Correction of strain errors induced by small rigid-body motions in electronic speckle pattern interferometry measurement

Abstract

Electronic speckle pattern interferometry (ESPI) can provide non-contact, high-precision, and full-field displacement measurement. This technique is particularly useful for determining deformations on objects with high strain gradients and complex shapes. However, small rigid-body movements of the object being measured can spoil the accuracy of the ESPI measurement and cause dramatic strain errors. Based on the theory of geometric optics, formulae for evaluating the theoretical strain errors caused by different modes of rigid-body motion were explicitly derived. The proposed formulae were validated by comparing the theoretical strain errors generated from a set of known rigid-body motions with the strains obtained by ESPI. The experimental results agreed well with the theoretical results. The results further revealed that the out-of-plane rigid-body motion had the strongest influence on the strain errors among the rigid-body movements considered. An out-of-plane movement of 0.01 mm could generate a strain error of approximately 40 μ ϵ. Finally, a correction procedure to eliminate the strain errors has been proposed. The effectiveness of the procedure was demonstrated with a four-point bending test on a concrete beam. This study on the quantitative evaluation and correction of strain errors due to various rigid-body movements is very useful for the practical applications of the ESPI technique in deformation measurement.