Abstract: Based on the visual triaxial compression servo-control test system and the three-dimension digital image correlation technology (3D-DIC), the generalized stress relaxation tests under different rheological regions were carried out, and the evolution of strain in the rock surface was discussed. The experimental results show that if sandstone tends to fail during the rheological process, the axial and radial strain concentration area would gradually appear in strain fields. The axial strain concentration area concentrates from the layered discrete distribution to the position of the upcoming crack, while the radial strain concentration area is about to form with the centered crack. The differential evolution rates of axial and radial strains at different areas are positively correlated with the overall strain evolution rate. The development of axial and radial strain in cement near the crack is going through three stages: deceleration, constant velocity and acceleration. The strain in areas away from the crack may increase firstly and then decrease. And the isochronous curve of the strains at different positions indicates the area where the crack is about to form. As the rheological direction coefficient ? changes from 0.3, 0, ?3.0, ∞, and 3.0 in sequence, the mean and variance of the overall evolutions of the axial and radial strain fields increase during the rheological process, indicating that the evolution rate is accelerated, the strain concentration phenomenon is more significant, and the difference between the evolution of the strain field before and after the sandstone failure also increases.

Key words: generalized stress relaxation, three-dimension digital image correlation, surface strain field, strain evolution