SAMPE Journal - November/December 2021 - 89

Temperature, T(K)
Percent of max moisture content
0.999Mm
0.99Mm
0.95Mm
0.9Mm
Time, Years
1/T*103
(1/K)
Figure 2. Diffusivity of moisture covers wide range (~2 Decades)
for traditional (Carbon/Epoxy) composites exposed to ambient and
elevated temperatures.
als by George S. Springer (SAMPE Fellow). Figures 2-4
are representative moisture diffusion characteristics
that resulted from extensive studies (experimentally
and analytically) by numerous investigators on carbon
fiber composite laminates. Figure 2 shows thew linear
dependence of diffusion coefficient over the temperature
range from 300K-375K (roughly 80F-215F) which
most often are reasonable range limits for majority of
service lives. Obviously, the vast majority of composite
structures livelihood has a residence life more toward
the 300K/80F range of use which is at least a decade
(or more) lower than operational higher temperature,
short-term exposures.
The Springer references1,2,3
conducted exposures and
calculations on typical " thin " laminates of carbon/epoxy
traditional quasi-isotropic materials to their maximum
moisture content levels as shown in Figure 3 (experimental
data extrapolated to various temperature ranges).
One notes with these tests, as well as numerous
others, that uncoated samples, unsealed samples, exposed
to moisture - take very long times to equilibrate
to exterior RH moisture exposure. Years - often decades
of time - are required to see moisture transport to saturation
levels typically above ambient storage levels. It
has though, been presumed that the porosity and void
content level is low to non-existent. If porosity, void
content or delaminations exist, then those provide significant
moisture ingress paths which can be structurally
detrimental.
Finally, moisture transition into composite laminates often
are subjected to periodic temperature exposure cycles
during their service lives from ambient to high and
back to ambient temperatures. Because the diffusion
coefficient and moisture content levels continuously
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Figure 3. Considerable time required for full moisture saturation (90.099.9%)
even in thin laminated structures.
Range of moisture distribution
after long term exposure
Moisture
Boundary
Layer
Figure 4. Cyclical moisture profile establish near exterior surfaces
predominantly as opposed to in-depth saturation.
vary in such cycling, the actual distribution cannot be
easily simulated accurately by a constant environmental
test. Figure 4 notes that analytically one would more
accurate note that the exterior air temperature and
moisture level (RH) would develop a boundary layer effect.
As such, moisture effects beyond such boundary
layers are small and essentially " steady state " over service
life times.
Future Tech Tidbit columns will address the significance
of micro-cracks, porosity, void content, and fiber type
and directional dependence of moisture transport.
References
1. " Environmental Effects on Composite Materials - Vol. 1, " George S.
Springer (Editor), Technomic Publishing, 1981, ISBN 87762-300-7
2. " Environmental Effects on Composite Materials - Vol. 2, " George S.
Springer (Editor), Technomic Publishing, 1984, ISBN 87762-348-1
3. " Environmental Effects on Composite Materials - Vol. 3, " George S.
Springer (Editor), Technomic Publishing, 1988, ISBN 87762-551-4
NOVEMBER/DECEMBER 2021
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SAMPE JOURNAL | 89
Diffusivity, D(mm2/s)
Temperature, °F
http://www.sampe.org
SAMPE Journal - November/December 2021

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