Optical full-field strain measurement method from wrapped sampling Moiré phase to minimize the influence of defects and its applications

For strain measurement in the case of complex situations such as when defects (including cracks, notches and stains) exist, the traditional phase unwrapping algorithm will bring great error in strain distributions around defects. In this study, a simple local phase unwrapping algorithm was proposed to minimize the influence of defects on full-field strain measurement. From the specimen grid images before and after deformation, the Moiré phases are first acquired by the sampling Moiré technique. The wrapped Moiré phase difference is then calculated to determine the strain distributions by only unwrapping the phase difference at the boundaries of the wrapped phase. In other words, the partial differentials of the Moiré phase difference are corrected by local phase compensation for strain calculation. The accuracy of the developed strain measurement method was verified from numerical simulations. As applications, this method was successfully used in microscale strain distribution measurements of an aluminum specimen with a prefabricated crack and several grid defects under tensile loading, and a titanium alloy specimen with a prefabricated notch and an emerged irregular crack under tensile-fatigue loading. The local phase unwrapping algorithm can also be integrated with geometric phase analysis for strain measurement.