Second-order phenomena in strain measurement by speckle techniques

The contrast degradation due to tilt between exposures limits the amount of tilt, the number of fringes and the spatial resolution. The effect was derived quantitatively and verified experimentally. Mutual influences between system parameters cause ambiguity in the measurement results. The main effects are imaginary displacement resulting from combined deflection and initial slope, and imaginary strain resulting from deflection. These effects were demonstrated experimentally. Formulas were developed that enable the designer of the experiment to choose the desirable parameters of the set-up. They point out the difficulties in using the method in ‘field measurements’, primarily because of the finite dimensions of the lens and the photographic plate.