For decades, the “crossed polarizer” system, sometimes known as a “strain viewer” or a “plane polariscope,” has been used to extract residual stresses from the inside of plastic and glass parts. However, its use is highly subjective and the results depend strongly upon interpretation. For these reasons, crossed polarizers are not commonly employed as a primary quality control tool. As the following example highlights, a variation of the crossed polarizer stress measurement system developed by Polaritek Systems, Inc. is able to rapidly provide objective and quantitative measurements of residual stress.
As an example, consider a plastic (PET) injection-molded preform that is intended to be converted into a plastic bottle and will be inspected between crossed polarizers. The residual stresses present at the preform stage will affect the performance of the resulting bottle. A typical fringe pattern using crossed polarizers is shown in Figure 1(a). The magnitude of the retardation (which is proportional to stress) for a given color fringe, called an isochromatic, may be estimated by consulting a color chart. Black fringes represent regions of zero retardation, and thus zero stress. However, crossed polarizers also reveal another type of black fringes, called an isoclinic, which indicate regions where the direction of the stress is parallel to the orientation of the optic axis of the polarizer. For most practical purposes, the direction of stress is not meaningful to a manufacturer. However, since both sets of black fringes are simultaneously revealed, it is difficult or impossible to distinguish regions of zero stress from the isoclinic fringes so the isoclinic fringes serve to mask the actual stresses. Rotating the part between the crossed polarizers alters the location of the isoclinic fringes – but for each orientation there is a corresponding set of black isoclinic fringes which falsely appear to be zero stress and mask the true stress. The end result is that the results from stress analysis obtained with crossed polarizers is often misleading and inconclusive.
A stress measurement system from Polaritek Systems is fundamentally better in that it is able to generate a quantitative map of stress in a matter of seconds. Polaritek utilizes additional optical elements to generate a circular polariscope which reveals exclusively isochromatics without any influence of isoclinics. Compare the images shown in Figure 1(b) – especially the center of the preform exhibiting stresses hidden behind an isoclinic fringe in Figure 1(a). The digitized image is fed into powerful image processing algorithms which generate a true stress map of the sample, shown in Figure 1(c). This stress map removes much of the ambiguity and subjectivity which has traditionally surrounded stress analysis with crossed polarizers. These stress maps are a reliable and quantitative representation of residual stress which now present a viable pathway towards integrating residual stresses into quality control and process control procedures.
Another example is the use of the Polaritek system on the base of a plastic (PET) bottle. Figure 2 shows images of the bottle base that has been sectioned and viewed on-axis. The gate is visible at the right edge of each image. Figure 2(a) shows an example of local crystallinity which is clearly visible in a Polaritek circular polariscope. Figure 2(b) shows a local region of high stress. Images of each sample between crossed polarizers are given at three different orientations. It can be readily observed how, at each angle, the isoclinic fringes partially or fully obscure the parts of the stress pattern – and most importantly the defect in question. Defects which are significant stress raisers typically cause the isoclinic fringes to radiate outwards from the defect. Consequently, the most severe defects are often hidden behind the isoclinic fringes at every orientation.
 When analyzing a part with complex geometry and multiple surfaces, some assumptions must be made in order to calculate a magnitude of stress. This is true for both a crossed-polarizer system and a Polaritek stress measurement system. In this example Polaritek assumes that both surfaces of the preform through which light is transmitted possess equal stress and that, for the region of interest near the center of the preform, the radius of the curvature of the preform is gross enough to be ignored.