# What happens to an elastic material being stretched and unstretched over and over? Does it lose it’s elasticity or remain unaffected?

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I know that a rubber band snaps if it’s pulled too far or if it gets pulled too fast. I know that some materials lose their elasticity if they’re kept in a stretched state for too long. But what happens to a material being stretched to a reasonable length and then unstretched periodically like the rubber band on a punch balloon? Does it lose it’s elasticity faster than a band that wasn’t being stretched or does it keep working normally?

In: Physics

Young’s modulus or the modulus of elasticity defines three zones. The point at which each zone is reached differs per material, depending on its properties. In the first zone, stretching and unstretching is harmless. The stretched material always goes back to its original position (Hooke’s law). At some point, you stretch it too hard so that the molecular structure breaks a bit. So, it does unstretch, but not completely anymore. When you pull even harder, it will break

The rubber band will eventually snap due to a phenomenon called fatigue.

There are many ways materials can fail. What you described in the first part of your post is loading the material to its failure point.

Depending on the material, there will be a range of loads that is elastic. In the elastic zone, removing the load causes the material to go back to its original state completely.

If you load the material beyond the elastic region, you enter the plastic region. Removing the load after reaching the plastic region causes the material to go back in shape but not to the original shape. There will he some amount of permanent deformation.

If you load the material beyond the plastic region, you eventually reach failure at which point the material breaks.

However, there are other ways materials can fail.

Even if you are in the elastic region, applying cyclic loads will eventually create microscopic cracks. Cyclic loading means applying a load, then removing it, then applying it again, removing it, and so on. These cracks will grow with the application of more cyclic loads. Eventually the material will fail. This failure mode is called fatigue.

If the cyclic load is very small, it can take million or billions of load cycles before a material fails.