48‐4: Analysis of Fracture Mechanism in Sand Paper Drop Test for Cover Glass

Abstract

Fracture mechanism of cover glasses on mobile devices in a drop event was investigated through high speed camera observation and conventional fracture surface analysis. Drop tests were performed for various chemically strengthened glasses with different mechanical properties and stress profiles. These were dropped on sandpapers with grit size of 180. Based on the observations, we confirmed that the fracture of cover glasses in the drop tests is caused by two consecutive steps. The first one is nucleation of a crack due to the contact with the protrusions of the sandpaper, which causes a strong tensile stress localized around the protrusions. The second one is the propagation of the crack due to a stress caused by bending of the smart phone during the rebound process after the first contact with the sandpaper. According to these investigations, we established a prediction formula for the “fracture drop height” for the sandpaper drop tests. It enables us to understand key parameters to improve the fracture drop height. For example, in addition to the stress induced by chemical strengthening, the mechanical properties of the cover glass and the body of the smartphone are key factors to improve the fracture drop height. We believe the knowledge in this study is important for improving the strength of the cover glasses in the future.