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National Advisory Committee for Aeronautics, Technical Notes - Photoelastic Analysis of Three Dimensional Stress Systems Using Scattered Light

A method has been developed for making photoelastic_
analyses of three~dimensional stress systems by utilising
the polarization phenomena associated with the scattering
of light. By this method, the maximum shear and the di—
rections of the three principal stresses at any point
within a model can be determined, and the two princi_al _
stresses at a free—bounding surface can be separately eval-
uated. ' Polarized light is projected into the model through
a slit so that it illuminates a plane section. The light
is continuously analyzed along its path by scattering and
the state of stress in the illuminated section is obtained.'
By means of a series of such sections, the entire stress'
field may be explored. The method was used to analyze_the
stress system. of a simple beam in bending. The results
were found to be in good agreement with those expected from
elementary theory.

A new method for the photoelastic analysis of three~
dimensional (spatial) stress systems was reported in de-
tail at the Eastern Photoelastic Conference at Cornell
University in May 1939. A brief description of the method
is outlined in reference 1. Following the announcement of
the method, the H. A. G. A. constructed a simple polari_scope
suitable for preliminary work and began -the study of the
application of the method to aircraft structures, power
plants, and other related problems. J5“

Several investigators have attempted to develop a
truly three~dimensional method of photoelastic analysis.
With the exception of the fixation method reported in refé
erence 2, these attempts have lacked general usefulness.
In the present paper, the basic principles underlying the
proposed three—dimensional method are discussed and a con—
crete example is worked out to show its application.

The nhotoelastic method (references 3, 4, and 5) is
based on the fact that many materials, notably certain
plastics, become doubly refractive (birefringent) when sub—
jected to stress. The amOunt of this birefringence may be
studied with a polariscope and the characteristics of the
stress system obtained. A model of a machine part or of a
structural part may therefore be made from such a material
and subjected‘hr1nrbfipropriate loading system; and—the
state of stress within it can then be determined. If the
materials from which the model and its prototEEE'aréffiade
possess-similar (proportional) elastic properties, the
stress systems will.coincide in direction and—be prayer—
tional_in magnitude provided that the deflections are such
that the geometry remains substantially undisturbed.