Role of Residual Stresses on Mechanical Properties of Obsidian: Naturally Occurring Glass

Abstract

Obsidian is a natural glass that is formed by the rapid cooling of viscous lava. Obsidian's mechanical properties in structure mechanics are visible in aspects such as fracture pattern, elastic modulus, hardness and fracture toughness when compared to soda lime glass. Obsidian forms sharper edges than any best surgical steel can do. The reason for the sharpness is speculated because of its structure and the way it fractures. Being an igneous rock, obsidian illustrates conchoidal fracture. As this natural glass cools very rapidly, the residual stresses in obsidian affects the way in which it fractures. This research is focused on studying the effect of residual stresses in obsidian and understanding the consequence of thermal annealing on the mechanical properties of obsidian. Results from nanoindentation show that obsidian has a higher hardness compared to soda-lime glass. Microstructures of obsidian and soda lime glass are also compared. The presence of alumina in obsidian, allows obsidian to have less non-bridging oxygen than soda-lime glass. Thermal annealing of obsidian shows an increase in apparent elastic modulus and hardness around Tg measured from nanoindentation experiments. This apparent increase is speculated because of the presence of residual stresses in obsidian.