DIRECT DETERMINATION OF 2ND-ORDER DERIVATIVES IN PLATE-BENDING USING MULTIPLE-EXPOSURE SHEAROGRAPHY

A technique, called multiple-exposure shearography, is presented in th is paper for direct determination of second-order derivatives of displ acements in a plate. The optical set-up is similar to that of conventi onal shearography. Four exposures of the illuminated test specimen on the same photographic film are made. The first exposure is made when t he specimen is in its unloaded state, and the second after an incremen tal load is applied. The third exposure is made when the specimen is s hifted slightly whilst it is still under load. Finally, the fourth exp osure is made after the specimen is unloaded. The fringe pattern, reco nstructed in the same way as in conventional shearography, is related to the second-order derivative of displacements. Hence, the curvatures and twist of the deflected surface are computed directly from the fri nge orders.