Mechanical Properties of SolidsIntroduction
In Chapter 6 we treated bodies as rigid: hard objects whose shape and size never change, however we push them. That was a useful simplification, but no real solid is perfectly rigid. Bodies can be stretched, squashed and bent, and even a thick steel bar gives a little under a large enough force. To change the shape or size of a solid, you need a force.
Riya is kneading atta for rotis. She presses her thumb into the dough and the dent stays there. Sina, meanwhile, has found Aman's chest expander and is pulling its spring. The colours in this chapter stay the same throughout: the solid body is blue, forces are purple, and the change in shape (and strain) is green.
Pull each one by its purple handle, then let go.
The spring went back to exactly its old length. The property of a body by which it tends to regain its original size and shape when the deforming force is removed is called elasticity, and the change it went through is an elastic deformation. The dough has no such tendency: it stays stretched for good. Substances like this are plastic, and the property is plasticity. Putty and wet clay come close to ideal plastics.
Spring Back or Stay Put?
Every solid falls somewhere between the spring and the dough. Sort these by what happens when you let go after a small push or pull.
Engineers need to know exactly how elastic their materials are. A building cannot be designed without the elastic properties of steel and concrete, and the same goes for bridges, cars and ropeways. Can an aeroplane be very light and still strong enough? Can an artificial limb be lighter but stronger? Why does a railway track have the shape of an I? Why is glass brittle while brass is not? The answers start with one question: how do simple loads deform different solids? That is what this chapter is about.