This chapter follows on from the previous chapter and looks at ﬁ brereinforced materials. The concept of ﬁ bre-reinforced materials had its origin in nature in the structure of wood. In metallic structures, the building unit is the crystal whilst a polymer is an agglomeration of large numbers of long thread-like molecules. A glass consists of a mass of fairly large silicate units which are too sluggish in their movements to be able to crystallise. In living matter, both plant and animal, the simplest unit is the cell. As a tree grows, the wood tissue forms as long tube-like cells of varying shapes and sizes. These are known as tracheids but we may regard them as ﬁ bres which are arranged in roughly parallel directions along the length of the trunk ( Figure 24.1 ). They vary in size from 0.025 to 0.5 mm in diameter and from 0.5 to 5.0 mm in length and are composed mainly of cellulose. The ﬁ brelike cells are cemented together by the natural resin lignin ; so wood can be regarded as a naturally occurring composite material in which the matrix of lignin resin is reinforced and strengthened by the relatively strong ﬁ bres of cellulose.