The Use of Digital Image Correlation (DIC) to Investigate the Effect of Layering and Bedding Plane Orientations on Young's Modulus and Poisson's Ratio Measurements of Buckinghorse Shale Samples

Abstract

ABSTRACT Anisotropy due to heterogeneities in rocks, such as shale, affects the mechanical properties of the rock depending on their orientation and arrangement. For instance, layering and pre-existing cracks can create potential weak planes. Hence, a measurement technique that can capture spatially detailed deformation, such as digital image correlation (DIC), is important for the investigation of the effect of rock heterogeneities. This technique enables a larger field of view than conventional strain gauges measurements. In this study, the effect of bedding planes on the macroscopic failure patterns of shale samples from the Buckinghorse Formation was investigated using unconfined compressive strength (UCS) tests. The DIC analysis showed areas of displacement discontinuity and localized strain caused by bedding plane orientation, and that layering has a significant effect on the Young's modulus computation based on the location of sampling and the length of measurement. With load perpendicular to the bedding, the Young's modulus showed significant variance even when measuring across large portions of the sample. As the virtual extensometer length increased, localized effects were smoothed, and the values approached the strain and Young's moduli from the LVDT instrumentation. While the variance of the calculated Poisson's ratio measured along different loading orientations appeared consistent, the stress-strain plots using horizontal virtual extensometers (lateral strain) presented more details on lateral differences with respect to location. Hence, it was shown that local variability in stiffness could potentially affect the mechanical characterization of rocks with strong heterogeneities. DIC analysis, which captures this variability, should be used to complement conventional laboratory instrumentation. INTRODUCTION Rocks have various heterogeneities that may affect their mechanical properties. Shale exhibits a transverse isotropic elastic response due to layering from sedimentation. The Buckinghorse shale formation, which is a dark gray shale formation found as part of the Lower Cretaceous shales in northeastern British Columbia (BC) (Chalmers and Bustin, 2008) was selected to be used in this study.