GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53
2nd Revised Pages310 • Chapter 9 / FailureFigure 9.17 Photomicrograph of a
craze in poly(phenylene oxide). (From
R. P. Kambour and R. E. Robertson,
“The Mechanical Properties of
Plastics,” inPolymer Science, A
Materials Science Handbook,A. D.
Jenkins, Editor. Reprinted with
permission of Elsevier Science
Publishers.)rate (i.e., rate of deformation), and (3) a triaxial stress state (which may be introduced
by the presence of a notch).Impact Testing Techniques
Charpy, Izod tests Two standardized tests,^6 theCharpyandIzod,were designed and are still used to
impact energy measure theimpact energy,sometimes also termednotch toughness. The Charpy
V-notch (CVN) technique is most commonly used in the United States. For both
Charpy and Izod, the specimen is in the shape of a bar of square cross section,
into which a V-notch is machined (Figure 9.18a). The apparatus for making V-notch
impact tests is illustrated schematically in Figure 9.18b. The load is applied as an
impact blow from a weighted pendulum hammer that is released from a cocked
position at a fixed heighth. The specimen is positioned at the base as shown. Upon
release, a knife edge mounted on the pendulum strikes and fractures the specimen at
the notch, which acts as a point of stress concentration for this high-velocity impact
blow. The pendulum continues its swing, rising to a maximum heighth′, which is
lower thanh. The energy absorption, computed from the difference betweenhand
h′, is a measure of the impact energy. The primary difference between the Charpy and
Izod techniques lies in the manner of specimen support, as illustrated in Figure 9.18b.
Furthermore, these are termed impact tests in light of the manner of load application.
Variables including specimen size and shape as well as notch configuration and depth
influence the test results.
Both plane strain fracture toughness and these impact tests determine the frac-
ture properties of materials. The former are quantitative in nature, in that a specific
property of the material is determined (i.e.,KIc). The results of the impact tests, on
the other hand, are more qualitative and are of little use for design purposes. Impact
energies are of interest mainly in a relative sense and for making comparisons—
absolute values are of little significance. Attempts have been made to correlate plane
strain fracture toughnesses and CVN energies, with only limited success. Plane strain(^6) ASTM Standard E 23, “Standard Test Methods for Notched Bar Impact Testing of Metallic
Materials.”