Acoustic Emission Monitoring of CFRP Laminated Composites ...

8 Chapter 2. Carbon Fibre-Reinforced Polymer Composites
possible to manufacture cheaper bres. Recent years have seen an increasing
interest in the use of carbon bres for applications in the automotive,
aerospace and biomedical industries. Their popularity stems largely from
their advantageous material properties such as high strength-to-weight ratio,
fatigue strength, corrosion and heat resistance.
The scope of this thesis is limited to carbon bre-reinforced epoxy composites,
also referred to as carbon bre-reinforced polymer (CFRP) composites.
Hence, the discussion in the remainder of this chapter is limited
to bre-reinforced polymer (FRP) composites.
A composite material is a nonhomogeneous material made by combining
two or more dierent materials. Each of the constituent materials retains
its own distinctive properties, while the composite will have properties
which are a complex function of the proportion of each constituent and
their interactions. When used in composites, carbon bres are used to
reinforce materials such as metals, carbon, ceramics, and polymers. The
reinforced material is referred to as the matrix. The composite will have
properties which cannot be achieved with the matrix (cured resin) and the
bres independently. 26
The material properties of an epoxy resin are vastly dierent from carbon
bres. Hence, the properties of a carbon/epoxy composite can be split
into matrix dominated and bre-dominated properties. Many of the important
properties of a composite are determined by the matrix. The matrix
transfer loads internally to all the bres, determines the maximum service
temperature, supports bres under compression and provides resistance to
damage growth. 27
In addition, the matrix provides interlaminar shear and
impact tolerance. 28, 29 The bre-dominated property is tensile strength. 29
Hence, the matrix dominates the transverse and compressive properties,
but the bres dominate the axial tensile properties.
Laminated composites are constructed by adding thin layers of sheets
and resin. The bres come as chopped strand mats, woven and unidirectional
sheets. Chopped strand mats consist of chopped bres that are
evenly distributed and randomly orientated. The bres are held together
with a binder. The strength of the mats is equal in all directions in the
plane of the laminate. Woven sheets are formed from interlacing yarns,
and are strongest in the direction of the bres, i.e. in two directions. The
orientation of all bres in a unidirectional sheet is in the same direction.