Material property tests of Smooth-on Vytaflex60 liquid rubber

During the fall semester of the 2005-2006 academic year, I decided to try to produce some scale models of structural systems out of rubber. I had in mind to use these models to demonstrate structural behavior to undergraduate students, thinking that seeing models deform under load would be far more convincing than the standard line diagrams of deformed shapes that we typically use in engineering classes. Peter Chomowicz, Professor of Environmental Design at the Maryland Institute College of Art, agreed to help me build the models, provided I could design them to demonstrate the proper structural response. One of the main difficulties in designing the models was a lack of information on the material properties of the liquid rubber we would use to form the models. Smooth-on, the supplier of the rubber, called Vytaflex60 in their product line, was able to provide quantities such as the density, 0.0376 lb/cu.in., the rupture strain 480%, and the shore A hardness, or durometer, 60. They could not provide the elastic modulus, the critical material property necessary for design of the structural models. Though I did not look that hard, I could not find any literature pointing to a direct relationship between shore A hardness and elastic modulus. The only test result I was able to find, at www.moldeddimensions.com/stressstrain.htm, indicated that a urethane rubber with a shoreA hardness of 65 had a modulus, for elongation up to about 200% of 200 psi. Faced with a lack of material property data and facing some difficulties in designing the models to demonstrate very specific types of behavior, Peter and I decided to forge ahead and build a small arch bridge, spanning 20 inches, out of vytaflex60. I would then perform material tests on the rubber, and use this first model to evaluate the accuracy of the calculation methods I was using in my design. Figure 1 shows this model, along with a scale factor. The model was designed using an elastic modulus of 200 psi and a weight density of 0.0376 lb/cu.in. To make the model, Peter built a beautiful mold, and demolded the bridge after a day of curing. From excess material, he also cut two material samples that I would test to determine the elastic properties of Vytaflex60. On March 17th, Ben Schafer, also of Johns Hopkins Civil Engineering, and I performed two tension tests on the material sample shown in Figure 2 in