Introduction

Throughout history, mankind has used composite materials to achieve combinations of properties that could not be achieved with individual materials. The Bible describes mixing of straw with clay to make tougher bricks. Concrete is a composite of cement paste, sand, and gravel. Today, poured concrete is almost always reinforced with steel rods. Other examples of composites include steel-belted tires, asphalt blended with gravel for roads, plywood with alternating directions of fibers, as well as fiberglass-reinforced polyester used for furniture, boats, and sporting goods. Composite materials offer combinations of properties otherwise unavailable. The reinforcing material may be in the form of fibers, particles, or laminated sheets.

Fiber-Reinforced Composites

Fiber composites may also be classified according to the nature of the matrix and the fiber. Examples of a number of possibilities are listed in Table 13.1.

Various geometric arrangements of the fibers are possible. In two-dimensional products, the fibers may be unidirectionally aligned, at 90°to one another in a woven fabric or cross-ply, or randomly oriented (Figure 13.1.) The fibers may be very long or chopped into short segments. In thick objects, short fibers may be random in three dimensions. The most common use of fiber reinforcement is to impart stiffness (increased modulus) or strength to the matrix. Toughness may also be of concern.

Elastic Properties of Fiber-Reinforced Composites

The simplest arrangement is long parallel fibers. The strain parallel to the fibers must be the same in both the matrix and the fiber, εf = εm = ε.