Page 291 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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Fracture Mechanisms in Nonmetals 271

Composite materials usually consist of a matrix and a reinforcing constituent. The matrix is often
soft and ductile compared to the reinforcement, but this is not always the case (see Section 6.2).
Various types of reinforcement are possible, including continuous fibers, chopped fibers, whiskers,
flakes, and particulates [20].
When a polymer matrix is combined with a strong, high-modulus reinforcement, the resulting
material can have superior strength/weight and stiffness/weight ratios compared to steel and
aluminum. Continuous fiber-reinforced plastics tend to give the best overall performance (com-
pared to other types of polymer composites), but can also exhibit troubling fracture and damage
behavior. Consequently, these materials have been the subject of extensive research over the past
40 years.
A variety of fiber-reinforced polymer composites are commercially available. The matrix material
is usually a thermoset polymer (i.e., an epoxy), although thermoplastic composites have become
increasingly popular in recent years. Two of the most common fiber materials are carbon, in the
5
form of graphite, and aramid (also known by the trade name, Kevlar), which is a high-modulus
polymer. Polymers reinforced by continuous graphite or Kevlar fibers are intended for high-
performance applications such as fighter planes, while fiberglass is an example of a polymer
composite that appears in more down-to-earth applications. The latter material consists of randomly
oriented chopped glass fibers in a thermoset matrix.
Figure 6.14 illustrates the structure of a fiber-reinforced composite. Consider a single ply
(Figure 6.14(a)). The material has high strength and stiffness in the fiber direction, but has relatively
poor mechanical properties when loaded transverse to the fibers. In the latter case, the strength and
stiffness are controlled by the properties of the matrix. When the composite is subject to biaxial
loading, several plies with differing fiber orientations can be bonded to form a laminated composite
(Figure 6.14(b)). The individual plies interact to produce complex elastic properties in the laminate.
The desired elastic response can be achieved through the appropriate choice of the fiber and matrix
material, the fiber volume, and the lay-up sequence of the plies. The fundamentals of orthotropic
elasticity and laminate theory are well established [21].

6.1.3.1 Overview of Failure Mechanisms

Many have attempted to apply fracture mechanics to fiber-reinforced composites, and have met
with mixed success. Conventional fracture mechanics methodology assumes a single dominant
crack that grows in a self-similar fashion, i.e., the crack increases in size (either through stable or
unstable growth), but its shape and orientation remain the same. Fracture of a fiber-reinforced
composite, however, is often controlled by numerous microcracks distributed throughout the mate-
rial, rather than a single macroscopic crack. There are situations where fracture mechanics is
appropriate for composites, but it is important to recognize the limitations of theories that were
intended for homogeneous materials.
Figure 6.15 illustrates various failure mechanisms in fiber-reinforced composites. One advan-
tage of composite materials is that fracture seldom occurs catastrophically without warning, but
tends to be progressive, with subcritical damage widely dispersed through the material. Tensile
loading (Figure 6.15 (a)) can produce matrix cracking, fiber bridging, fiber rupture, fiber pullout,
and fiber/matrix debonding. Ultimate tensile failure of a fiber-reinforced composite often involves
several of these mechanisms. Out-of-plane stresses can lead to delamination (Figure 6.15 (b))
because the fibers do not contribute significantly to the strength in this direction. Compressive
loading can produce the microbuckling of fibers (Figure 6.15(c)); since the polymer matrix is
soft compared to the fibers, the fibers are unstable in compression. Compressive loading can also
lead to macroscopic delamination buckling (Figure 6.15(d)), particularly if the material contains
a preexisting delaminated region.

5 Kevlar is a trademark of the E.I. Dupont Company.