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 lm 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. *subhash@ufl.edu

Measurement of stress build‐up of ion exchange strengthened lithium aluminosilicate glass

“…The validity of the measured stress profiles was tested by the criteria of stress equilibrium (difference in the areas of compressive and tensile zones) and by comparison with Na + ion concentration profiles. Our results are compared to analogous studies by Varshneya et al, Jannotti et al and Inaba et al…”

“…Figure compares DOL from our measurement (SCALP‐04 and SCALP‐05), Varshneya's DOL values and Na + diffusion depths. Varshneya's DOL values for samples strengthened for 4 and 8 hours were, respectively, 49% and 34% deeper than the SCALP‐04 DOL values.…”

“…Over the past decade, a great deal of research on chemically strengthened lithium aluminosilicate (LAS) glass has been published by Varshneya and Jannotti . Earlier stress studies of the same type of glass were conducted by Donald and Hill .…”

Gradient scattered light method for non-destructive stress profile determination in chemically strengthened glass

“…Sane and Cooper [16] showed that the stress profile in a sliced sample will show a tension maximum, which was not present in the unsliced plate. Jannotti et al [17] measured the stress profile in a chemically strengthened lithium aluminosilicate glass, but the sample had to be specifically prepared for this purpose. The two opposite surfaces of the glass sample that were perpendicular to the light propagation direction were ground and polished off to remove the layers that could influence the measurement result.…”

“…The proposed method of stress profile measurement is demonstrated for a chemically strengthened lithium aluminosilicate (LAS) glass, which is known for its ultra-high surface compression stress (*1000 MPa) and large case depth (up to 1000 lm) [2,17].…”