Evaluating Stress in Glass and Laminated Glass

Stress in glass may reveal information about the likelihood of failure. Failure can include non-compliance with intended specifications (causing concerns over predicted failure) as well as the more common actual fracture failure. In general, two modes of analysis are common: evaluation of direct photoelastic behaviour (transmitted and reflected), and indirect photoelastic behaviour by light scattering. Polarography uses direct photoelasticity to assess stress fields in a material. Photoelasticity is method which allows experimental stress analysis of two- and three-dimensional components to be performed by analyzing the photoelastic fringes, i.e., isochromatics and isoclinics. These photoelastic fringes are created by the changes in optical properties of a normally isotropic material to one aligned with stresses from an applied force (active or residual), and the resulting strains. Photoelastic analysis can performed in both transmitted light using a suitable sample (such as a glass pane) or in reflected light using a reflective coating behind a transparent specimen. Either way, these methods allow a full-field view of stress patterns. For indirect photoelastic behaviour, the intensity of a scattered light source (laser beam) depends on the birefringence caused by the stresses. The polarization state of the laser beam is optically modulated using a compensator, and the scattered light intensity pattern along the laser path length can be recorded by camera. From the intensity pattern, the optical retardation distribution can be calculated using well-known methods. The stress can be calculated from the retardation. Examples of glass windows (heat strengthened, annealed, and ballistic resistant) as well as a few other glass objects will be used to illustrate the insight gained.