Fracture Strength
Fracture strength is one
of the most commonly cited properties for structural ceramics
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.
The primary motivation for testing materials in
uniform, uniaxial stress fields is the need to control the stress-state
variable to characterize the mechanical behavior of the material at given
stress levels. Common methods of controlling the stress states include the
application of uniaxial and uniform compressive or tensile stresses to
uniformly shaped volumes of material. Use of uniaxial stress tests (tension or
compression) has been limited, especially in regard to brittle, structural
ceramics, because of the need for elaborate specimen preparation, the need
for specialized testing equipment (including specimen grips), and the
difficulty of achieving the necessary uniform stress state. Therefore, the
flexure bar has traditionally been the popular testing arrangement for
ceramics, because of the ease of fabrication of the specimen geometry, the
efficient use of material, the simplicity of gripping and loading, and the
seemingly straightforward analysis.