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Strain can be directly observed using e.g. a ruler. Can (internal) stress be directly observed?

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Strain is difficult to observe with the naked eye. Many material either plastically deform or break instead of showing visible strain. With the help of polarization filters it is no problem to visualize strain in transparent materials.

This is an image of a plastic ruler under strain viewed through polarization filters:

enter image description here

A high number of bands corresponds to a large strain, areas that only show a single color are less strained. Further details are nicely explained in the wikipedia article on photoelasticity.

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    $\begingroup$ So your answer is "no", since this displays strain and not stress. The two are related. But if you would take a transparent material with a different stress tensor depending on position within the material, then the strain can be very different than the stress. $\endgroup$ – fibonatic Feb 3 '14 at 12:35
  • $\begingroup$ Piezo materials change their electrical, and in some cases, optical, properties under strain. Whether that counts as "directly observed" I dunno. $\endgroup$ – Carl Witthoft Feb 3 '14 at 12:51
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    $\begingroup$ @fibonatic: Yes, stress is similar to a force and I can't think of a direct display. The response of a material though can nicely be visualized. One 'indirect' way might be to break a piece of glass. If it shatters into thousand pieces it was stressed but it is more difficult to observe that without breaking it. $\endgroup$ – Alexander Feb 3 '14 at 13:49
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Stress does not exist without an associated strain. How could it in an actual existing material? Phenomena like piezoelectricity are associated with strains too.

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