From: Alan Thomson (abthomson@goes.com)
Date: Tue Dec 24 2002 - 11:35:08 CST
Jason A. Cotting writes...
<snip>
Take a paper clip and unfold it. Now work it back and forth bending it. It
will eventually break at the point it is being bent at. This is probably
what happens to the water proofing on tents. This is known as fatigue
failure.
This is more important in a material that fails plastically as opposed to
tent fabric that has more elasticity. So maybe folding a tent isn't that
much of an issue for the fabric.
Comment...
Sorry, the engineer in me could not resist - I also insist on "concrete"
when everyone says "cement", but that's another thread...
Jason should have stopped at fatigue failure. Elastic refers to the
distortion of a material and it's returning to it's original shape when
the load is removed, plastic refers to distortion beyond the elastic range
(material does not return to it's original shape when the load is removed,
but does not break). When a single large load is applied, materials break
when the end of the plastic range is reached. Elasticity is a measure of
the extent of distortion that is not permanent when the load is removed,
plasticity is a measure of how much permanent distortion a material will
undergo before breaking. Brittle materials fail shortly after entering
the plastic range.
Fatigue failure occurs in the elastic range due to the repetition of low
levels of loading. Since the waterproofing does not fail the first time
the tent is folded, but after repeated foldings, I'd agree with him that
it is a type of fatigue failure.
By the way, I have always folded and rolled my tents, but this thread has
me reconsidering that....
Happy holidays everyone.
- Al Thomson, Troop 236, Schooley's Mountain NJ
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