# Elastic limit of a spring

1. Apr 21, 2006

### foofoo

Hi everybody.

First of all, sorry for my English. It's not my mother tongue.

We made the Hooke's Law experiment at school (high school). Everything worked out great. We had to find the spring constant and we did. But our teacher gave us an assignment for extra credits! We have to find the elastic limit of our springs without damaging it. I know we could get lots of springs and test them with know weigths (trial and error), but I don't think that's what she wants.

I've been googling and I've found some websites that talk about the elastic limit of springs and show plots where you can identify the elastic limit point. They talk about plastic deformation and how stuff crack in different ways. I've found topics about stress and strain, but I haven't found a specific example applied to what I need. I found a site that talked about if we stretch a wire, the amount it stretches by depends on its length, its diameter and the material it’s made of. I think something similar applies to springs.

Could anyone tell me which concept I should look for? A book's reference would be great? Does anyone know a book where I could find an example like this?

Thanks all.

2. Apr 23, 2006

### lightgrav

The important point is that the Hooke's Law linear relation (L = Lo + k F),
which is reversible, is only valid for "small" deformations.
Larger deformations will be non-linear, and not reversible.
That is, if you increase Force little-by-little, and plot the slope as you go,
the slope (dL / dF) will change just as the elastic limit is reached.

This WILL make a permanent change in the spring.
But if that change is small enough, next year's students won't notice that your spring is different from the springs that were not abused this year.

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