Photoelastic fringes were directly measured to fully characterise high magnitude, steep compressive stress gradients in an ion-exchanged glass, trade named Iron-Armor. Initially, using a thick (9.9mm) rectangular bar and circular polariscope arrangement the overall residual stress profile in a bulk specimen was determined. However, due to the relatively large thickness of the specimen, the high density of fringes (steep stress gradient) close to the edge of the specimen became too diffused to allow an accurate count of fringe order. A thinner (0.71mm) specimen was then used along with a polarising light microscope to enhance the fringe contrast. This arrangement yielded approximately four isochromatic fringes, representing a maximum surface compressive stress of 984MPa, which rapidly decreased to ~300MPa within 25um depth from the strengthened surface. Also, the case-depth of the ion-exchange process was found to be 0.8mm. Thus, the technique was able to directly capture the extremely high residual compressive stresses generated in an ion-exchanged glass. The current technique utilised for residual stress measurement is more objective and straightforward to implement than what is specified in ASTM standard C-1422, particularly for those specimens having steep stress gradients just below the strengthened surface.
