Chapter 11 Deformation Measurement within a Volume of Translucent Yield Stress Material Using Digital Image Correlation A. McGhee and P. Ifju Abstract This paper introduces a method of determining in-plane deformation of a translucent yield stress material (YSM) at any depth using digital image correlation. A 2D plane of uniquely shaped speckles are introduced to a volume of the YSM using a 3D printing technique. A cylindrical object, is dragged through the 2D plane at four different speeds each with four different diameters. The displacements caused by the cylinder were found and analyzed. Keywords 3D printing • Carbomer 940 • Digital image correlation • Soft matter • Yield stress material 11.1 Introduction A robust method of measuring the in-plane displacements within a translucent yield stress material (YSM) has been developed. Although the ability to measure deformation within a yield stress material is nothing new, the techniques employed previously have many limitations which restrict the type of analysis that can be done. Previous experiments have used glass beads suspended in a volume of the YSM which are illuminated with a laser sheet which allow for particle tracking of the glass beads. Since the glass beads are all shaped the same, the displacement of these beads can only be detected if the displacement between an image pair is very low [1]. Large displacements can be measured with this method, but requires many correlations between sequential image pairs to be correlated and summed together; where each correlation introduces some amount of error which would stack up throughout the summation. Our method, however, utilizes uniquely shaped speckles which allow for large scale displacements to be measured directly and with low error. The methods described here can be used with all types of yield stress materials as long as the speckle pattern can be imaged. The yield stress material used in this experiment is called Carbomer 940 which when introduced to water and a base such as NaOH creates a volume of individually swollen microgel particles with a cross section of about 10 m. These microgel particles jam together to give the gel material properties of a solid, yet the ability of these particles to shear due to some stress give the gel fluid like properties as well. The amount of stress needed for the particles to shear is called the yield stress, and this yield stress is highly dependent on the rate of the shear event. Rheological analysis show that the Herschel-Bulkley model provide the best fit for the shear rate versus the yield stress [2]. 11.2 Experimental Method Digital image correlation (DIC) usually measures the deformation of a surface, but we are interested in the in-plane displacement within a medium. To do this, uniquely shaped speckles made of tinted polyethylene glycol (PEG) are printed into a volume of YSM using a 3D printer with an injection needle attachment (Fig. 11.1). Above the speckle pattern a layer of white paint premixed with the YSM is printed and acts as a background. Cylinders of four different diameters, 0.41, 0.81, 1.27, and 1.82 are moved through the speckle plane at speeds of 0.01, 0.1, 1, and 10 mm/s with the cylinder tip far from the speckle plane. A. McGhee • P. Ifju ( ) Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, 32611, USA e-mail: ifju@ufl.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_11 77
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