GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53
2nd Revised Pages332 • Chapter 9 / FailureViscoelastic creepis the term used to denote the creep phenomenon in polymeric
materials. It is one of the topics of discussion in Section 7.15.SUMMARY
Fundamentals of Fracture
Ductile Fracture
Fracture, in response to tensile loading and at relatively low temperatures, may occur
by ductile and brittle modes, both of which involve the formation and propagation
of cracks. For ductile fracture, evidence will exist of gross plastic deformation at the
fracture surface. In tension, highly ductile metals will neck down to essentially a point
fracture; cup-and-cone mating fracture surfaces result for moderate ductility. Cracks
in ductile materials are said to be stable (i.e., resist extension without an increase
in applied stress), and inasmuch as fracture is noncatastrophic, this fracture mode is
almost always preferred.Brittle Fracture
For brittle fracture, cracks are unstable, and the fracture surface is relatively flat and
perpendicular to the direction of the applied tensile load. Chevron and ridgelike
patterns are possible that indicate the direction of crack propagation. Transgranu-
lar (through-grain) and intergranular (between-grain) fractures are found in brittle
polycrystalline materials.Principles of Fracture Mechanics
The significant discrepancy between actual and theoretical fracture strengths of brit-
tle materials is explained by the existence of small flaws that are capable of amplifying
an applied tensile stress in their vicinity, leading ultimately to crack formation. Frac-
ture ensues when the theoretical cohesive strength is exceeded at the tip of one of
these flaws.
The fracture toughness of a material is indicative of its resistance to brittle frac-
ture when a crack is present. It depends on specimen thickness and, for relatively
thick specimens (i.e., conditions of plane strain), is termed the plane strain fracture
toughness. This parameter is the one normally cited for design purposes; its value is
relatively large for ductile materials (and small for brittle ones), and is a function of
microstructure, strain rate, and temperature.Brittle Fracture of Ceramics
At room temperature, virtually all ceramics are brittle. Microcracks, the presence of
which is very difficult to control, result in amplification of applied tensile stresses and
account for relatively low fracture strengths (flexural strengths). This amplification
does not occur with compressive loads, and, consequently, ceramics are stronger in
compression. Fractographic analysis of the fracture surface of a ceramic material
may reveal the location and source of the crack-producing flaw.Fracture of Polymers
Fracture strengths of polymeric materials are also low relative to metals. Both
brittle and ductile fracture modes are possible, and some thermoplastic materials