Mechanical Properties of Glass

Abstract

This chapter focuses on the mechanical properties and behavior of glasses below their glass transition temperature. In this temperature range, they are usually seen as perfectly brittle materials: materials that deform only elastically, until they break. This chapter explores the behavior of glasses from the domain from elasticity to fracture. We first review the most widely used methods for measuring the elastic moduli of glasses, and the state of the art regarding knowledge of the relationship between elastic moduli and short- to medium-range order in glasses. We then discuss nonlinear elasticity in glasses, and how temperature and pressure impact on elastic moduli. But glasses can also deform plastically under high levels of compressive stress, particularly under sharp contact (indentation), because of high pressure and shear stress. This plastic deformation is highly dependent on glass composition; it results from densification and shear flow, two mechanisms that are affected by the loading path, temperature and strain rate. We examine how plasticity occurs under sharp contact (e. g., indentation, scratch), until damage appears (cracks). Finally, we examine the practical strength of glasses, which is highly dependent on resistance to surface damage as well as to crack propagation (fracture toughness). The important role of moisture is also discussed, as it is responsible for subcritical crack propagation, and thus lower durability of glass parts.