Crack Opening Displacement Technique to Evaluate the Magnitude of Residual Stress in Glasses
Chad Watson and Bill Knowlton
Tempering and ion exchange are processing techniques used in glass production that strengthen glass by inducing a surface and near-surface compressive residual stress. Current methods to measure residual stress are destructive and indirect; therefore, it is paramount to develop a method that can accurately measure the residual stress of a glass component. In this study, crack opening displacement (COD), a fracture mechanics-based method, is applied to untreated, thermally tempered and ion exchanged glass. The untreated glass will be used as a baseline to determine the residual stress each processing technique produces. Additionally, a comparison will be made between ion exchanged and tempered glass to validate the COD method. Vickers indentations were used to induce microcracks (10-150 microns) at varying loads to better understand the relationship between COD, processing induced residual stress and environmentally induced stress. Indents created crack profiles which were measured using atomic force microscopy. A fracture mechanics model utilizing the crack radius and width will be used to extract a residual stress. The results should display trends that indicate a difference in residual stress with processing. Higher levels of residual stress are expected to be seen in glass that has been treated via thermal tempering or ion exchange. The outcome of the study will be used to define a methodology to measure residual stress, allowing ease of application in numerous glass components.
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