| The fracture toughness, the resistance of a material to crack propagation, is a property that is vital to the characterization and use of brittle materials. In recent years, the high hardness and low density of many glasses and ceramics has led to the replacement of metal components with ceramic or glass in many critical applications. The vulnerability of brittle materials to catastrophic fracture makes accurate characterization of the fracture toughness a necessity. Historically there has been a great deal of debate on the proper method of measuring the fracture toughness of brittle materials. The ASTM standard 1421C was only recently published in 1999 and offers three different specimen geometries for determining the fracture toughness of a brittle material. While standard methods for measuring the low rate fracture toughness of materials have only recently been developed, there remains very little consensus for the testing of materials at dynamic loading rates. Understanding dynamic fracture toughness is very important in the development of both transparent and traditional opaque ceramic armor materials, as well as in many civilian applications where a brittle material may undergo high loading rates. A Split Hopkinson Pressure Bar was used to expand the current ASTM standard into the dynamic regime by comparing two standard quasi-static specimen configurations: the surface crack in flexure, and the chevron notch. Using these methods, the rate effects of a soda-lime glass was studied over loading rates between 0.06--1.5 * 106N/s. Fracture toughness values were measured between 0.66--2.22MPa m as the increased loading rate caused the fracture toughness of the material to increase. |
