A laser-speckle strain gauge

A new non-contact and automatic method of measuring surface strain has been developed. It makes use of the speckle displacement that is detected by cross correlating the signals from a photodiode array. According to a theory the difference between the speckle displacements for a pair of symmetrically incident laser beams is proportional to the surface strain parallel to the plane of incidence and independent of the translational and rotational components. This relation is verified by experiments using a metal specimen subject to uniaxial strain that is measured by a resistance gauge. The gauge length of the present method is given by the laser spot size and the sensitivity is of the order of the array pitch divided by sensor distance. They are 1 mm and 20 microstrain, respectively, in the experiments.