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Are there any recommended clamps for holding samples of a viscoelastic material in place? My goal is to have the clamps hold the rectangular samples along lines drawn parallel to edges of the samples for elastic testing.

Hand screw clamps appear to be completely inappropriate as the material just squishes out of position. But, I think most clamps would suffer this weakness.

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Indeed, clamps are not very appropriate for materials with a high Poisson's ratio for exactly that reason. A few better alternatives may be:

  • Drill a hole through the sample near each extremity. Pass a steel bar through the hole and then clamp onto the bar. This has the problem of generating significant shear stresses around the holes during the experiment, such that the rupture may be due to shear instead of tension as expected. The mode of rupture should make it easy to identify how the element ruptured.
  • If the material is castable (liquid which hardens), a modified version of the above can be used, where multiple small steel threads in both directions perpendicular to the force are embedded into the material during casting. The clamps can then be tied to the threads. This helps distribute the transfer force, reducing the shear stress at any given point.
  • Depending on the material's properties, something like concrete's split cylinder test may be possible.
  • Flexure tests may also work, though their results may not be exactly equal to those of true tension tests.
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If practical rolling the strap sample a few times around a double cylinder mechanism which are designed to lock into each other with tension similar to seat belt or the old military uniform service belts would be the best setting.
This clamp on elastic material would not perform evenly, it will allow more slip on the ends. So it has to be installed far enough to allow for redistribution of strain and has to be calibrated for establishment of the origin datum after loading.

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