GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53
2nd Revised Pages306 • Chapter 9 / FailureBendingOriginOriginImpact or point loadingInternal pressureOriginTorsionOrigin(a) (b)(c) (d)Figure 9.13 For brittle ceramic
materials, schematic
representations of crack origins
and configurations that result
from (a) impact (point contact)
loading, (b) bending, (c) torsional
loading, and (d) internal pressure.
(From D. W. Richerson,Modern
Ceramic Engineering, 2nd edition,
Marcel Dekker, Inc., New York,- Reprinted fromModern
Ceramic Engineering, 2nd edition,
p. 681, by courtesy of Marcel
Dekker, Inc.)
failure analysis normally focuses on determination of the location, type, and source
of the crack-initiating flaw. A fractographic study (Section 9.3) is normally a part
of such an analysis, which involves examining the path of crack propagation as well
as microscopic features of the fracture surface. It is often possible to conduct an
investigation of this type using simple and inexpensive equipment—for example, a
magnifying glass, and/or a low-power stereo binocular optical microscope in con-
junction with a light source. When higher magnifications are required the scanning
electron microscope is utilized.
After nucleation, and during propagation, a crack accelerates until a critical (or
terminal) velocity is achieved; for glass, this critical value is approximately one-half
of the speed of sound. Upon reaching this critical velocity, a crack may branch (or
bifurcate), a process that may be successively repeated until a family of cracks is
produced. Typical crack configurations for four common loading schemes are shown
in Figure 9.13. The site of nucleation can often be traced back to the point where a
set of cracks converges or comes together. Furthermore, rate of crack acceleration
increases with increasing stress level; correspondingly, degree of branching also in-
creases with rising stress. For example, from experience we know that when a large
rock strikes (and probably breaks) a window, more crack branching results [i.e., more
and smaller cracks form (or more broken fragments are produced)] than for a small
pebble impact.
During propagation, a crack interacts with the microstructure of the material
and with the stress, as well as with elastic waves that are generated; these interac-
tions produce distinctive features on the fracture surface. Furthermore, these fea-
tures provide important information on where the crack initiated, and the source of
the crack-producing defect. In addition, measurement of the approximate fracture-
producing stress may be useful; stress magnitude is indicative of whether the ceramic
piece was excessively weak or the in-service stress was greater than anticipated.
Several microscopic features normally found on the crack surfaces of failed ce-
ramic pieces are shown in the schematic diagram of Figure 9.14 and also the photomi-
crograph in Figure 9.15. The crack surface that formed during the initial acceleration