Nano and Neutron Science Applications for Geomechanics

Recent advances in experimental characterization techniques offer unique opportunities to evaluate mechanical properties of geomaterials. In this paper, authors introduce two such techniques that have significant potential to impact geomechanics community. The technique of using Instrumented indentation testing using nano-indenter to evaluate hardness and modulus of individual sand particles is introduced. Use of the measured data from nanoindenter on individual sand grain in modelling its assembly using numerical methods such as the discrete element method (for example PFC-2D) is addressed. Modulus for two silica sands with varying particle shape is presented for the depth of indentation in the range of 100 to 1000 nanometers. Use of neutron science for solving relevant problems to geotechnical engineering is also described in this paper. Neutrons are subatomic particles with no electric charge having duality and interacts with atomic nuclei, while X-ray interacts with electrons. Neutrons thus have significantly higher penetration power for most geo-materials when compared to X-rays and are sensitive to hydrogen. Example initial use of neutrons for solving a new class of problems including non-destructive evaluation of strain at particle level while the assemblage is subjected to target stress conditions, and imaging applications including flow through partially saturated porous medium are presented. 1 Nanoindentation for mechanical characterization of individual sand particle High stresses may occur in granular materials such as pile end bearing, high earth or rock fill dams, or foundations of offshore gravity structures. These high stresses can lead to particle breakage and thus, for the consideration of wider range of geotechnical situations, it is of relevance to quantify and analyze accurately the mechanical properties such as hardness and elastic modulus at the particle level. During the last decade, there was a revolutionary development in the field of depth-sensing nanoindentation technique, by which mechanical properties of small volumes of homogeneous material can be determined accurately. It has the potential to play a significant role in elucidating the mechanisms associated with many particulate mechanics issues of geotechnical engineering community. In this technique, an indenter probe of appropriate geometry is placed in contact with the sample surface and then pushed into it. The resistance to indentation and the indent depth are continuously monitored throughout the experiment, and by analyzing the load-displacement curve, both hardness and elastic modulus of the material can be measured directly and precisely. Nanoindentation technique is now an accepted and proven technique for mechanical characterization of materials in a wide variety of disciplines (Dutta et al. 2004; Penumadu et al. 2003; Tsui and Pharr 1999). In this research, mechanical properties of individual sand particles were obtained using the continuous stiffness measurement (CSM) option of nanoindentation technique. 1.1 Depth Sensing Indentation The depth sensing indentation instrument principally consists of three basic components consisting of an indenter of target geometry, a force actuator, and a displacement sensor as shown in Figure 1. The indenter is mounted on a rigid column through which the axial force is transmitted. For present testing, a Berkovich indenter (a threesided pyramid) (Hay and Pharr 2000) was used. The force is imposed on the indenter by passing current through a coil that sits within a circular magnet. The indenter displacement is measured by the voltage difference of the capacitive plate arrangement of the displacement sensing system as shown in Figure 1. The plate-and-indenter assembly is supported by two leaf springs that are designed to have very low stiffness in the vertical direction and very high stiffness in horizontal directions. The theoretical force and displacement measurement resolution of this system is 50 nN and <0.01 nm respectively. The material hardness (H) and the elastic modulus (E) are two important mechanical properties