Experimental Mechanics @ Michigan State University
	EM Basics: 3D Photoelasticity
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When light is transmitted through a three-dimensional model the effect of the stresses along the light path are integrated to produce the photoelastic pattern. To analyse stresses at a point the stress freezing technique is utilised. In this technique a three-dimensional model is made using epoxy resin. The model is loaded in an oven. The temperature is raised in the oven to the glass transition temperature of the epoxy resin which is usually about 135 - 140 ºC. The temperature is then reduced gradually at 1ºC per hour to room temperature. After this cycle the load is removed and the model sliced. These slices are viewed directly in a polariscope and analysed in the same way as a two-dimensional model. Epoxy resins have a biphasic chemical structure, i.e. two structures. At room temperature one structure, the less flexible, bears the load. Above the glass transition the second structure bears the load, due to the temporary collapse of the first structure. On cooling the first structure reforms around the loaded second structure. When the load is removed the second structure is held in its deformed state by the first structure. The component can be sliced without disturbing this 'stress frozen' structure. Loading the first structure generates very few photoelastic fringes. Whilst the second structure produces the majority of the fringe pattern.
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