The Competing Effect of Initial Crack Depth Versus Chemical Strengthening Parameters on Apparent Fracture Toughness of Sodium Aluminosilicate Glass

Abstract

The widespread use of sodium aluminosilicate glass in many engineering applications due to its mechanical and optical properties (transparency, dielectric, etc.), has become common in recent years. However, glass, a brittle material, has its vulnerability to fracture. Processes such as heat treatment (heat tempering) or chemical strengthening through ion-exchange have been used to create residual stress profiles on the glass, in a bid to improve its fracture strength. However, failure still occurs, which is mostly catastrophic and expensive to repair. Therefore understanding, predicting, and eventually improving the resistance to damage or fracture of chemically strengthened glass is important to designing new glasses that would be tougher while retaining their transparency. The relationship between the glass residual stress parameters such as the compressive stress (CS), depth of compression layer (DOL), and central tension (CT) versus apparent (effective) fracture toughness for different crack depth was investigated in this study using a Silicon Carbide particle blast plus ring-on-ring (RoR) test method. The results also showed that improving the fracture resistance of glass via chemical strengthening requires a proper combination of CS, DOL, and CT, which is particularly dependent on the initial/existing crack (flaw) depth. It was determined that for a damage event involving the introduction of a shallow crack depth, the criterion for optimal resistance to fracture, in terms of apparent fracture toughness, is weighted more towards a high CS, than deep DOL while for a deep flaw damage event, it is more weighted towards deep DOL, than a high CS. These results provide a valuable piece of information in the design of a more robust glass in engineering applications.