Abstract

Traditionally the measurement of strain from optical methods such as the in-plane moire technique is carried out by obtaining the displacement data from a fringe pattern and subsequent differentiation. In this paper, an electrooptic method is proposed for direct measurement of strain from moire fringes on test specimens by a moving-fringe procedure. A simple, real-time strain evaluation is achieved by directly processing intensity signals to obtain phase information from the moving fringes created by grating translation. The strain value at a point is directly proportional to the phase difference between the signals from two closely spaced photodetectors in the fringe field. The specimen carries a bonded grating over which a reference grating is translated at a constant velocity. The detector output variations due to fringe movement are sent through signal conditioning units to a programmable counter interfaced with a small computer. The measured phase difference is used for direct calculation and display of strain values. Preliminary results indicate that an accuracy better than 50 microstrains is easily achieved and is largely independent of the range of strain measurement. This optical strain-gauge technique using only noncoherent light offers the advantages of quick, realtime, remote, and selectable-point strain evaluation.

© 1985 Optical Society of America