Principal strain resolution by photoelastic means

Claim 1

A PHOTOELASTIC STRAIN INDICATOR SYSTEM FOR DETERMINATION OF THE MAGNITUDE OF A GIVEN PRINCIPAL STRAIN ACTING AT THE SURFACE OF A LOADED WORKPIECE, WHICH SYSTEM CONSISTS ESSENTIALLY OF: A TESTPIECE OF FORCED-BIREFRINGENT MATERIAL ADHESIVELY BONDED TO THE WORKPIECE SURFACE, A REFLECTOR INTERPOSED BETWEEN THE TESTPIECE AND THE WORKPIECE, AND AN INDICATOR; SAID INDICATOR COMPRISING A SUPPORTING STRUCTURE AND, MOUNTED UPON SAID STRUCTURE, A SOURCE OF POLARIZED LIGHT, AN ANALYZER FOR SAID POLARIZED LIGHT, AND LIGHT DIRECTING ELEMENTS; SAID LIGHT DIRECTING ELEMENTS DIRECTING LIGHT ALONG A PATH FROM SAID SOURCE THROUGH SAID TESTPIECE TO SAID REFLECTOR AND, UPON REFLECTION BY SAID REFLECTOR, THROUGH SAID TESTPIECE TO SAID ANALYZER, THE PORTIONS OF SAID PATH WITHIN SAID TEST PIECE BEING NORMAL TO THE GIVEN PRINCIPAL STRAIN AND AT AN ANGLE 0 WITH RESPECT TO THE OTHER PRINCIPAL STRAIN DEFINED BY 0=COS-1U1/2, U BEING POISSON''S RATIO FOR THE MATERIAL OF THE TESTPIECE; WHEREBY OBSERVABLE FORCED-BIREFRINGENCE IS DIRECTLY RELATED TO THE MAGNITUDE OF THE GIVEN PRINCIPAL WORKPIECE STRAIN.