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Fracture Toughness Testing of Metals 329
initiates at P . A test on a fully ductile material, such as steel on the upper shelf, produces a load-
i
displacement curve like Figure 7.29(c); a maximum load plateau occurs at P . The specimen is
m
still stable after maximum load if the test is performed in displacement control. Three types of
CTOD result, d , d , and d , are mutually exclusive, i.e., they cannot occur in the same test.
c
m
u
As Figure 7.29 illustrates, there is usually no detectable change in the load-displacement
curve at the onset of ductile crack extension. The only deviation in the load-displacement
behavior is the reduced rate of increase in load as the crack grows. The maximum load plateau
(Figure 7.29(c)) occurs when the rate of strain hardening is exactly balanced by the rate of
decrease in the cross section. However, the initiation of crack growth cannot be detected from the
load-displacement curve because the loss of cross section is gradual. Thus d must be determined
Ic
from an R curve.
7.6 DYNAMIC AND CRACK-ARREST TOUGHNESS
When a material is subject to a rapidly applied load or a rapidly propagating crack, the response
of that material may be drastically different from the quasistatic case. When rapid loading or
unstable crack propagation are likely to occur in practice, it is important to duplicate these conditions
when measuring material properties in the laboratory.
The dynamic fracture toughness and the crack-arrest toughness are two important material prop-
erties for many applications. The dynamic fracture toughness is a measure of the resistance of a material
to crack propagation under rapid loading, while the crack-arrest toughness quantifies the ability of a
material to stop a rapidly propagating crack. In the latter case, the crack may initiate under either
dynamic or quasistatic conditions, but unstable propagation is generally a dynamic phenomenon.
Dynamic fracture problems are often complicated by inertia effects, material rate dependence,
and reflected stress waves. One or more of these effects can be neglected in some cases, however.
Refer to Chapter 4 for an additional discussion on this subject.
7.6.1 RAPID LOADING IN FRACTURE TESTING
Some testing standards, including ASTM E 399 [8] and E 1820 [4] include annexes for fracture
toughness testing at high loading rates. This type of testing is more difficult than conventional
fracture toughness measurements, and requires considerably more instrumentation.
High loading rates can be achieved in the laboratory by a number of means, including a drop tower,
a high-rate testing machine, and explosive loading. With a drop tower, the load is imparted to the
specimen through the force of gravity; a crosshead with a known weight is dropped onto the specimen
from a specific height. A pendulum device such as a Charpy-testing machine is a variation of this
principle. Some servo-hydraulic machines are capable of high displacement rates. While conventional
testing machines are closed loop, where the hydraulic fluid circulates through the system, high-rate
machines are open loop, where a single burst of hydraulic pressure is released over a short time interval.
For moderately high displacement rates, a closed-loop machine may be adequate. Explosive loading
involves setting off a controlled charge that sends stress waves through the specimen [21].
The dynamic loads resulting from impact are often inferred from an instrumented tup. Alter-
natively, strain gages can be mounted directly on the specimen; the output can be calibrated for
load measurements, provided the gages are placed in a region of the specimen that remains elastic
during the test. Crosshead displacements can be measured directly through an optical device
mounted to the cross head. If this instrumentation is not available, a load-time curve can be converted
to a load-displacement curve through momentum transfer relationships.
Certain applications require more advanced optical techniques, such as photoelasticity [22, 23] and
the method of caustics [24]. These procedures provide more detailed information about the deformation
of the specimen, but are also more complicated than global measurements of load and displacement.