Fracture mechanisms in soft rock: Identification and quantification of evolving displacement discontinuities by extended digital image correlation

Deformation of rocks at shallow depths (i.e., at low confining pressures) is primarily driven by the evolution of new fractures or the propagation and coalescence of pre-existing ones. In this paper, fracture evolution from inclined flaws (cuts) in a soft rock deformed under plane-strain uniaxial compression is analyzed using high-resolution digital photographs and Digital Image Correlation (DIC). DIC provides a full-field measurement of the kinematics and thus allows the analysis of non-homogeneous deformation. However, standard DIC approaches do not handle correctly fractures, as they are displacement discontinuities. Therefore, an extended DIC method is developed that allows automatic tracing of discontinuities and their quantification in terms of the displacement jumps along their length. The analysis of spatial and temporal fracture evolution using this method enhances the understanding of the phenomena in that it provides experimental quantification of fracture mode (opening/closing and shearing).