Utilizing Stress Transmissions in Bonded Granular Materials to Determine Grain Contact Stiffness in Sandstone

An experimental investigation of stress transmission through bonded particulate structures with visible stress pattern and stress-strain data from rock matrix akin to the problem of rock fracturing remains a challenge and this is addressed in the present work. The stress transmission through sandstone under mechanical loading is analyzed experimentally by applying a thin coating of birefringent material. The retardation of the light components that reflect from the surface of the birefringent on sandstone samples was measured. By using a reflection type optical tomography, the maximum shear stress under the external loading indicates stressed point on the surface of the sample which was related to grain displacement. The strain induced birefringence occurs due to the anisotropy within the grains in the sandstone which result isochromatic fringes. Hence the shear stress map on the surface of the sandstone sample was visualized. This information is used further to evaluate modulus at micro scale and the yield strength. The stress measurement made from photo-stress experimental techniques was compared with the ultrasound sensors, strain gauges and bulk strength devices. Analogous to this experiment, simulations using Discrete Element Modeling (DEM) were performed using the measured grain-scale parameters as inputs. The boundary and initial conditions for the experimental conditions were used to simulate force distribution for sandstone under compression. The stiffness ratio associated with the grain-to-grain cementation in experiment agrees excellently with the simulations. This makes it possible to visualize and understand how micro-scale behavior contributes to the bulk strength characteristics of cementations in materials even when they are opaque.