Measuring displacement derivatives by electronic speckle pattern shearing interferometry (ESPSI)

Electronic speckle pattern shearing interferometry (ESPSI), also known as shearography, is a whole-field optical technique used to measure approximately the fields of displacement derivatives. The accurate measurements of these derivatives have two problems: first, although ESPSI results are approximately equal to the derivatives, they are equal to the derivatives only if the shear distance tends to zero, hence, if experimental data rendered by ESPSI are taken directly as equal to the derivatives, the measurements may carry an important shearing error; second, ESPSI yields values relative to a reference value at a specific location of the field that can be very difficult to determine accurately. In this paper, we propose a general procedure to compensate the shearing error and to introduce the reference by adding two quantities to the values rendered by ESPSI. As an example, we measured a displacement derivative field induced on a metallic sheet specimen by applying tensile load.