Page 335 - Mechanical Behavior of Materials

336 Chapter 8 Fracture of Cracked Members

Figure 8.2 Crack (light area) growing from a large nonmetallic inclusion (dark area within)
in an AISI 4335 steel artillery tube. The inclusion was found by inspection, and the tube was
not used in service, but rather was tested under cyclic loading to study its behavior. (Photo
courtesy of J. H. Underwood, U.S. Army Armament RD&E Center, Watervliet, NY.)

The study and use of fracture mechanics is of major engineering importance simply because
cracks or cracklike flaws occur more frequently than we might at first think. For example, the
periodic inspections of large commercial aircraft frequently reveal cracks, sometimes numerous
cracks, that must be repaired. Cracks or cracklike flaws also commonly occur in ship structures,
bridge structures, pressure vessels and piping, heavy machinery, and ground vehicles. They are also
a source of concern for various parts of nuclear reactors.
Prior to the development of fracture mechanics in the 1950s and 1960s, specific analysis
of cracks in engineering components was not possible. Engineering design was based primarily
on tension, compression, and bending tests, along with failure criteria for nominally uncracked
material—that is, the methods discussed in Chapters 4 and 7. Such methods automatically include
the effects of the microscopic flaws that are inherently present in any sample of material. But they
provide no means of accounting for larger cracks, so their use involves the implicit assumption
that no unusual cracks are present. Notch-impact tests, as described in Section 4.8, do represent
an attempt to deal with cracks. These tests provide a rough guide for choosing materials that resist
failure due to cracks, and they aid in identifying temperatures where particular materials are brittle.
But there is no direct means of relating the fracture energies measured in notch-impact tests to the
behavior of an engineering component.
In contrast, fracture mechanics provides materials properties that can be related to component
behavior, allowing specific analysis of strength and life as limited by various sizes and shapes of
cracks. Hence, it provides a basis for choosing materials and design details so as to minimize the
possibility of failure due to cracks.
Effective use of fracture mechanics requires inspection of components, so that there is some
knowledge of what sizes and geometries of cracks are present or might be present. For example,
periodic inspections are commonly performed on large aircraft and bridges so that a crack cannot
grow to a dangerous size before it is found and repaired. Methods of inspection for cracks include
not only simple visual examination, but also sophisticated means such as X-ray photography and
ultrasonics. (In the latter method, reflections of high-frequency sound waves are used to reveal the