Photoelasticity
Some materials, including glass, celluloid, Bakelite and some other
plastics, become doubly refracting when subjected to stress. If a piece of such material is placed
between two crossed Polaraids the stress patterns can be observed. Different colours of light are
affected differently and some very beautiful effects can be obtained. Plastic models of components
such as gears, turbine blades and hooks can be made and the stress patterns in them observed to
check their design.
These patterns can be seen in car windscreens. The patterns stored in
them are due to the stresses produced during their manufacture.
Student investigation
Photoelastic stress patterns may be observed very easily as follows.
Place a piece of Polaroid on the top of an overhead projector and blank off the rest of the transmitted light. Put the object under test, such as a protractor or clear plastic ruler, on top of the Polaroid and place a second piece of Polaroid on top of the specimen. The projection on the screen will show the photoelastic patterns clearly.
Alternatively a slide projector may be used, one piece of Polaroid being placed in the slide holder and the second piece taped on the front of the projector lens. The test object can then be hung between the two. Using this method differing loads may be applied to the bottom of the specimen and the resulting change in the stress pattern observed.
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