Figure 1 tabulates theoretical weathered glass breakage stress, with red text highlighting values for typical flaw size and Critical Stress Intensity Factor ranges.Ĭritical glass strength values are established based on material testing and statistical models to standardize safe working limits. Newly manufactured glass is typically stronger than “weathered” glass, which has additional flaws due to handling and exposure to moisture and/or cycled loading conditions. Moisture and tensile load cycling can accelerate flaw growth. Surface flaws concentrate stresses and, combined with a low Critical Stress Intensity Factor, reduce glass strength. However, unlike ductile steel, glass fails suddenly with little elastic deformation and no significant plastic deformation it breaks in a sudden, brittle manner.įigure 1. With its relatively high Critical Stress Intensity Factor range, ductile steel alloys will undergo significant plastic deformation, stretching under tensile loading before breaking. Steel alloys have a value between 50 and 200 MPa-m ½ (between 45.5 and 182.2 Ksi-in ½), up to 400-times that of glass. The Mode 1 (i.e., crack opening – Figure 1) Critical Stress Intensity Factor for glass is between 0.5 and 1.0 Megapascals – square-root meter (MPa-m ½) or between 455 and 910 pounds-force per square inch square-root inch (psi-in ½). Fracture Toughness is measured by the Critical Stress Intensity Factor, K IC. Basic Glass Fracture Mechanicsįracture Toughness is a material’s resistance to the opening of a sharp surface imperfection (e.g., a crack or fissure) where propagation occurs suddenly, uncontrollably, and quickly. The severity and distribution of these microscopic “flaws” control the tensile strength of glass in combination with its characteristic Fracture Toughness. This is because, despite its apparent transparency, smoothness, and clarity, freshly manufactured glass is proliferated with surface imperfections unobservable to the naked eye. Glass tensile strength, however, is typically in the range of 10% its compressive strength. Glass ideally can develop up to 17 Gigapascals (GPa) or 2,466 kilopounds-force per square-inch (ksi) compressive strength. Despite the difference in molecular geometry, the theoretical compressive strength of glass is in the same high range as metals and ceramics and metal materials. Whereas glass and ceramics share multiple performance properties, ceramics can have solid molecular geometry (from ionic bonding). In crystalline solids, ionic bonds transfer electrons between ions, typically breaking between 600 to 4000 kJ/mole. Glass covalent bond breakage energy is in the high range of 435 kJ/mole. Covalent bonds share electrons between atoms, typically breaking between 150 and 400 kilojoules per mole (kJ/mole). Unlike solid material crystalline lattice structures with ionic bonds, such as ceramics, glass is an amorphous solid (i.e., not a supercooled fluid) relying on covalent bonds. An annealing process slowly cools the material to release internal stresses, resulting in the most common manufactured form of architectural flat glass. Heated to 1500 degrees Celsius, the mixture forms into a continuous ribbon “floated” on a long bath of liquid tin. Soda-lime silica flat glass sheets consist of silica sand, sodium carbonate, lime, metal oxides, and recycled glass. The objective is to provide information to engineers investigating glass breakage and tips for specifiers to avoid glass breakage problems. ![]() ![]() This article discusses basic fracture technology of flat glass in architectural and structural glass assemblies, illustrating characteristic fracture surfaces and crack patterns. The crack patterns in broken glass and their fracture surface details describe the origination and energy intensity that caused breakage.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |