High-accuracy digital speckle correlation method for rock with dynamic fractures

The digital image correlation and the calculation of correlation coefficient are the basic principle and the key algorithm of the digital speckle correlation method (DSCM). The image block correlation near the fracture zone will change much before and after the occurrence of cracks in the rock specimen. So the common image block construction method using the measured pixel point as the centre of image block will cause many errors. In order to solve the problem, a simple and efficient method named OP & FB (One pixel point and five pixels block) is put forward to eliminate the fracture impact, whose function has been implemented by coding in the self-developed software package PhotoInfor . The application results in axial compression for rock with two holes show that (1) the OP & FB method can solve the deformation measurement problem and increase the measurement accuracy for the rock with dynamic fractures induced by compression; (2) the OP & FB method can be used as a general high-accuracy method in DSCM for materials with or without dynamic fractures; (3) the relation between the fracture evolution and the axial load of rock specimen during axial compression is obtained, and local big shear strain occurs near the peak load while the critical damage of rock mass structure arises approaching the residual stress stage; and (4) the DSCM with OP & FB method can obtain the relation of the deformation on the wall of hole within the rock specimen and the crack information of surrounding rock mass. So it will provide powerful tools for establishing a forecasting technique for internal rock mass crack by measuring the displacement of the wall of the hole, and it has obvious meaning in the monitoring of tunnel engineering construction.