7.4 Design for
Strength
Manufacturers and users currently assess
the mechanical loading capacity of ceramic products in four
ways:
- by experience (intuition),
- through the analytical calculation of stress distribution
or
- the numerical calculation of stress distribution, or
- again through the calculation of stress distribution
and, based on this, the local distribution of failure probability
obtained through statistical methods.
The effort required for the strength-oriented design of
a part increases from 1 to 4. The degree of utilisation of
the material properties also increases however, through which
some engineering solutions may become possible for the first
time.
The design of ceramic parts for long-term use or subject to
fatigue stress can be carried out using the same process.
However, since the failure of these materials is generally
the result of crack initiation and crack growth phenomena,
the application of fracture mechanics in combination with
numerical calculation methods and statistical failure models
is recommended.
For many less critical applications, intuitive knowledge and
approximate calculation are used in practice for the design
of the part as well as the choice of material. The conditions
of use are also taken into account, and the part designed
accordingly. Intuitive knowledge develops out of years of
experience of both the design of ceramic parts and of their
behaviour in operation. Many basic rules for the design of
ceramic materials (for example, compressive stress is OK,
tensile stresses are critical, notches are critical, etc.)
are a result of this experience. However, a mathematical description
is not given here.
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