82 A. McGhee et al. ß z(F(t)) F(t) thick glass polydimethylsiloxane (PDMS) light source black tape absorbs light aimed at camera Instron universal tes ng machine Fig. 12.1 A cartoon of the experimental setup. The steel sphere is connected to a universal testing machine which applies a load at a constant rate. The rays of light are shown escaping the material at angles greater than ˇ and otherwise reflecting internally until the light hits the steel ball and reflects downwards towards the camera 60 mm 60 mm contact boundary 0.0 2.5 surface deflection (mm) Fig. 12.2 A surface deflection field from the DIC analysis is presented with the contact boundary from the FTIR analysis shown as a white ring Using a Nikon D7200 camera mounted directly underneath the specimen, the contact area between the PDMS surface and the steel ball was imaged. The aforementioned experimental setup can be seen in Fig. 12.1. This experimental procedure was done on three different PDMS samples which has a modulus range of 240 kPa to 2.1 MPa. 12.3 Results The results of the analysis shows the outline of the contact boundary overlaid on top of the surface deformation as seen in Fig. 12.2. This deformation field shows the surface deflection in the z direction, with positive values corresponding to indentation and negative values to budging. The raw data of the displacement was larger than the values presented due to the fact that the images were taken through a glass and PDMS surface. To correct for this magnification, a plot of the maximum deflection from the DIC analysis was plotted against the displacement measurements from the universal testing machine and the slope of the given line was found to be 1.35; by dividing the DIC displacement by this slope the surface deflection measurement was corrected.
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