Page 277 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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6 Fracture Mechanisms

in Nonmetals

Traditional structural metals such as steel and aluminum are being replaced with plastics, ceramics,
and composites in a number of applications. Engineering plastics have a number of advantages,
including low cost, ease of fabrication, and corrosion resistance. Ceramics provide superior wear
resistance and creep strength. Composites offer high strength/weight ratios, and enable engineers
to design materials with specific elastic and thermal properties. Traditional nonmetallic materials
such as concrete continue to see widespread use.
Nonmetals, like metals, are not immune to fracture. Recall from Chapter 1, the example of the
pinch clamping of a polyethylene pipe that led to time-dependent fracture. The so-called high-
toughness ceramics that have been developed in recent years (Section 6.2) have lower toughness
than even the most brittle steels. Relatively minor impact (e.g., an airplane mechanic accidentally
dropping his wrench on a wing) can cause microscale damage in a composite material, which can
adversely affect the subsequent performance. The lack of ductility of concrete (relative to steel)
limits its range of application.
Compared with the fracture of metals, research into the fracture behavior of nonmetals is in
its infancy. Much of the necessary theoretical framework is not yet fully developed for nonmetals,
and there are many instances where fracture mechanics concepts that apply to metals have been
misapplied to other materials.
This chapter gives a brief overview of the current state of understanding of fracture and failure
mechanisms in selected nonmetallic structural materials. Although the coverage of the subject is
far from complete, this chapter should enable the reader to gain an appreciation of the diverse
fracture behavior that various materials can exhibit. The references listed at the end of the chapter
provide a wealth of information to those who desire a more in-depth understanding of a particular
material system. The reader should also refer to Chapter 8, which describes the current methods
for fracture toughness measurements in nonmetallic materials.
Section 6.1 outlines the molecular structure and mechanical properties of polymeric materials,
and describes how these properties influence the fracture behavior. This section also includes a
discussion of the fracture mechanisms in polymer matrix composites. Section 6.2 considers fracture
in ceramic materials, including the newest generation of ceramic composites. Section 6.3 addresses
fracture in concrete and rock.
This chapter does not specifically address metal matrix composites, but these materials have
many features in common with polymer and ceramic matrix composites [1]. Also, the metal matrix
in these materials should exhibit the fracture mechanisms described in Chapter 5.
6.1 ENGINEERING PLASTICS

The fracture behavior of polymeric materials has only recently become a major concern, as
engineering plastics have begun to appear in critical structural applications. In most consumer
products made from polymers (e.g., toys, garbage bags, ice chests, lawn furniture, etc.), fracture
may be an annoyance, but it is not a significant safety issue. Fracture in plastic natural gas piping
systems or aircraft wings, however, can have dire consequences.
Several books devoted solely to the fracture and fatigue of plastics have been published in recent
years [2–6]. These references proved invaluable to the author in preparing Chapter 6 and Chapter 8.

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