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and that is the whole point...If all things being equal and size for size, acrylic and polc are the same strength then given equal fastenings, the elongation absorbs energy and creates a much safer material. If the force causes deflection, then a rigid material will fail. Lets be blunt, I think it fair to say that there are two causes of failure, under normal use. One : Point impact, somthing hitting it, Ie a block, stray object etc. OR two a surface load, Ie a large volume of water, a person or similar large area impacting it. In a crappy situation a point inpact becomes more likely. (going forward to stop a madly flogging sail). If at total deflection both materials have the same strength given equal size, why would you go for rigid? To quote the Japanese saying (and also car crash testers) "The strong branch that moves with the wind will survive longer than the ungiving"...Further 100 % is a right angle to its fixed plane. I would love a plastics chemist to step in here. The question I have is "If you had a equaly well fixed piece of the two materials at say 300 mm square, and you subjected them to point of failure stress from high impact water, which would fail first ?
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