Page 305 - Forensic Structural Engineering Handbook

9.16 CAUSES OF FAILURES

Accelerated Testing. Durability is often quantified by the performance of a material
under an accelerated weathering test. In such a test, the material of interest is subjected to
magnified environmental loads. These harsh conditions applied over a relatively short test
period are extrapolated to predict the behavior of the material under normal weathering for
a longer period of time. The “acceleration factor” is the ratio between the test period and
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the associated real service time. Therefore, the exposure time until material failure during
an accelerated weathering test multiplied by the acceleration factor yields the theoretical
design life of the material.
While such data may be very useful in predicting the durability of components, acceler-
ated testing remains highly subjective. Testing may not reproduce all of the ancillary envi-
ronmental loads contributing to deterioration of a material, such as the effects of atmospheric
pollution or biological attack. In addition, there is no industry consensus on the acceleration
factor for the most commonly used artificial weathering devices, because correlation data is
rarely available from real-world tests (non-accelerated actual weathering over many years)
to directly compare the long-term behavior.
Even given reliable acceleration factors for such equipment, there remains variation in
the repeatability and reproducibility of accelerated weathering tests. In addition, many stan-
dardized test methods do not set criteria to determine when the test subject has “failed”;
they only recommend a standard practice for conducting the testing. Therefore, several pre-
cautions are necessary to produce meaningful results using accelerated weathering tech-
niques. A reliable accelerated testing program would correlate the results of laboratory
testing to conditions observed in the field or to a control specimen exposed to non-accelerated
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weathering. It is also important to define the scope of the testing, with more limited or tar-
geted testing likely to achieve more meaningful results; for example, some chemical prod-
uct manufacturers will perform testing to determine whether their product is likely to stain
specific adjacent materials at the site.
An example of a broader and more difficult scope of testing may be to determine the
durability of a specific paint for use on the exterior of a building. The substrate, environ-
mental exposure, and water-tightness of the walls comprising the painted surfaces will
impact the performance of the product in service. These additional factors must be consid-
ered in developing a meaningful accelerated weathering test.
The various combinations of different degradation factors must be taken into account so
that laboratory test results can represent adequately the environmental loads that govern the
in-service performance of a material. Identifying the factors most likely to affect a building
component for a given installation may serve to exclude certain products from use prior to
testing. In most accelerated testing, only one or two environmental loads are magnified at a
time. Therefore, complex interactions affecting a material may not be adequately captured
in a laboratory test alone.
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According to ASTM E632, the following steps are necessary to produce meaningful
results from accelerated testing:
1. Identify the attributes and properties critical to the service life of a material or
component.
2. Identify the type and range of degradation factors affecting the test subject in service.
3. Identify likely failure mechanisms.
4. Select various test methods.
5. Define performance requirements and the scope of the testing program.
6. Perform pretesting to refine the test methodology.
7. Perform the predictive service life test.
8. Correlate accelerated test data with in-service test data or observations.