Enhanced analysis of dynamic indentation data for shale mechanical properties mapping

Surface measurements are extensively used in many industries and research facilities to help shed light on a material's mechanical properties without the need for traditional, time consuming, and expensive laboratory tests. In addition to being typically fast and nondestructive, surface measurements offer an advantage over traditional testing in that they can be used to produce large data sets at spatial resolutions that allow for the near continuous characterization of variability and heterogeneity over a wide range of scales. However, a common limitation of surface measurements is that they are often limited to providing correlation-based indexes and lack a full understanding of the physical interaction between the probe and the measured material. This work reports on the analysis of data sets obtained with a dynamic indentation system designed to probe locally homogeneous aggregates at a spatial resolution of the order of a few millimeters. The dynamic indenter system's output is based on time-resolved force data recorded during the free fall of a precisely machined mass onto the material of interest. While results from contact mechanics allow for extracting an elastic stiffness in a first pass analysis, the complex response often observed in geomaterials suggests that more information can be derived from the data. To that effect, we acquired a large number of force vs. time data sets on shale as well as a on a number of standard brittle and ductile materials (e.g. plastic, aluminum, fused quartz) using two indenter tips, namely spherical and cube corner. Based on a kinetic energy transfer analysis of the free falling mass, we show that the force vs. time data can be converted into force vs. displacement at a suitable precision to allow it to be analyzed within the general framework of static indentation methods. Our approach is validated by comparing the data obtained from the reference materials with photographs and white light interferometry imaging of the divots left by the cubic indenter. Once transformed using the proposed approach, the method offers a means to quantify the amount of energy dissipated through inelastic processes without the need for imaging of the divots, thus allowing for routine automated use of dynamic indentation for geomechanical profiling of shale core. We also suggest the possibly to identify associated failure modes by carefully comparing the results obtained with differently shaped tips. KEY WORDS: rock surface measurement, dynamic indentation, elastic stiffness, fine-scale