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When measuring Young's modulus in a material, does the geometry of the material actually matter? I have seen several references recommend that I use cylindrical pieces. But, wouldn't the tests work just as well using non-cylindrical(ex. rectangular pieces)?

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Young's modulus of a material doesn't depend on geometry. It is a mechanical property of material and depend on its structure. But, we cannot determine Young's modulus of a material by its structural properties experimentally. We (in your case) want to determine $E$ (Young's modulus) by using $E=\large{\frac{PL}{A\delta}}$ in a tension test ($P$ is the tension force,$L$ is the initial length, $A$ is the cross sectional area and $\delta$ is the elongation). So, we should make our experiment conditions ideal as much as possible. For example, we should use a shape that doesn't make stress concentration as much as we can and for this aim cylindrical pieces are better than rectangular pieces.

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  • $\begingroup$ Is there a simple explanation why stress concentration is reduced in cylindrical pieces compared to rectangular pieces? $\endgroup$ – user29305 Jul 5 '16 at 9:38
  • $\begingroup$ @AidanRocke Stress concentration in the sharp edges increases. Mathematical calculations of stress concentration are complex and I should confess that I am not familiar with them. Sorry! $\endgroup$ – lucas Jul 6 '16 at 2:25

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