Elasticity and plasticity

What happens when you pull a rubber band? It changes shape, gets deformed – by the force you apply. And when you let go, it contracts again. It regains its original shape when you stop pulling. Materials that do this are called elastic materials.

If you pull some chewing gum, it will also deform. But when you stop pulling it stays in a new, stretched-out form. This kind of material that keeps its new shape, is called a plastic material. How about this tennis ball? If you make a dent in the surface with your finger it rebounds again when you remove the pressure.

The surface of the ball is elastic. What happens when you drop the ball? When it hits the floor, it is compressed. Since the surface of the ball is elastic, it pops back out again. The surface pushes against the ground, making the ball fly upwards.

It’s the elasticity of the tennis ball that makes it bounce so well. If you push your finger into a piece of clay instead the indentation remains. Clay is a plastic material. When the clay hits the floor, its surface is also flattened, but since it stays in that shape, the clay hardly bounces at all. A metal spring is also elastic.

If we extend it and let go it returns to its original length. The further the spring is extended, the stronger the force needed to pull it. This relationship between extension and force can be shown in a graph. If we don’t use any force at all, the spring is at its starting point, its equilibrium position. As we start extending the spring, the force keeps increasing.

If the spring’s extension is doubled, the force is also doubled. In the graph, this gives us a straight line – a spring has a linear relationship between extension and force. However, when the spring gets extended too far, something happens. The spring keeps extending, but the force doesn’t increase at the same rate. It is no longer a linear relationship.

This point is called the spring’s extension limit. And when we stop pulling, the spring doesn’t return all the way to its original length. When the spring is stretched beyond its extension limit, it is not completely elastic any more. Many elastic materials have an extension limit. Like this metal wire.

As long as it is subjected to a small force, it is elastic. But as this wire reaches its extension limit, a small section becomes thinner: the wire develops a neck. And this necking is not reversed when the force is removed. Therefore, it is a plastic deformation. A force can deform an object.

Materials that retain their new form are plastic materials. Materials that regain their original shape after being deformed are elastic materials. Many elastic materials have a limit where they cease to be completely elastic, an extension limit. If they pass this extension limit, they behave as plastic materials, becoming permanently deformed.