Elastic deformation

1. An object is elastically deformed when it returns to its original shape and size once the deforming forces are removed.
2. While the forces act, the object is still distorted, but the change in shape is temporary.
3. A rubber band, a steel spring and a trampoline mat all deform elastically when they are stretched by modest forces.
4. A tennis ball deforms elastically when it is squashed against a racket, then springs back to a round shape as it leaves the strings.
5. In an elastic deformation, the forces pull the particles of the material slightly further apart, or push them slightly closer together, without changing how they are arranged.
6. When the deforming forces are removed, the forces between the particles pull them back to their original positions, so the object recovers its shape.

Plastic deformation

1. An object is plastically deformed when it does not return to its original shape and size after the deforming forces are removed.
2. Plastic deformation is also called inelastic deformation, and the change in shape is permanent.
3. A lump of modelling clay, a drinks can crushed underfoot and a paper clip bent open all deform plastically.
4. A car's crumple zone is built to deform plastically in a collision, so the front of the car stays crushed afterwards.
5. In a plastic deformation, layers of particles in the material slide past one another into new positions.
6. When the forces are removed, nothing pulls the particles back to where they started, so the object keeps its new shape.
7. The word plastic describes the type of deformation, not the material, so a steel spring can deform plastically just as a plastic carrier bag can.

Elastic or plastic

1. Whether a deformation is elastic or plastic depends on what happens after the forces are removed, not on how far the object stretched while it was loaded.
2. Most objects deform elastically for small forces and plastically for large forces.
3. A steel spring stretched gently springs back to its original length, but the same spring stretched very hard ends up permanently longer, with its coils pulled apart.
   1. A coiled spring toy pulled too far down a staircase stays open in the overstretched section and never coils neatly again.
4. An object can show both types of deformation at once, recovering most of its stretch but keeping a small permanent extension.
5. The permanent extension is the length after all the forces are removed minus the original length.
   1. A spring that is $10.0cm$ long, stretched to $18.0cm$ and then released, might shrink back only to $10.6cm$, leaving a permanent extension of $0.6cm$.

Loading and unloading

1. Loading means adding masses to an object one at a time, so the stretching force increases in equal steps.
2. Unloading means removing the masses one at a time, so the stretching force decreases in the same steps.
3. Comparing the lengths measured during loading and unloading shows which type of deformation has happened.
   1. If the object has the same length at each load on the way down as on the way up, and finishes at its original length, the deformation is elastic.
   2. If the object finishes longer than it started, it has been plastically deformed.
4. A steel spring usually gives almost identical readings during loading and unloading, provided it has not been overloaded.
5. A rubber band behaves differently, because it is longer at each load during unloading than it was during loading, even though it returns to almost its original length at the end.