Deformation of Display for Handheld Devices during Drop Impact

Chemically strengthened glass is widely used as a cover glass in consumer electronic devices, such as smartphones, tablets, and notebook computers. A cover glass protects the Liquid Crystal Display (LCD) in these devices from field abuse, such as scratches and impacts in device drop. Therefore, it is important to investigate the protective glass response under dynamic impact loading. The pendulum ball drop test has been applied in the past to understand glass panel's dynamic behavior upon impact with a steel ball. Excellent correlation in the deformation of the glass panels between measurement and modeling has been obtained in the past. However, compared to deformation, strain is more oscillatory in nature and differs in frequency of response due to wave propagation and contact complexities. Hence, measurement of strain needs extended care, and needs to be further investigated. In this study, a steel ball is made to freely fall and drop on to the surface of the glass. The glass panel is simply supported over cylindrical steel supports. This arrangement minimizes the wave propagation complexities and uncertainties in the measurements. Care is taken in the fixture design to ensure that glass receives most of the impact energy, and no or minimal impact energy is lost in the fixture due to its compliance. Two synchronized high-speed cameras monitor the impact of the ball from the bottom of the glass. Images captured at high frame rates are analyzed using 3D Digital Image Correlation (DIC) technique, and the full field dynamic response of the test subject is captured. Efforts were made to spray speckle pattern on the sample as fine as possible so that small facet size and step could be maintained, hence measurement accuracy is increased, but at the same time, data obtained is more prone to noise, and therefore, proper filtering technique was applied to take care of this problem. Finally, a Finite Element Model (FEM) is developed that uses ABAQUS to provide a comprehensive understanding of the dynamic response of the glass. Excellent correlation in deformation and strain are obtained between the measurements and predictions for different energy drops. With the validated finite element model deformation and strain measurements can be implemented on product level drop.