Fractures in cheese

Overview:
Learn how fractures grow by pulling on a piece of cheese

Equipment:
Pre-sliced American cheese (the smooth, unnaturally yellow stuff that comes individually wrapped in plastic) works best

Safety:
Don't eat the cheese if you're lactose intolerant!

How to do the experiment:
First, take a slice of cheese and pull on the edges. It should tear apart. Eat it. Get a new slice.

Now, make a small incision in the middle of the cheese slice with your fingernail or a butterknife, parallel to the edge of the cheese slice. Then pull on the two cheese edges parallel to the incision (so that you're pulling in a direction perpendicular to the incision). Watch how the small defect you've introduced into the cheese slice concentrates the tearing. Observe the shape of the propagating fracture, especially the pointed tips where the tearing is taking place, and how the fracture tips move faster as the fracture gets bigger. Eat the torn up slice and get a new one.

Now try repeating this, only this time make two incisions near the middle of the cheese, maybe about an inch apart, and make them offset diagonally from each other (see picture below). Now when you pull on the cheese, fractures will begin to propagate from each of these defects. As the tips of these fractures begin to propagate past each other, they will begin to curve toward each other, and eventually link up into a single fracture.

    +------------+
    |            |
    |    |       |
<-  |            |  ->
    |      |     |
    |            |
    +------------+

Explanation:
What you are doing is creating tension fractures, an important experiment for understanding how things pull apart. Like your slice of cheese, the crusts of the Earth and other planets sometimes get pulled on by tectonic forces. This can create tension fractures, some of which will link together to form larger faults. As people who live in earthquake-prone areas know, big faults can be bad news for the people living nearby! Tension fractures are also seen as deep cracks on glaciers, or as the magma-filled dikes which supply molten rock to the "curtain of fire" eruptions in Hawaii. A more everyday example is cracks in the surface of an asphalt road. If you look at these cracks while you're walking down the road, you may find patterns of cracks much like the ones you produced in your cheese experiments.

When you pull on a piece of cheese, you are creating tensional stress throughout the volume of the cheese. If there is a defect in it (like the incision you made), the stress cannot be transmitted across that defect (the walls of the incision can't pull on each other), so the stress that would normally be transmitted across the defect is instead concentrated around the edges of the defect. To visualize this, try drawing a square like your piece of cheese, and then draw evenly spaced lines from one side to the other, parallel to the direction you are pulling. Don't let any of the lines cross the fracture...instead, make them curve around the nearest edge of the fracture. The concentration of lines you get around the edges of the fracture represents the concentration of tensional stress. This concentration of stress means that the cheese will want to split apart around the edges of the incision. The bigger the fracture gets, the more stress will be concentrated at the tip of the fracture. This is why it gets easier to pull on the cheese as the fracture grows. When the tips of two fractures go past each other, the direction of tensional stress that the fracture tips "see" changes because the stress cannot be transmitted in a straight line across that gap; it is curved around by both of the fracture tips. To visualize this, try drawing the piece of cheese as it looks as the fractures start to bend. Draw the lines across it as you did before, and see how the stress direction is bent between the fractures. This is what makes the fractures bend toward each other and link up into a larger "fault."

Useful References:
Remember what your cheese looked like after the two-fracture experiment? Check out the area in the upper left quadrant of the image at http://www.jpl.nasa.gov/galileo/europa/p48527.html

Also, some of the Earth Sciences links in the MadSci Library.

Further comments: