Callister - An introduction - 8th edition

636 • Chapter 16 / Composites
Longitudinal
direction
Transverse
direction
Figure 16.8 Schematic
representations of
(a) continuous and
aligned, (b) discontinuous
and aligned, and
(c) discontinuous and
randomly oriented
fiber–reinforced
composites.
(a) (b) (c)
load is achieved only at the axial center of the fiber. As fiber length l increases, the
fiber reinforcement becomes more effective; this is demonstrated in Figure 16.7b,
a stress–axial position profile for l l c when the applied stress is equal to the fiber
strength. Figure 16.7c shows the stress–position profile for l l c .
Fibers for which l W l c (normally l 15l c ) are termed continuous; discontinuous
or short fibers have lengths shorter than this. For discontinuous fibers of lengths
significantly less than l c , the matrix deforms around the fiber such that there is
virtually no stress transference and little reinforcement by the fiber. These are essentially
the particulate composites as described earlier. To effect a significant
improvement in strength of the composite, the fibers must be continuous.
16.5 INFLUENCE OF FIBER ORIENTATION
AND CONCENTRATION
The arrangement or orientation of the fibers relative to one another, the fiber
concentration, and the distribution all have a significant influence on the strength
and other properties of fiber-reinforced composites. With respect to orientation,
two extremes are possible: (1) a parallel alignment of the longitudinal axis of the
fibers in a single direction, and (2) a totally random alignment. Continuous fibers
are normally aligned (Figure 16.8a), whereas discontinuous fibers may be aligned
(Figure 16.8b), randomly oriented (Figure 16.8c), or partially oriented. Better
overall composite properties are realized when the fiber distribution is uniform.
Continuous and Aligned Fiber Composites
Tensile Stress–Strain Behavior—Longitudinal Loading
Mechanical responses of this type of composite depend on several factors, including
the stress–strain behaviors of fiber and matrix phases, the phase volume
fractions, and the direction in which the stress or load is applied. Furthermore,
the properties of a composite having its fibers aligned are highly anisotropic, that
is, dependent on the direction in which they are measured. Let us first consider
the stress–strain behavior for the situation wherein the stress is applied along