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I was looking for some shear strength properties of common polymers/plastics.... I have not found any usefull information yet. I know polymers aren't that linear elastic, but I expected to find some range of shear properties. I would like to use it for a stress evalution of a general cross section at shear.

Can someone help me with some numbers or tell me why it is impossible to give such figures?

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  • $\begingroup$ If the dominant failure mechanism is tensile, not shear, for example, then possibly only tensile strengths are provided. In this case, you'd have to analyze the stress state to find the magnitude and orientation of the maximum tensile stress. $\endgroup$ – Chemomechanics Aug 2 '18 at 17:33
  • $\begingroup$ Welcome to engineering stack exchange. This is a great question that challenges how polymers compare to steels in normal engineering use. Turns out, like many things, the devil is in the details. $\endgroup$ – Mark Aug 7 '18 at 18:34
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The reason it is impossible to give you figures is that nobody has run the tests to verify the shear strength. Why? Wouldn't it be easy to get a block of polypropylene and run a test? Well, that test would only be valid for that supplier of polypropylene, and only for that thickness.

This uncertainty in testing results from several interesting factors:

  1. Unlike steels, who's unique properties can be determined by specifying a set additive concentration, a polymer's properties depends on chain lengths as well as cross linking, which come down to manufacturing methods.
  2. This means that a "standard" polymer is never - standard. For example, polypropylene is usually broken into homopolymer, copolymer, and random-block copolymer. All of these can be made in long and short chain. And the co-polymer block added can greatly influence the degree of cross-linking. Each specifically manufactured chain-length polymer is typically called a resin, with a manufacturer's trade name.
  3. Even these wildly varying property differences can be subtly influenced by the extrusion method, not unlike how annealing changes the property of a steel.
  4. A manufacturer will not run unnecessary tests to bring a resin to market. If a resin will not be needed to operate in shear, there is no need to run a test on shear strength.
  5. Any engineers concerned with shear have learned to convert the stress via Mohr's circle to tensile stress and estimate using a very conservative factor of safety using those methods.
  6. A shear strength estimate would typically be made under the presumption that the ratio of shear strength to tensile strength would be similar to the ratio between shear modulus and tensile modulus, and utilize the Poisson's ratio (which typically remains constant regardless of cross-linking or chain length).
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