Page 94 - Instrumentation Reference Book 3E

Photo-elasticity 79

4.9 Photo-elasticity
For many years, the phenomenon of photo-elas-
ticity has been employed in experimental studies
of stress distribution in bodies of various shapes.
The effect made use of is the birefringence that
occurs in some materials and its dependence on
stress.
Light is in general made up of components
polarized in different directions; if in a material
the velocities of these components are different,
the material is said to be birefringent. The birefrin-
gence is increased (often from zero in stress-free
material) by stress, commonly being proportional
to the difference in stress in the two directions of
polarization. The effect was originally discovered
in glass; synthetic materials, notably epoxy resins,
are much more commonly used now.
In practice, a model of the structure to be
examined is made out of photo-elastic material.
This is placed in a rig or polariscope such as the
one shown in Figure 4.17, and loaded in a way
that corresponds to the load imposed on the ori-
ginal. By recombining components of the light
ray with polarizations parallel to the two princi-
pal stresses, fringes are produced, and their posi-
tion and number give information about strain in
the model. The first-order fringe occurs when
there is a phase difference of 360" between the
Figure 4.16 Distortion of a pattern under large strain. components; the nth order when there is a 360"n
Courtesy, South Australian Institute ofTechnology. difference.

Figure 4.17 Polariscope in use. Courtesy, Sharples Stress Engineers Ltd.