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What happens when you put something under stress?

Strain is the response of a material to stress. It is defined as the change in length of the material under stress (L' −L₀) divided by the original length (L₀). For a material under tension, the material may show an incremental increase in length. For a material under compression, the material may show an incremental decrease in length.

One way to demonstrate strain for yourself is to use compressible packing foam (beams) or insulation (tubes). Draw regular grids on the foam (as shown below). What happens to the grid spacing as you squish, stretch and bend the foam? When you bend the foam, you can see a combination of compressive and tensile stresses on opposite sides of the bend.

Mechanical Properties of Materials

How do mechanical engineers describe the way a material behaves when under stress? One measurement is called the elastic modulus, and is defined as stress divided by strain. It is a measure of how much strain is produced by a given amount of stress on a material. Another measure is Poisson's ratio, which describes how a stress applied along one dimension of a material affects the other dimensions.

Here's a demonstration you can try for yourself. As one example, compare tootsie rolls vs. jolly ranchers. What's the difference? How does each respond to tensile and compressive forces? If you squish a tootsie roll, what happens to it? Materials like tootsie rolls, taffy and caramel deform plastically (change shape permanently). They also get lots wider as they are squished shorter, or narrower if they are stretched longer. How about a jolly rancher? These are brittle, so they do not deform plastically before they break by fracturing. Other materials you might try are silly putty (see Bouncy Polymer Chemistry for a recipe for making your own) and ice cubes.