The Equipment Design Process
333
other hand, with an operating temperature in the ambient zone, the
selection process becomes more complicated because of the greater
number of potential materials that will meet the requirements.
2. Stress. Once the operating range of usage has been established,
stress and ductility are the next most prominent mechanical properties that must be considered. Here, the first decision that must be
made is whether we are dealing with yield or ultimate strengths and
whether these strengths should be those determined in tension, torsion, or compression.
3. Ductility. The ductility is usually taken into consideration at the same
time that the stress requirements are being evaluated because, in general, the higher the strength, the lower the ductility of the material. If
both properties are important, some compromises must be made.
4. Toughness. The fracture toughness must be considered in addition to
the ductility, particularly if shock loading is believed to be an operating condition. If it is not known whether shock, or impact loading,
could occur, then err on the conservative side.
5. Modulus of elasticity. The modulus is an important design parameter,
and where weight is a consideration, the specific modulus is a more
useful number. It must be realized that the inherent modulus of a
material cannot be altered without changing its composition. For
metals, the modulus of a soft metal is identical to that of the same
strain-hardened metal. The moduli of polymers are very small and
not well defined. Perhaps the composites are the only materials that
can be designed to a desired modulus. Ceramics have the highest
moduli of all groups because of their strong covalent-ionic bonds.
6. Physical properties. The coefficient of thermal expansion and the thermal and electrical conductivities are the most prominent physical
properties that one must usually know. It should be kept in mind
that these properties do vary with temperature, and one must select
values that were determined over the temperature range of interest.
Density and specific heats are physical properties that are of lesser
importance, although the density must be considered in terms of the
specific strength and specific modulus.
7. Corrosion resistance. This is one of the most difficult and at the same
time one of the most important material characteristics with which
to deal. The corrosion resistances of the most often-used materials
of construction to many media and environments have been compiled and discussed in various publications. One such compilation
is Corrosion Resistance Tables of metals, nonmetals, coatings, linings,
plastics, elastomers, and fabrics by Philip A. Schweitzer (published
by Marcel Dekker). It contains data on 874 corrodents. Another book
333
other hand, with an operating temperature in the ambient zone, the
selection process becomes more complicated because of the greater
number of potential materials that will meet the requirements.
2. Stress. Once the operating range of usage has been established,
stress and ductility are the next most prominent mechanical properties that must be considered. Here, the first decision that must be
made is whether we are dealing with yield or ultimate strengths and
whether these strengths should be those determined in tension, torsion, or compression.
3. Ductility. The ductility is usually taken into consideration at the same
time that the stress requirements are being evaluated because, in general, the higher the strength, the lower the ductility of the material. If
both properties are important, some compromises must be made.
4. Toughness. The fracture toughness must be considered in addition to
the ductility, particularly if shock loading is believed to be an operating condition. If it is not known whether shock, or impact loading,
could occur, then err on the conservative side.
5. Modulus of elasticity. The modulus is an important design parameter,
and where weight is a consideration, the specific modulus is a more
useful number. It must be realized that the inherent modulus of a
material cannot be altered without changing its composition. For
metals, the modulus of a soft metal is identical to that of the same
strain-hardened metal. The moduli of polymers are very small and
not well defined. Perhaps the composites are the only materials that
can be designed to a desired modulus. Ceramics have the highest
moduli of all groups because of their strong covalent-ionic bonds.
6. Physical properties. The coefficient of thermal expansion and the thermal and electrical conductivities are the most prominent physical
properties that one must usually know. It should be kept in mind
that these properties do vary with temperature, and one must select
values that were determined over the temperature range of interest.
Density and specific heats are physical properties that are of lesser
importance, although the density must be considered in terms of the
specific strength and specific modulus.
7. Corrosion resistance. This is one of the most difficult and at the same
time one of the most important material characteristics with which
to deal. The corrosion resistances of the most often-used materials
of construction to many media and environments have been compiled and discussed in various publications. One such compilation
is Corrosion Resistance Tables of metals, nonmetals, coatings, linings,
plastics, elastomers, and fabrics by Philip A. Schweitzer (published
by Marcel Dekker). It contains data on 874 corrodents. Another book