At any point in a loaded component there is a stress acting in every direction. The directions in which the stresses have maximum and minimum value for the point are known as principal directions. The corresponding stresses are known as maximum and minimum principal stresses. When polarised light enters a loaded transparent component it is split into two beams. Both beams travel along the same path, but each vibrates along a principal direction and travels at a speed proportional to the associated principal stress. Consequently the light emerges as two beams vibrating out of phase with one another which when combined produce an interference pattern. The polarised light is produced by the polariser in the polariscope and the analyser performs the combination. The interference pattern is observed in the polariscope, and the fringes are contours of principal stress difference which are known as isochromatics. When plane polarised light is used black fringes known as isoclinics are superimposed on the isochromatic pattern. Isoclinics indicate points at which the principal directions are aligned to the polarising axes of the polariser and analyser.
When analysing components which are essentially two-dimensional it is possible to make a model from a flat transparent plate. The model is then loaded at room temperature in a polariscope. When the model is unloaded the photoelastic fringe pattern will disappear. The major advantages of this procedure are its simplicity and the ability to modify the model to produce an optimised design with the lowest possible maximum stress.
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.
It is possible to apply a reflection coating to a real engineering component and to bond to this coating a thin sheet of photoelastic material. When the component is loaded the surface strain in the component is transmitted into the photoelastic sheet generating stress in it. The resulting fringe patterns can be observed by illuminating the component with polarised light and viewing it through a polariser. This technique allows the stresses in a component to be fund directly. It avoids the need to scale stresses from models to real components which is necessary in two- and three- dimensional photoelasticity. The major disadvantage of the technique is that only surface stress data can be obtained.