Photoelasticity
birefringence - polarization - stress analysis
What it shows:
Normally isotropic substances can become birefringent when under
stress. This property can be used in stress analysis.
How it works:
To use birefringence in stress analysis, the sample is placed between
two crossed Polaroids. The first Polaroid produces a linearly
polarized light source for the sample. This source has components
split into ordinary and extraordinary rays; the differing velocities
of these rays in the sample creates a phase difference which is color
dependant.. The second Polaroid takes components of the ordinary and
extraordinary rays; if the components are in phase, constructive
interference occurs, and the color is seen. If out of phase, the color
is not seen. As the level of birefringence is proportional to the
stress, differently stressed areas will appear different colors.
1. A V-shaped piece of thin Plexiglass that can be squeezed to show stress at the elbow.
2. A block of urethane, 10x7x2cm, cast on mercury, that can be squeezed and twisted.
3. Plexiglass block whose holes have been plugged with tightly fitting
metal pins. The stress caused the plastic by the insertion of the pins
is a model for riveting metal plates in aircraft or ships.
4. C-clamp tightened on a Plexiglass block shows the compressive stress pattern.
The thickness of the material will also dictate which color emerges in phase.
Cellophane is drawn in the manufacture of sticky tape, so is birefringent.
Because the stress is uniform along its length, it is a single color. We have a
sheet of glass with several strips of the tape crisscrossing; here a double layer
of tape is a different color because a new color is in phase. Rotate the strips
90° to obtain the complementary colors. Old cellophane bags do the same thing.
Setting it up:
We've found the best way to display is to use two Polaroidstot, one
circular (diameter 22cm) that sits on an OHP and the other square (28x20cm) which
is on 12cm legs so that it straddles the other. There is space between them so
that the sample can be placed and played with. All the above samples are available.
Comments:
Really beautiful colors abound (better when viewed directly so invite your audience up at
the end). Clear audio cassette boxes are another good source of stressed plastic. Rating ***
