Photoelastic Measurement of High Stress Profiles in Ion‐Exchanged Glass

Abstract

Photoelastic fringes were directly measured to fully characterize high magnitude, steep compressive stress gradients in an ion‐exchanged glass, trade named Ion‐Armor™. Initially, using a thick (9.9 mm) 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 (0.71 mm) specimen was then used along with a polarizing light microscope to enhance the fringe contrast. This arrangement yielded approximately four isochromatic fringes, representing a maximum surface compressive stress of 984 MPa, which rapidly decreased to ~300 MPa within 25 μm depth from the strengthened surface. Also, the case‐depth of the ion‐exchange process was found to be 0.8 mm. Thus, the technique was able to directly capture the extremely high residual compressive stresses generated in an ion‐exchanged glass. The current technique utilized 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.