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.
Tension
Use of ceramics at elevated temperatures in many applications has necessitated the
development of methods for determining the uniaxial tensile strength of specimens
uniformly stressed at both ambient and elevated temperatures. This is because in a
flexure test, failure often initiates at the surface unless volume-distributed flaws are
larger than the surface flaws. Direct tension strength specimens subject many more
volume flaws to the maximum stress, and consequently, tensile strengths are usually
lower than flexural strengths. If the design application is for a component with a large
volume, then the tensile specimen data are clearly preferred since there will be less
scaling of strength for size and many more volume flaws are sampled in the tension