Page 392 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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372 Fracture Mechanics: Fundamentals and Applications

FIGURE 8.14 Effect of notch radius on the Izod
impact strength of several engineering plastics. Taken
from D 256-03, “Standard Test Methods for Deter-
mining the Izod Pendulum Impact Resistance of
Plastics.” American Society for Testing and Materials,
Philadelphia, PA, 1988.

energy required to fracture the specimen is inferred from the initial and final heights of the pendulum
(Figure 7.37). In the case of the Izod test, the specimen is a simple cantilever beam that is restrained
at one end and struck by the pendulum at the other. One difference between the metals and plastics
test methods is that the absorbed energy is normalized by the net ligament area in plastics tests,
while tests according to ASTM E 23 report only the total energy. The normalized fracture energy
in plastics is known as the impact strength.
The impact test for plastics is pervasive throughout the plastics industry because it is a simple
and inexpensive measurement. Its most common application is as a material-screening criterion.
The value of impact strength measurements is questionable, however.
One problem with this test method is that the specimens contain blunt notches. Figure 8.14
[17] shows the Izod impact strength values for several polymers as a function of notch radius. As
one might expect, the fracture energy decreases as the notch becomes sharper. The slope of the
lines in Figure 8.14 is a measure of the notch sensitivity of the material. Some materials are highly
notch sensitive, while others are relatively insensitive to the radius of the notch. Note that the
relative ordering of the materials’ impact strengths in Figure 8.14 changes with notch acuity. Thus
a fracture energy for a particular notch radius may not be an appropriate criterion for ranking
material toughness. Moreover, the notch strength is often not a reliable indicator of how the material
will behave when it contains a sharp crack.
Since Izod and Charpy tests are performed under impact loading, the resulting fracture energy
values are governed by the short-time material response. Many polymer structures, however, are
loaded quasistatically and must be resistant to slow, stable crack growth. The ability of a material
to resist crack growth at long times is not necessarily related to the fracture energy of a blunt-
notched specimen in impact loading.
The British Standards Institution (BSI) specification for unplasticized polyvinyl chloride (PVC-U)
pipe, BS 3505 [18], contains a procedure for fracture toughness testing. Although the toughness
test in BS 3506 is primarily a qualitative screening criterion, it is much more relevant to structural
performance than the Izod impact test.
Appendices C and D of BS 3506 outline a procedure for inferring the toughness of a PVC-U
pipe after exposure to an aggressive environment. A C-shaped section is removed from the pipe of
interest and is submerged in dichloromethane liquid. After 15 min of exposure, the specimen is
removed from the liquid and the surface is inspected for bleaching or whitening. A sharp notch is
placed on the inner surface of the specimen, which is then dead-loaded for 15 min or until cracking