ABSTRACT
Fracture strength is one of the most commonly cited properties for structural ceramics. The importance of fracture strength as a characterization descriptor can be traced to several factors. First, fracture strength and fracture strength distribution are often used as critical indicators of the success of materials development programs. Specific examples are provided by recent research activities related to the processing of advanced ceramics for automotive gas turbine applications [1-3]. Figure 4.1 illustrates recent improvements in the mechanical behavior of a hot isostatically pressed (HIPed) silicon nitride as reflected by the strength distribution. These data are not only useful from the standpoint of processing, but may serve as the basis for the selection of several candidate materials for a single target application. Second, the design of ceramic components for advanced structural applications which are based upon probabilistic failure methodology [4,5] requires the characterization of the fracture strength distribution and its dependence on specimen volume. Finally, the measurement of the time, load-rate, or stress dependencies of fracture strength is often used in fundamental studies of slow crack growth. For example, the resistance to slow crack growth may be evaluated by mea suring the fracture strength as a function of loading rate (dynamic fatigue) [6],
A number of techniques and methodologies have been developed for the measurement of fracture strength. Most of these techniques equate the fracture strength to the maximum stress (tensile or compressive) at fracture. Consequently, in order for a particular load and specimen geometry to be useful for the determination of fracture strength, the stress distribution must be well established. A complicating factor in the determination of fracture strength is that the strength of ceramic materials is quite sensitive to size, shape, and surface finish. This sensitivity is largely responsible for the wide variation in strength values often reported for a given material. To be viable, the test methodology must therefore account for these effects.
