Chapter 13 Towards Measuring Intergranular Force Transmission Using Confocal Microscopy and Digital Volume Correlation Kimberley Mac Donald and Guruswami Ravichandran Abstract We aim to show the feasibility of using confocal microscopy imaging techniques for Digital Volume Correlation (DVC) and analysis of granular mechanics experiments. The first part of this study validates the DVC and confocal microscopy imaging methods for a general problem of uniaxial compression of continuous media. The second part investigates the specific problem of granular mechanics. Intergranular force transmission will be analyzed from 3D image stacks captured using confocal microscopy. DVC methods will be used to analyze the full 3D grain motions and deformations. The intergranular forces will be determined inversely using the Granular Element Method (GEM). Preliminarily results show that confocal microscopy is a useful volumetric imaging method for DVC analyses and shows promise in furthering the study of intergranular force chains and shear bands. Keywords Confocal microscopy • Digital Volume Correlation (DVC) • Granular mechanics • Intergranular forces • Shear bands 13.1 Introduction Intergranular force transmission is of critical importance in analyzing and predicting shear band formation in granular packings. Numerous 2D studies have demonstrated the relationship between shear band slip and the breaking of intergranular force chains. These studies used a variety of analysis methods including photoelasticity and Digital Image Correlation (DIC) [1, 2]. There have also been some studies of 3D granular packings using neutron diffraction and X-ray diffraction methods, which involve considerable effort [3, 4]. These studies demonstrate that further investigation of three-dimensional effects may improve our understanding of the mechanics of granular materials. A potential alternative to neutron and X-ray diffraction methods that allows for capture of volumetric 3D images is confocal microscopy. Commonly used in biological sciences, confocal microscopy uses a series of pinholes to capture images of unique points in 3D space. This imaging method has been used in mechanical studies of cell-matrix interactions [5]. Analysis of 3D studies is often achieved through Digital Volume Correlation (DVC), which is a natural extension of 2D DIC to volumetric space and has been used effectively in both continuum and granular studies [6]. When combined with force inference methods such as the Granular Element Method (GEM), it becomes a particularly powerful tool for analysis of mechanical experiments [7]. 13.2 Materials and Methods 13.2.1 Specimens A preliminary study of uniaxial compression was carried out on a hydrogel polymer cylinder. Polyacrylamide specimens were prepared using AMRESCO® ACRYL/BIS™ 19:1 solution with ammonium persulfate (APS, AMRESCO®) and tetramethylethylenediamine (TEMED, Alfa Aesar) as sources of free radicals and as a catalyst respectively. Prior to polymerization, specimens are embedded with 2 m carboxylate modified FluoSpheres® (580 nm excitation, 605 nm peak K. Mac Donald ( ) • G. Ravichandran Division of Engineering and Applied Science, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA, 91125, USA e-mail: kmacdonald@caltech.edu © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_13 85
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