The study of environmentally assisted
cracking (EAC) in its most basic sense involves the consideration
and evaluation of the inherent compatibility between a material
and the environment under conditions of either applied or residual
stress. This is a very broad topic encompassing many possible
combinations of materials and environments. However. it is also
a critical consideration because equipment, components, and structures
are intended to be used under specific conditions of environment
and stress. Furthermore, the materials used in construction typically
have a multitude of manufacturing and process variables that may
affect materials performance. Testing for resistance to EAC is
one of the most effective ways to determine the interrelation
of material, environmental, and mechanical variables on the cracking
process.
The grand dimensions of this
subject immediately limit attempts to make simplistic application
of only a single method of testing for all cases. Factors such
as,
- material type,
- process history,
- product form,
- active cracking mechanism(s),
- loading configuration and geometry, and
- service environment conditions,
to name a few, can have major consequences
in determining the type of specimen and test condition to be utilized.
The prudent approach to selection of testing methods is usually
to assess these considerations along with a survey of previous
experiences provided from prior investigations for similar applications
or from those found in the published literature.
It can be said that there is no single perfect testing technique
for the evaluation of EAC. However, the evaluation of materials
for EAC typically involve the use of the specimen and technique
that takes into account as many necessary factors as possible
for the particular material and environment under consideration.
In some cases, this may mean the use of
- More than one type of test specimen
- Various alternative configurations of the same specimen
- Alternative test techniques with the same specimen (e.g. crevices
applied potential, constant load, and slow strain rate)
Most of all, it is important to provide a link between the results
of laboratory evaluations and real-world service applications.
This is often developed through studies involving:
- Integrated laboratory and field or in-plant tests
- Correlation of laboratory data with service experience
- Reviews of published literature on the service performance
of similar materials
In any case, the evaluation of EAC susceptibility using laboratory
testing methods can provide data resulting in an increased confidence
level. This often allows for an optimization of the materials
of construction. By this it is meant that the allowance for unpredictable
service performance can be reduced resulting in a lower material
cost, reduced downtime, and a reduction in the number of costly
failures.
Also see Stress Corrosion Cracking, Hydrogen
Embrittlement, Liquid Metal Embrittlement,
Constant Load-Deflection tests, SSR Tests,
Fracture Mechanics Tests
