The Rock Strength Device : A scratching apparatus to determine rock properties F. Dagrain 1 , T. Richard 2 , C. Germay 3 1 Faculté Polytechnique de Mons – Department of Mining Engineering, Rue du Joncquois, 53 – 7000 Mons – Belgium 2 CSIRO Petroleum, 26 Dick Perry Avenue, Technology Park – Kensington WA 6151 – Australia 3 EPSLOG SA – Rue M. Moutschen, 30 – 4020 LIEGE – Belgium Fabrice.Dagrain@fpms.ac.be Abstract: The present paper presents a portable apparatus, the Rock Strength Device (RSD), and the methodology to assess mechanical properties of cohesive granular material from "partially-destructive" scratching tests with rectangular cutters (the material surface is just scratched at shallow depth). A correlation between the intrinsic specific energy (measured in the scratch test) and the uniaxial compressive strength has been built on more than 200 rock samples, and a correlation between the friction angle at the wear flat/rock interface and the internal friction angle of the material is given for 20 different rocks. The test provides also a “continuous” log of the material properties along the groove with a wide range of practical applications. Keywords: rock cutting, scratching test, rock properties, specific energy, compressive strength, friction angle. I. INTRODUCTION Previous papers (Almenara and Detournay, 1992; Detournay et al., 1995; Adachi et al., 1996; Richard et al., 1998) have presented strong experimental evidence that rock strength properties can be assessed from “scratch” tests. Experiments indicate that rock cutting is associated with a “ductile” or a “brittle” mode of failure depending on the depth of cut (Richard et al., 1998). The ductile mode takes place at shallow depth of cut and is associated with plastic flow, while the brittle mode occurs above a threshold depth of cut and is characterized by the propagation of tensile crack. In the ductile mode, the forces mobilized on the cutting face are found to be proportional to the cross-sectional area of the cut. The coefficient of proportionality, called the intrinsic specific energy ε, appears well correlated to the uniaxial compressive strength, q, of the material. The forces mobilized on the wear flat of a blunt cutter are dependant of a friction law characterized by the friction angle ϕ and the contact stress σ. The friction angle underneath the cutter appears well correlated with internal friction angle φ of the material being traced. A portable device was developed at the University of Minnesota in the late 1990’s in order to conduct quick and precise scratch test and measure accurately the magnitude and inclination of the force acting on the cutter. Over the years, the apparatus was improved, the experimental procedure was refined and optimized and tests were conducted on numerous materials: sedimentary rocks but also some construction materials (Van Parys and Dagrain, 2006). The paper describes the experimental setup and the methodology. We present results of scratching, uniaxial and triaxial compression tests carried out in different laboratories, and establish a correlation between ε and q, and a correlation between ϕ and φ. Finally, we present several practical applications associated to the logging features of the scratch test. II. EXPERIMENTAL SETUP AND PROCEDURE The “scratching” test consists in tracing a groove at constant shallow depth, (d < 1 mm), on the surface of a sample with a rectangular cutter of width w. The normal and tangential components of force F applied to a cutter are measured while cutting. The cutter is inclined by an angle θ = 15° and is displaced at constant horizontal velocity v. The cutters used are sharp or blunt. In the case of blunt cutters, the wear flat is characterized by its length and its area A f = w . Figure 1: Scratching test setup.