比田井 洋史

Abstract Wheel scribing on glass generates a vertical crack with a periodic stripe pattern beneath the wheel (hereafter referred to as the first crack). After the passage of the scribing wheel, sometimes seconds later, the first crack is repropagated with a smooth surface (the second crack). The second crack propagates to 90% or more of the glass thickness under suitable scribing conditions, facilitating the breaking process. The mechanism of secondary crack propagation has not been sufficiently explained in previous studies. Therefore, this study used analytical and experimental methods to examine stress distribution and crack propagation behavior during wheel scribing. Finite element analysis suggests that the increase in the stress intensity factor contributing to the propagation of the second crack was due to not only the crack opening force but also the bottom deformation of the glass specimen. An analytical model accounting for the bottom deformation can simulate the characteristic behavior, such as rapid deepening when the scribing load exceeds a specific threshold value, of the second crack. This study indicates that the elastoplastic deformation caused by wheel contact induces the deformation of the entire specimen, and the state of the bottom constraints is important for controlling the second crack.

The effects of the ambient temperature and relative humidity on crack propagation behavior during wheel scribing were investigated. A chamber was built to allow dynamic observation of crack propagation behavior in a controlled atmosphere. A developed miniature scriber was installed in the chamber, and the crack propagation behavior was observed from lateral and back sides during wheel scribing under various atmospheric conditions. As a result, the median crack propagation rate increased with relative humidity. We speculated that this was caused by the stress corrosion of glass. Although stress corrosion is considered to be more reactive at higher temperatures, the results of scribing at different temperatures showed that higher temperatures did not necessarily increase median crack propagation. This is due to the formation of lateral cracks before the median cracks have fully propagated. These results suggest that the interaction between multiple cracks should be considered when discussing the effects of temperature and humidity in wheel scribing.