Page 294 - Materials Science and Engineering An Introduction

266 • Chapter 8 / Failure

tests are relatively complicated, and we will not attempt to provide detailed explana-
tions. In brief, for each test type, the specimen (of specified geometry and size) contains
a preexisting defect, usually a sharp crack that has been introduced. The test apparatus
loads the specimen at a specified rate, and also measures load and crack displacement
values. Data are subjected to analyses to ensure that they meet established criteria be-
fore the fracture toughness values are deemed acceptable. Most tests are for metals, but
some have also been developed for ceramics, polymers, and composites.

Impact Testing Techniques
Prior to the advent of fracture mechanics as a scientific discipline, impact testing tech-
niques were established to ascertain the fracture characteristics of materials at high
loading rates. It was realized that the results of laboratory tensile tests (at low loading
rates) could not be extrapolated to predict fracture behavior. For example, under some
circumstances, normally ductile metals fracture abruptly and with very little plastic
deformation under high loading rates. Impact test conditions were chosen to represent
those most severe relative to the potential for fracture—namely, (1) deformation at a
relatively low temperature, (2) a high strain rate (i.e., rate of deformation), and (3) a
triaxial stress state (which may be introduced by the presence of a notch).
6
Charpy, Izod tests Two standardized tests, the Charpy and the Izod, are used to measure the impact
energy (sometimes also termed notch toughness). The Charpy V-notch (CVN) tech-
impact energy
nique is most commonly used in the United States. For both the Charpy and the Izod,
the specimen is in the shape of a bar of square cross section, into which a V-notch is
machined (Figure 8.12a). The apparatus for making V-notch impact tests is illustrated
schematically in Figure 8.12b. The load is applied as an impact blow from a weighted
pendulum hammer released from a cocked position at a fixed height h. 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 concen-
tration for this high-velocity impact blow. The pendulum continues its swing, rising to a
maximum height h , which is lower than h. The energy absorption, computed from the
difference between h and h , is a measure of the impact energy. The primary difference
between the Charpy and the Izod techniques lies in the manner of specimen support,
as illustrated in Figure 8.12b. These are termed impact tests because of the manner of
load application. Several 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 have been used to de-
termine the fracture properties of materials. The former are quantitative in nature, in
that a specific property of the material is determined (i.e., K Ic ). The results of the impact
tests, however, 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—abso-
lute values are of little significance. Attempts have been made to correlate plane strain
fracture toughnesses and CVN energies, with only limited success. Plane strain fracture
toughness tests are not as simple to perform as impact tests; furthermore, equipment
and specimens are more expensive.

Ductile-to-Brittle Transition
One of the primary functions of the Charpy and the Izod tests is to determine whether
ductile-to-brittle a material experiences a ductile-to-brittle transition with decreasing temperature and,
transition if so, the range of temperatures over which it occurs. As may be noted in the chapter-
opening photograph of the fractured oil tanker for this chapter (also the transport

6 ASTM Standard E23, “Standard Test Methods for Notched Bar Impact Testing of Metallic Materials.”