What is happening on the atomic level when materials get squashed or stretched?

Question

When you stretch a rubber band, or a t-shirt or any material; what is happening on the atomic level that allows some materials to extend whilst others break?

Likewise: what is happening when you squash sand or plasticine? What is happening that allows it to bind together regardless of how it’s deformed?

In: Physics

Answer 1

In events where there’s only a physical change (i.e., no chemical reactions), the effects are happening on an intramolecular level.

Intramolecular means: interactions between molecules. In addition to the forces that bind atoms together into molecules, there are forces that bind molecules together into solid objects. When you apply a mechanical force to an object, you’re stretching or squishing those bonds. Oftentimes the result is that the molecules align themselves in a new way.

The following points apply mainly to polymers (including rubber and plasticine, I believe.)

Sometimes a long, folded molecule will unfold and extend when you apply a tensile force.

Sometimes a material will be composed of many long chains that all attract each other weakly, and when you pull on the material with enough force, the chains will slide past each other. (The chains go from being aligned “side-by-side” to “head-to-toe”, lengthening the material.)

Sometimes a material will be composed of long, randomly oriented molecular chains, and when you pull on the material, the chains align themselves in parallel.

Conversely, when materials are compressed, molecules can fold up, squish closer together, and reorient themselves to accommodate the force.

Metals are a little different. Their crystalline structure means their constituent molecules/atoms align themselves into planes. The planes are able to slide past each other in response to a force, but defects in the crystal lattice can hinder that sliding motion, decreasing the metal’s malleability/ductility and causing it to break instead.

Metals are also usually comprised of sectors of differently oriented crystal structures called grains. These grains can also slide past each other and align with each other in response to a stress.