One
of the most common and straight forward methods utilized in EAC tests is the
use of a constant, applied tensile load that acts as the driving force to EAC.
Typically, a tension specimen is employed and specimens are loaded to various
levels of applied stress as defined by ASTM G-49. A typical tension specimen
and exposure cell is shown in Fig. 1. A distinction is usually made between the procedure
of running these types of tension tests with regard to the methods employed for
applying the load. A constant load is usually applied using a dead weight
fixture. In its simplest form, a hanging weight is suspended from the specimen if
small loads are required. Alternately,
for higher loads, a simple lever can be utilized to magnify the applied load.
The stress (S) on an unnotched specimen is calculated with the following
formula: S = P/A where P = load on the specimen and A= specimen cross-sectional
area. For the case of dead weight loading, a constant load is produced on the
specimen. However. once cracking initiates in the specimen, the cross-sectional
area is reduced so the applied stress actually increases. Therefore. in these types
of tests on susceptible material, the specimen often fails soon after
initiation of cracking and little information on crack propagation is obtained.
The effects of the corrosion and stress can be focused at a single location by
the use of a single notch in the specimen gage section.
An
alternative to the constant load tensile test is the constant deflection
tensile test. In this case, the dead weight loading mechanisms is replaced by a
spring, proof ring, or other compliant device. This type of loading
configuration is usually much simpler and allows for more specimens to be
tested in a limited area such as a laboratory exhaust hood. However, in this
case the load will decrease as the crack propagates through the specimen. If
insufficient compliance is available in the loading fixture, the crack will
stop prior to specimen fracture. Therefore, for deflection-controlled tests. it
is important to obtain information regarding the load/deflection relationship
for the geometry of fixture used. Normally, the important aspects of testing
are as follows:
- Check deflections both before
and after tests.
- Examine the specimens closely
for sub-critical crack growth.
- Calibrate the fixture
periodically.
- If only limited compliance is
available, break specimens in air after testing to locate any sub-critical
cracks and the reduction in load-carrying capacity they cause.
Other Constant Deflection Specimens
There
are a variety of specimens that utilized constant deflection. These include
- Bent beam specimens (2-, 3-,
and 4-point loading) per ASTM G38 (see Fig. 2)
- C-ring specimens per ASTM G3R
(see Fig.
3)
- U-bend specimens per ASTM G30
(see Fig.
4)
Each
type of specimen has a compliance dictated by the specimen dimensions and the
modulus of elasticity of the material bring tested. For purely elastic
deflections, simple stressing equations can be utilized to relate applied
tensile stress to specimen deflection. These are provided in the standard test
method documents. One of the limitations inherent to deflection-controlled
specimens is the nonlinearity in the stress vs. strain relationship once the
elastic limit is surpassed. For determination of deflections on specimens
stressed to high percentages of the engineering yield strength (i.e., beyond
the yield point of the material), it is common to utilize a strain gauged
calibration specimen. This specimen can be used to measure the exact
stress/deflection relationship for the combination of material and specimen
geometry being used.
Once
the stress/deflection relationship for the specimens to be used is determined,
it is typical to stress several specimens at various levels of stress. This
allows for assessment of the susceptibility to cracking as a function of
applied stress. This can be performed either by monitoring time to failure as a
function of applied stress or by evaluation of the failure/no-failure
performance at a fixed test duration and level of applied stress. This latter
technique is particularly useful in quality assurance or lot release testing
where the combination of service experience and laboratory testing has provided
information regarding the level of laboratory performance required to determine
acceptable service performance. In some cases, constant deflection specimens
such as bent beams and C rings are not as severe as dead weight-loaded
specimens. This is usually due either to variations in susceptibility with
orientation or to limited compliance reducing the driving force for crack,
propagation.
U-bend
specimens are constant deflection specimens hut are normally not stressed to
various levels of deflections or load. They are severely plastically deformed
in their fabrication by bending a strip of material around a mandrel. Usually,
multiple specimens are utilized and failure/no-failure is monitored after
various durations of exposure. The plastic deformation in most cases provides a
mechanical inducement to the initiation of SCC. Therefore, it can accelerate
SCC in some systems that normally require an unacceptably long time of crack
initiation on other types of specimens.