ABSTRACT
A fracture toughness test measures the resistance of a material to crack extension. Such a test may yield either a single value of fracture toughness or a resistance curve, where a toughness parameter such as
K
,
J
, or
CTOD
is plotted against the crack extension. A single toughness value is usually sufficient to describe a test that fails by cleavage, because this fracture mechanism is typically unstable. The situation is similar to the schematic in Figure 2.10(a), which illustrates a material with a flat
R
curve. Cleavage fracture actually has a falling resistance curve, as Figure 4.8 illustrates. Crack growth by microvoid coalescence, however, usually yields a rising
R
curve, such as that shown in Figure 2.10(b); ductile crack growth can be stable, at least initially. When ductile crack growth initiates in a test specimen, that specimen seldom fails immediately. Therefore, one can quantify ductile fracture resistance either by the initiation value or by the entire
R
curve.
