mechanics testing techniques are typically utilized for evaluation of the
effects of metallurgical or environmental variables on EAC where the specimen
contains a sharp crack. One of the most common and relatively simple techniques
for incorporation of fracture mechanics techniques for the evaluation of EAC is
through the use of constant load or constant deflection specimens. In the case
of constant load specimens, a load is applied to a fracture mechanics specimen
using a directly applied dead weight or through a pulley or lever system to
magnify the dead weight load. These methods are analogous to those used for
constant load tensile specimens discussed previously.
most common types of specimens utilized for evaluation of EAC are the compact
tension (CT), single-edge notched bend (SENB), or double-cantilever beam (DCB)
varieties (see Figure 3). Normally, they are fatigue
pre-cracked prior to exposure to the environment to produce a sharp crack tip.
The fatigue pre-cracking must be performed at a low enough stress intensity to
minimize the plastic zone ahead of the crack. This is usually accomplished
through load-shedding techniques whereby an initially high peak load is used to
initiate the fatigue precrack and the cyclic load is then decreased as the
precrack approaches its desired length. In the testing of precracked specimen,
it has been found that excessive initial stress intensity of precracked
specimens can produce a large plastic zone that can act as a barrier to EAC
initiation. In these cases, the effect will be to produce nonconservative data.
stress intensity at the tip of the crack can be calculated using standard
equations as given in ASTM E399 for CT and SENB specimens and in NACE TM0177,
Method D for the DCB specimen. As shown for the DCB specimen, side grooves can
be utilized to assist in keeping the crack growing in a planar fashion under
plane strain conditions. In some cases the crack will tend to grow out of plane
resulting in an invalid test. The important consequence of using side grooves
is that the equation for the CT or DCB must contain a correction factor that
accounts for the geometry and dimension of the side grooves.
weight-loaded specimens are often used to monitor time to crack initiation and
can be used to monitor crack growth rate vs. stress intensity K. This is
normally performed by measurement of crack opening displacement, which can be
related to crack length for a particular specimen geometry using compliance
techniques. In the latter case, however, provisions must be made to monitoring
crack opening displacement at a rapid rate because the crack growth rate will
tend to increase with increasing K as the crack proceeds through the specimen.
attractive alternative to dead weight-loaded specimens is constant deflection
specimens. In this situation, either CT or DCB specimens are loaded to an
initial level of crack tip stress intensity by deflection of the arms of the
specimen. This deflection is obtained either by inserting the wedge into the
specimen or by tightening a bolt arrangement that deflects the arms of the
specimen. The initial stress intensity must be above the threshold stress for
EAC which allows cracking to initiate. Once cracking initiates, it proceeds
while the stress intensity decreases as the crack propagates through the
specimen. Thus, this type of test is commonly referred to as a decreasing K
test and is extensively utilized for evaluation of EAC in its various forms.
Once the stress intensity at the crack tip reaches a value insufficient to
sustain crack growth, crack growth will stop. These conditions of load and
crack length can be used to define the threshold stress intensity using the
appropriate equations for either the CT or DCB specimens.
the period required to run a decreasing K test is very long. An alternative is
to use a rising load test whereby the fracture mechanics specimen is subjected
to an increasing load. In this case, the crack open displacement and load are
monitored simullaneously and the results are analyzed in a similar manner to
conventional fracture mechanics tests. One of the difficulties in the
interpretation of rising load tests is that the threshold stress intensity
obtained by this method often differs from that determined by decreasing K
tests. The dynamic strain rate in the rising load test can complicate the
interpretation of the test result particularly if HE or HEC are the prevalent
cracking mechanisms. In these cases, a loading rate is usually selected that
results in data that correlate with the results of service experience in terms
of resistance to EAC.