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The fractional fringe order in a photoelastic experiment is readily determined by Tardy’s method of compensation, which requires the knowledge of orientation of the principal-stress directions at the point of question. The orientation is given by an isoclinic parameter in a plane polariscope and may be in error since isoclinics are not generally distinct and narrow.
It is shown, by considering the intensity expression for the transmitted light in a quasicircular polariscope, that there is no simple relationship between the angle of rotation of the analyzer, the relative retardation introduced by the stressed model, and the condition for the extremum of transmitted light when the polarizing axes of the polarizer and analyzer are not parallel to the principal-stress directions before rotating the analyzer. In fact, the relationship is complicated for an arbitrary orientation of the polarizing axes; the retardation condition for the extreme values of the transmitted light is independent of rotation of the analyzer when the polarizing axes are inclined at 45° to the direction of the principal stresses.
In practice, when the errors of aligning the polarizing axes with the principal-stress directions are small, such that the squares and higher powers may be omitted, Tardy’s compensation technique gives accurate results.
© 1967 Optical Society of America
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