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Other than higher creep and relaxation moduli, why are polymerics rarely used for large scale structures, architectural, oil rigs, pipes, or other? It seems like, although they have weaker mechanical properties, they have better corrosion resistance etc.

Are polymerics just cheaper?

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    $\begingroup$ What are "polymerics"? $\endgroup$ – Olin Lathrop Feb 5 '16 at 15:42
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    $\begingroup$ I think that you answered your own question. The only benefit that you state is corrosion resistance. The mechanical properties are worse. $\endgroup$ – hazzey Feb 5 '16 at 16:26
  • $\begingroup$ Please add more details and explanation to your question to make it clearer. $\endgroup$ – hazzey Feb 5 '16 at 16:26
  • $\begingroup$ Here's a nice report which provides technical specs as to how and why plastic fails. sintef.no/globalassets/project/ffs/dokumenter/seminar-juni2010/… $\endgroup$ – GisMofx Oct 28 '17 at 16:00
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Polymers are in fact used - wood is a very common construction material (although perhaps one should consider it a composite?).

I don't really have a straightforward answer, but polymers (= plastics) are also prone to degradation caused by the environment: UV light is bad for them, water might cause them to swell and go soft, they are prone to creep. Also, as you say, they are simply weak (lower Young's modulus). Their range of operating temperatures are very bad too: you can subject metals to fairly wide range of low and high temperatures without changing their properties very much, but this span is much narrower for polymers.

Like wood, they are easy to ignite (unless flame retardants are used which on the other hand they often are) but unlike wood, many plastics lose their capacity to carry loads at higher temperatures (just 100-200 °C). This also happens with metals, but they yield at much higher temperatures than polymers (plastics).

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    $\begingroup$ Wood is definitely a biocomposite, though it is made primarily from two biopolymers. It has strong cellulosic fibers running along the grain and weaker lignin holding the fibers together in the transverse direction. This is why it splits more readily along the grain. Low Young's modulus does not make a material weak, but rather flexible. Elasticity is derived from atomic scale bond strength while bulk strength is derived from microstructural features. Otherwise good answer! $\endgroup$ – wwarriner Feb 5 '16 at 17:14
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per pound, steel is far cheaper than plastic and far stronger. to make a plastic structure as strong as a steel one would require a very large amount of plastic which would cost far more than if steel were used. Corrosion of steel can be managed by coatings and alloying composition.

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    $\begingroup$ As in Stainless - doesn't rust :_ just stains less... $\endgroup$ – Solar Mike Oct 28 '17 at 7:45

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