Damage accumulation and rupture of network materials are discussed in this chapter. The chapter is divided into two parts addressing the rupture of networks without and with pre-existing defects. The relation between the network structure and the strength and ductility of materials without pre-existing cracks is defined, and examples from gels, cellulose networks, and nonwovens are provided. The effect of the fiber tortuosity, fiber aspect ratio, fiber preferential alignment, and the variability of fiber and crosslink properties on network strength is discussed. As in other materials, the size effect on strength is important in Network materials and a section is dedicated to statistical aspects of the strength. Failure under multiaxial loading conditions is compared with failure in uniaxial tension. The second part of the chapter presents an analysis of the propagation of cracks in networks. Pronounced notch insensitivity is observed in many materials and numerous examples are presented. Special attention is given to toughness and guidelines are provided to assist the toughening of quasi-brittle Network materials. A subsection is dedicated to the strength, toughness, and fatigue resistance of elastomers and gels.

This chapter presents a comprehensive overview of the mechanical behavior of Network materials, with emphasis on the structure–properties relation. Crosslinked and non-crosslinked Network materials are discussed in separate sections. The behavior of crosslinked networks in tension, shear, compression, and multiaxial loading is described. The effects of fiber tortuosity, fiber alignment, crosslink compliance, network connectivity, and variability of fiber properties on network stiffness and nonlinear behavior are discussed in detail. The size effect on linear and nonlinear material properties is evaluated in relation with network parameters. Three types of nonlinear behavior are identified, corresponding to networks that stiffen or soften continuously during deformation, and networks with an approximately linear response. Numerous examples of each type are presented, including collagen networks, fibrin and actin gels, elastomers, paper, and nonwovens. The response of non-crosslinked athermal networks, such as fiber wads, is studied in compression and tension. The effect of entanglements in athermal networks is analyzed and a parallel drawn with the mechanics of thermoplastics.

In many networks, fibers interact though surface interactions such as cohesion and capillarity, which cause fiber bundling. In adequate conditions, the same process leads to the formation of percolated networks of fiber bundles. These have a specific structure and their mechanical properties are quite different from those of regular networks of fibers and molecular filaments. Separate sections are dedicated to crosslinked and non-crosslinked networks with surface interactions. Surface interactions perturb weakly the structure of crosslinked networks, but have a significant effect on their mechanics. If the network is not crosslinked, surface interactions reorganize the network and define the resulting structure. The properties of networks of fiber bundles embedded in solvents (colloidal suspensions) and in the dry state (buckypaper) are discussed in separate sections.

In this chapter the principles of composite manufacture are discussed. The advantages and disadvantages of each method are considered in identifying a process for a particular artefact. Specifically, the need to use sophisticated fibre placement techniques in manufacture is described.

In this chapter we describe the resins used for the manufacture of composite artefacts. The concept of curing is discussed with respect to the chemistry of typical polymer matrices. The advantages and disadvantages of thermosets and thermoplastics are also discussed.

In the case of thermosets, the importance of thermoplastic and rubber toughening is considered. While we concentrate on polymer matrix materials, ceramic and metal matrices are referred to for completeness.

In this chapter, the analysis in Chapter 6 is extended to dynamic loading. The main aim is to provide sufficient knowledge for predicting the life of a composite structure.

Fibrous reinforcement of materials has been employed over many centuries to increase performance. Many early plastics materials of the late nineteenth and early twentieth centuries relied on ‘fibrous’ inclusions, while the development of glass fibres for polymer reinforcement in the 1930s introduced the material known as fibreglass. Eventually, with the development of boron fibres for metal reinforcement and the discovery of high-strength carbon fibres in 1964, the term composites came into general use. More recently, carbon nanotubes and related materials and graphene have led to the development of nano-composites. The Composites Age has arrived.

This chapter describes the synthesis of the principal fibres and provides the range of acicular reinforcing particles, nanofibres, nanotubes, and nanosheets. The properties of the most common fibres – carbon, glass, ceramics, and natural and advanced polymers – are considered. The differing grades and their structural property relationships are also discussed. Surface treatments for adhesion and compatibility are described.

Polymeric matrices absorb moisture, so here we examine how this affects the performance of a composite material. For an aerospace artefact, absorption and desorption is an important issue. For example, on the tarmac the relative humidity (RH) is high, whereas in flight the RH is low. Also, the ambient temperature can vary significantly, whereas the skin of a military aircraft may reach temperatures of 120 °C in flight. Therefore, we consider the effects of RH, temperature, and thermal excursions on moisture absorption and how they influence the micromechanics. Initially we can assume that the fibres are insensitive to water, which is realistic for most common reinforcements apart from aramid fibres.

Protocols for repair and recycling of composites are described. Future developments that embrace self-healing systems are also considered. End-of-life options such as fibre and matrix recovery are also discussed. The economics of differing approaches are briefly considered using a whole-life cost model.

In this chapter the micromechanics of unidirectional fibre composites (see Section 5.1) are extended to laminates, where strain transfer occurs at a matrix crack other than at a fibre-break. The stress distribution under load is also discussed to describe the accumulation of damage under differing loading conditions.

Since the discovery of carbon fibres in the 1960s, the applications have grown. Because of the high specific strength and stiffness, aerospace applications have dominated, especially initially in military aircraft. The intent here is to demonstrate how the choice of material has been identified. Most critical demonstrators have come from the field of aerospace because of the benefits of carbon fibres and the development of confidence in their use in safety-critical designs. The latter has involved much testing and durability studies. Middleton has provided several case histories detailing the development of composite applications in aircraft structures [1]. The use of composite components has increased with improved confidence in the durability and reliability of these materials and structures. The Airbus A380 was introduced in 2006 using a carbon-fibre-reinforced polymer (CFRP) centre wing box, while the fuselage employed an aluminium–glass fibre composite laminate (GLARE). The centre wing box is a critical carbon-fibre composite structure that joins the wings to the fuselage. Together with several other composite components, such as the horizontal and vertical stabilizers, keel beam, and pressure bulkhead, the total composite usage is 22% w/w. In 2011 the Boeing 787 Dreamliner employed carbon-fibre materials for the fuselage and wings. In total, the latter used 80–90% by volume or 50% by weight of composite materials. The Airbus A350, introduced in 2015, also uses CFRP for the fuselage and wings, and in total composite usage is 53% w/w.

This chapter describes the mechanical performance of a fibre composite. A number of variables that control deformation and fracture are discussed: continuous or discontinuous fibres; fibre angle; fibre length; the transfer of stress between matrix and fibre at a short fibre and/or a fibre-break; and the role of the matrix. Individual components can fracture independently and control the micromechanics; the redistribution of stress after these events is discussed.