Digital Moiré Method And Its Applications For Large Deformation Measurement

Digital moirémethods are effective experimental techniques in analyzing mechanical problems of soft materials which can endure large deformation. Introducing some novel mathematical theories and advanced image-processing techniques, this paper puts emphasis on the following aspects: study of digital moirémethods which adapt to large deformation measurement; systematically analysis of the mechanical parameters distributions near the crack tip of rubber-like material.A circular digital moirémethod which is feasible in measuring circular and radial displacements is proposed in this paper. Using coordinate transformation and its inverse operation, deformed gratings and experimental results can be easily analyzed both in Cartesian coordinate system and polar coordinate system by image-processing.The distributions of displacement near the crack tip of rubber-like materials are measured with the proposed digital moirémethod. The sector division model and the distribution rules are systematically investigated. The experimental results indicate that there are both expanding sector and shrinking sector near the crack tip. The boundary lines of sectors and strain distributions are also given. And the effect factors to sector division model near the crack tip are analyzed in detail. The strain singularity near the crack tip is also studied, and the result indicates that the singularity order of strain in the expanding sector is different from that of the shrinking sector.An easy method to prepare specimen gratings for large deformation measurement is presented. The specimen gratings are printed on the specimen and can eliminate the layer effect, which make it possible for gratings to endure large deformation with specimen. Combined the gratings preparing method with the inner-gratings moirétechnique, an inner grating digital moirémethod is proposed in this paper. Using wavelet transform in simulating moiréfringes, the digital fringes are reconstructed by the wavelet decomposition of logical moiré. Further more, phase-shifting technique and multiplied fringes technique are combined with the digital moirémethod to improve the sensitivity and accuracy of the measurement for large deformation. The validity of digital moirémethods presented in this paper is verified by typical experiments.