Multiple choiceQuestion 1 · Recall
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By the end you can define elastic deformation and plastic deformation, and decide which one has happened by comparing an object's length before loading, at maximum load, and after the load is removed.
Stretch a rubber band and let go. It snaps back to the length it started with. Nothing has changed permanently.
Bend a paper clip and let go. It stays bent. The shape has changed for good.
Both objects were deformed — their shape changed while a force acted. The difference is what happens afterwards, when the force is removed.
That single difference gives us the two key words for this topic.
Elastic deformation
Elastic deformation is deformation in which the object returns to its original shape and size when the deforming force is removed.
The change is reversible. The energy used to stretch the object is given back when it relaxes.
Plastic deformation
Plastic deformation is deformation in which the object does not return to its original shape and size when the deforming force is removed. A permanent change remains.
Plastic deformation is also called permanent deformation. Some of the energy put in is not recovered: it ends up heating the material.
A solid is held together by forces between its atoms. Those atoms sit at a separation where the forces pull and push equally, like a tiny spring at its natural length.
A small stretch pulls the atoms slightly apart. Let go and the interatomic forces push them back to where they were. That is elastic behaviour.
Stretch far enough and whole layers of atoms can slide past one another into new positions. The forces can no longer pull them back to the old arrangement, so the new shape stays. That is plastic behaviour.
The analogy is a pile of coins: nudge the pile gently and it settles back; push a layer so far that it slides over the one below, and it stays in its new place. The analogy stops at the atomic scale — atoms do not slide as whole blocks — but the idea of a one-way rearrangement is the same.
Worked example: Worked example: reading a length test
A wire has an unstretched length of .
A load stretches it to . When the load is removed, the wire is long.
Was the deformation elastic, plastic, or both? Give the permanent extension.
The two words describe what happens to parts of the deformation, not two separate types of object.
Once a material has been stretched beyond its elastic limit, some of the stretch is permanent and some is still elastic. Release it and the elastic part springs back, leaving the plastic part behind.
That is why the wire above ended up longer instead of returning to or staying at .