March 1, 2006

Stress Test

Method

Lots of materials can get stressed out, and you know what happens then. Failure! Overstressed things tend to break. You can easily detect stress points in transparent plastic with polarized sunglasses.

Put on a pair of polarized sunglasses. Hold the other pair in front of you and slowly rotate it clockwise. You will notice there is a position where all the light is blocked and the lenses of the pair you are holding look black. Keep holding the sunglasses in this position.

Put a piece of transparent plastic in front of one of the lenses. (Good sources of plastic to stress test are jewel boxes for CDs, cassette cases, clear plastic forks, or even torn plastic bags.) Look at areas that are bent, at edges, and at stamped designs. If you don't see any stress, apply some by bending, breaking, or tearing the plastic. In sunlight stressed areas will appear in a rainbow of colors with the most stressed areas appearing as black lines. Because the colors can interfere with the detection of the stresses, it is better to use a light source that is a single color, such as the sodium- or mercury-vapor bulbs in streetlamps. Only the black lines will show up under these lights.

Insider Information

Finding hidden stresses may not seem like an earthshaking discovery to you, but architects in California think it is. They worry about metal fatigue, a kind of weakness, in the structures they design for earthquake zones. Before a design is finalized, a clear plastic model is subjected to forces similar to an earthquake and then examined using polarized lenses. The weak points in the model predict problems that could occur in a real building.

Materials that are not transparent can also be examined for stress—but not with polarized lenses. Airplanes and bridges, which can develop metal fatigue, are regularly inspected with X rays.

Reprinted with permission from Don't Try This at Home! Science Fun for Kids on the Go by Vicki Cobb (www.vickicobb.com) and Kathy Darling. Text copyright © 1998 by Vicki Cobb and Kathy Darling. Published by William Morrow and Company, Inc.

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