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A rubber band that has a 5 cm diameter (Assume a perfect circle). The rubber band is then stretched. The rubber band returns to it's original shape. The rubber band now has a 6 cm diameter. What is this effect called?

Is there a material that returns to it's original shape exactly and always or is that impossible?

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This effect is called the elastic limit: https://en.wikipedia.org/wiki/Yield_(engineering)#Definition

Elastic limit (yield strength)

Beyond the elastic limit, permanent deformation will occur. The elastic limit is therefore the lowest stress at which permanent deformation can be measured. This requires a manual load-unload procedure, and the accuracy is critically dependent on the equipment used and operator skill. For elastomers, such as rubber, the elastic limit is much larger than the proportionality limit. Also, precise strain measurements have shown that plastic strain begins at low stresses.

As stated in the Wikipedia article, as the rubber band comes under higher degrees of stress, it will experience a stretching (strain) which it will recover from if the strain does not surpass the elastic limit.

Also remember that the strain experienced by the material is time-dependent, so if your rubber band is under constant stress (stretched around something for long time) it will be more deformed than if stressed the same amount for only a moment.

For more information on strain accumulating over time: https://en.wikipedia.org/wiki/Creep_(deformation)

Creep (deformation)

The rate of deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function.

All materials will experience this creep. If a material can be strained by stress (can be stretched), it can be deformed by experiencing a large enough cumulative strain.

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  • $\begingroup$ Thanks. I get it now. Does creep have a maximum? E.g. If I leave the elastic band stretched to it's max for a decade, will it stay at it's max stretch? $\endgroup$ – Questioning Oct 6 '17 at 2:42
  • $\begingroup$ Bear in mind that mind that while you are still applying the stress to the elastic for that decade, the creep effect is all the while causing permanent deformation in the elastic. This means that over time, the amount of stress required to reach maximum strain would decrease, such that, ultimately, the amount of stress required (if you continuously ramped it up to maintain the strain) to continue the permanent creep deformation would tear the elastic. $\endgroup$ – Kieran Moynihan Oct 6 '17 at 8:17
  • $\begingroup$ Reworded version of above: Starting by defining the “max stretch” as the stress point right before tearing the elastic. As the elastic band becomes permanently deformed over time due to the creep effect, the elastic limit will shift along with the deformation of the elastic while the max stretch will remain constant. Eventually the elastic limit will exceed the max stretch, and raising the stress to further strain the elastic would tear it. $\endgroup$ – Kieran Moynihan Oct 6 '17 at 8:23
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elastomers have fairly complex and time-dependent stress/strain and creep characteristics. in general, this is referred to as elastomeric stress/strain hysteresis, which ordinary engineering materials like most metals do not possess (at least for loads below the elastic limit, and at room temperature).

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