Digital Speckle Correlation Method And Its Application On Damage And Fracture Of Concrete

Digital speckle correlation method (DSCM) is an optical measuring technique, which was developed with the developing of photoelectron, digital image processing, and computer in the 80's of last century. DSCM possesses many advantages, such as no interference fringe processing, simple optical arrangements, high accuracy, big measuring range, etc., so it is well applied in machinery, civil engineering, material science, medicine, and so on. And DSCM promotes the devolopping of the subjects. The research work of this paper is mainly about the establishment of DSCM experimental system and its application.Four sub-pixel searching algorithms were discussed, which were the interpolation and fitting of correlation coefficient, the gradient algorithm considering the statistical characterization of micro-region (gradient algorithm), Newton-Raphson iterative algorithm, and gray-gradient iterative algorithm. And Newton-Raphson iterative algorithm was improved in this paper. The capabilities of the four algorithms were researched using simulated speckle images. The result shows that, the Newton-Raphson iterative algorithm improved in this paper has higher calculation accuracy, and the gradient algorithm has more calculation speed. There were two ways improving Newton-Raphson iterative algorithm in this paper. First, an effective simplified method was proposed for deducing the partial derivatives of correlation coefficient; second, the gradient algorithm was introduced to the estimation of iterative initial value, which increased the speed of convergence.A software program for DSCM's calculation was developed, which had user-friendly interface and well function. The program can implement the computing of displacement field, the smoothing of displacement field data based on finite element method, and the computing of stain field. The deformation of a three-point bending plexiglass beam was measured with DSCM, and the result shows that the experimental system is reliable and accurate.Cast iron and concrete three-point bending beam with pre-crack were tested during their destruction, and CCD camera collected the speckle images of the beams' surface. The deformation fields and crack opening displacement of the cast iron beam were measured with DSCM, and the evolution process of displacement and strain fields before abruption of the concrete beam were also obtained with DSCM. This provides a new experimental way for study of brittle fracture mechanics and prevention of quasi-brittle material's destruction.