Rick Girard wrote:
Paul, below is a post from the resident expert on composites
at the canard forum.
Patrick writes:
Has anyone ever done a realistic study as to what
the realistic
life expectancy is with cured resin? It seems that with each
passing day, we
are adding another day to the unknown expected life.
What I mean, is that if in say 3 years, we find out
that saf t poxy
or aero poxy has a life of 30 years, 40 years or
whatever.....
That "time life" might devastate our endeavors, or
at least the
value....knowing that we are building an aircraft good for
no more
than XX years....
Is there any factual data about this topic anywhere?
PLEEAAAZZZE don't start this rumor. This subject has been
studied
thoroughly. I will be more than happy to mail you a copy of
a paper
presented in 1980 by Grumman Aersopace titled REAL LIFE
AGING PROPERTIES OF
COMPOSITES. The paper was written after 20 years of data
collection on
various different types of fiber reinforced epoxy composite
materials
exposed to environmental extremes our hand built lover
affairs will never
see. The reason for the study was that up until that point,
only
accelerated aging tests had been conducted and results
indicated one can
expect considerable amounts of degradation. This was an
unexpected result,
and they needed verification and correlation from a "real
life" aging study.
I quote from the section - Discussion of Results -
Fiberglass Laminates.
"The data presently available from accelerated aging tests
on fiberglass
components invariably show considerable amounts of
degradation. Therefore,
it has been assumed by most users that such degradation must
be expected in
service. The data obtained under this program for specimens
machined from
parts exposed to various climatic conditions under service
loads show that
for properly protected structures the degradation for
periods of up to 20
years is either MINOR or NON-EXISTANT. (In many cases a
strength increase
was observed)." - end of quote. Caps added by this
author.
The phrase "properly protected" quite simply means -
painted.
I am 110% confident, that if they had tested the same
components today, the
conclusion would be the same except it would state for "up
to 41 years."
Because such a study takes so much time, the information if
provides is
extremely valuable. It also strongly indicates that one
should not depend
solely on accelerated aging test methods for this particular
application.
ANY ONE who wants a copy, send me your address and I'll drop
it in the snail
mail.
Gary Hunter
EAA Technical Counselor
I am not convinced. Paint coatings can deteriorate and lower pressure
at the higher altitudes can accelerate the aging process by increasing
the rate of out gassing. If you can smell a plastic part or composite
it is out gassing. NASA subjects organic parts of the space shuttle,
such as tires, to pre conditioning them by artificial aging in a vacuum
chamber with temperature cycling.
If it is out gassing the chemical composition is changing.
Ten year and twenty year old cars have plenty of problems with plastic parts.
Paul Lamar
All deterioration that I am aware if in composites is related to moisture
in the resin and is relatively minor reduction in strength. The oldest
homebuilt composites are 25 years old or even older and seem to be fine.
My Dad just junked a 48 year old fiberglass boat that was in usable
shape except that the transom was a thin layer of glass on both sides
of a 1" piece of plywood that finally rotted due to not being covered
in the bottom, eventually rotted from the inside out. The rest of the
hull seems to be fine, a bit "chalked" due to outside storage for nearly
50 years.
Grumman's tests showed not measurable difference in strength over a
very long period, lacking UV damage. I''l buy it.
The pix appear to show severe overload to me, std pullout in one case
and intact metal fittings with failures in the composites in each case.
At least consistent with flutter and not corrosion. Possibly the carbon
composite was weakened, but I doubt it. Too little info to be sure,
but I do agree that a weakened structure would be more prone to
flutter.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
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