COMPOSITES - Materials for Civil and Construction Engineers - page 446

Composites

Microscopic Macroscopic

Fiber-reinforced Particle-reinforced

FIGURE 11.1 A classification scheme of composite materials.

Although several definitions of composites exist, it is generally accepted
that a composite is a material that has two or more distinct constituent ma-
terials or phases. The constituents of a composite typically have significantly
different physical properties, and thus the properties of the composite are no-
ticeably different from those of the constituents. This definition eliminates
many multiphase materials that do not have distinct properties, such as many
alloys with components that are similar.
There are a number of naturally formed composites, such as wood, which
consists of cellulose fibers and lignin, and bone, which consists of protein
collagen and mineral appetite. However, in this chapter we discuss artifi-
cially made composites only.
Composite materials can be classified as microscopic or macroscopic, as
shown in Figure 11.1. The distinction between microscopic and macroscopic
depends on the type of properties being considered. This distinction seems
arbitrary, but normally microscopic composites include fibers or particles in
sizes up to a few hundred microns. On the other hand, macroscopic compos-
ites could have constituents of much larger size, such as aggregate particles
and rebars in concrete.

11.1 Microscopic Composites

Many microscopic composite materials consist of two constituent phases: a
continuous phase, or matrix, and the dispersed phase or reinforcing phase,
which is surrounded by the matrix. In most cases, the dispersed phase is
harder and stiffer than the matrix. The properties of the composite depend
on the properties of both component phases, their relative properties, and
the geometry of the dispersed phase, such as the particle shape, size, distri-
bution, and orientation.
As indicated in Figure 11.1, microscopic composites fall into two basic
classes: fiber-reinforced and particle-reinforced . This classification is based
on the shape of the dispersed phase. Figure 11.2 shows composites with con-
tinuously aligned fibers, random fibers, and random particles. The mecha-
nism of strengthening varies for different classes and for different sizes and
orientations of the dispersed shape.