractions (and weight fractions), assuming no voids and perfect fibre-matrix adhesion.

This chapter introduces the six basic steps needed to design and make a fibre-reinforced composite (FRC): (1) fibre and matrix selection; (2) mould preparation; (3) layup and consolidation; (4) curing (and post-curing); (5) demoulding; and (6) post-processing (finishing). These six steps are considered using a simple wet layup process for a flat unidirectional (UD) composite. The mechanical performance of the composite is estimated based on the rule of mixtures (RoM), inverse rule of mixtures (IRoM) and Kelly-Tyson (KT) models from Chap. 2. A description of the wet layup process is offered and discussed in the context of the options available to a composite fabricator. The chemical crosslinking processes for polyester, vinylester and epoxy are presented. Consideration is given to demoulding the FRC and to the post-processing options. The use of gelcoat, flow coat and paint are mentioned in the context of a broader discussion on surface finishing of FRC. The outcome of the chapter is a step-by-step design and manufacturing method that is easily replicated.

Vacuum bagging and prepreg moulding methods are introduced in this chapter. These advanced moulding techniques address some of the shortcomings of wet layup (hand lamination) but require additional equipment and consumables, as well as higher fabricator skill levels. The requisite equipment and consumables are described in detail. A step-by-step guide to vacuum bagging is offered and the storage and handling requirements of prepregs are considered. The advantages and limitations of these advanced methods are presented alongside their typical fibre volume (and weight) fractions. Finally, fibre volume fractions for wet layup are compared to the higher fractions typically obtained by vacuum bagging and prepreg moulding.

A selection of simple composite design (mechanics) equations were introduced in Chap. 2 to enable prediction of the mechanical properties of fibre-reinforced composites (FRCs). The rule of mixtures (RoM) and inverse rule of mixtures (IRoM) expressions were introduced for elastic moduli estimation, whilst longitudinal strength was considered via the Kelly-Tyson (KT) model. Chapters 3 and 4 then introduced wet layup, and vacuum bagging and prepreg moulding methods for composite manufacture. A carbon fibre-reinforced panels was fabricated using similar constituents (i.e. carbon fibres in an epoxy matrix) for each fabrication method. This chapter builds on these earlier chapters via a comparison of design (mechanical property) estimates and experimental data. Tensile tests are performed on multiple specimens cut from each of the panels. This chapter provides guidance on the accuracy of design estimates and offers support for the typical (rule of thumb) volume fractions suggested in Chaps. 3 and 4 for each manufacturing approach.

So far, this book has focused on flat, planar laminates in order to simply and easily illustrate composite design and manufacture considerations. However, creating most composite structures (or parts) requires a mould. This chapter considers composite design in the context of part mouldability (i.e. design for manufacture, DFM), and offers the composite designer some guidance on mould design and construction. In terms of part mouldability, special attention is given to draft angles (and undercuts), surface textures and sharp corners. Mould design and construction are examined in the context of mould durability; speed and ease of construction; and the mould tool fabrication cost. A step-by-step mould-making process is presented to enable the reader to better understand moulding considerations for laminates. A simple composite part (a frisbee) is then moulded to illustrate the fabrication process.

This chapter considers the manufacture of hollow sections. Mandrel lamination (wrapping) and bladder moulding are described and then used to create composite tubes. A step-by-step guide to tube manufacture is presented for prepreg moulding, but the fabrication methods introduced are more widely applicable (and hence, can be adapted for use in wet layup processes). Whilst the focus is on cylindrical tubes, the fabrication process can be simply modified to create square or rectangular hollow sections or more complex tubular structures. A helical spring and a bicycle handlebar are used as more complex examples. Demoulding methods for mandrel wrapped tubes are discussed. A specific focus is given to mechanical extraction and thermal (heating and cooling) mechanisms. The concept of mouldless composite construction is introduced. The importance of a mandrel’s coefficient of thermal expansion is considered in the context of demoulding using thermal methods.