Chapter 12 Surface Deformation with Simultaneous Contact Area Measurement for Soft Transparent Media due to Spherical Contact A. McGhee, D. Nguyen, and P. Ifju Abstract We present a method to measure surface deformations between a steel sphere and a flat PDMS surface. A sphere was chosen as the specimen to ensure the resulting deformation measurement can be compared to known theoretical models. A 36 mm diameter steel sphere was pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with a constant load rate controlled by an Instron testing machine. The modulus of the PDMS samples range from 241 kPa to 2.1 MPa. A digital image correlation technique was used to measure the surface deformation of the PDMS with increasing applied load. Keywords Digital image correlation • Frustrated total internal reflectance • Polydimethylsiloxane • Spherical contact • Soft matter 12.1 Introduction A method of investigating contact mechanics between two bodies using digital image correlation (DIC) through transparent media was developed in combination with frustrated total internal reflectance (FTIR). This combination of data enable deformation measurements with the respective contact measurement between the two surfaces. If the object causing the deformation is attached to a universal testing machine (UTM), the displacement and load measurements can be used to produce interesting results such as stress mapped to the contact plot. The challenges with this method include correction for the index of refraction through the transparent medium as well matching the contact measurement from FTIR to the surface deformation from DIC. This method was created at the University of Florida to measure the contact with corresponding deformation between a hard rough surface and a soft polydimethylsiloxane (PDMS) sheet. To improve and expand upon this method we use a simple spherical surface as the indenter so we can directly compare the results to known deformation models. Furthermore, since the steel ball sticks to the PDMS surface, we measure the pull off deformation and compare it to theoretical models. 12.2 Experimental Method Using an Instron-5969 to control for load rate, a 36 mm diameter steel ball was depressed into the PDMS and then pulled off. The surface deformation and contact area were measured during both the loading phase and pull off phase of the experiment. To measure the surface deformation, a speckle pattern was applied to the contact surface of the PDMS and imaged with stereoscopic cameras. The images were then analyzed using correlated solutions 3D DIC software. The contact area was measured using FTIR. In the FTIR method, a light source illuminates the surface of contact between two objects. Light will reflect internally in a medium until the angle made between the light ray and the surface is greater than some critical angle ˇ determined by the relative index of refraction between the boundary. If a body with a higher index of refraction is spaced within a distance equal to the wavelength of light, the light will become frustrated and will become visible on the surface. A. McGhee • P. Ifju ( ) Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, 32611, USA e-mail: ifju@ufl.edu D. Nguyen Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL, 32611, USA © 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_12 81
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