| Description |
Recently the use of ceramic and glass materials has taken a more important role in technological applications which were previously limited to metals. Ceramics and glasses have been of interest because of their superiority in resistance to corrosion and their stability at high temperatures. In many instances, the use of these materials has led to problems since their behavior under stress is difficult to predict due to inherent brittleness. Kies and Clark [1] have done work on glass properties as related to their use in underwater diving spheres for the Navy. They, as well as other researchers in the field, found that many important factors need to be considered, including time to failure, the speed of subcritical crack growth and the effects of strain rates. The properties and characteristics of a material, to a large extent, depend on subcritical crack growth. This is where the growth rate for a crack is relatively slow and does not cause immediate or sudden catastrophic failure. This can, however, lead to an early or unpredicted failure. One of the best ways to characterize the important properties of the materials is through the generation of the K-V diagram (stress intensity vs. crack velocity). The crack velocity, V, is related to the stress intensity factor, K1, by the relationship: V=AK^NI. From this diagram many of the important factors that depend on time can be determined. Unfortunately, it is difficult to characterize these properties in a laboratory setting due to the uncertainty in the nature of the intended use. Many of the presently used methods preclude testing of materials at high temperatures or in hostile environments. |
