The figure shows (i) the theoretically
estimated maximum values of strength for each strengthening method
applied and (ii) the strength of steel and non-ferrous metals
that are used in actual structures.
Strength is the most important factor for a structural material,
because higher strength makes it possible to reduce the weight
of the structure. However, the strength of a material is not
the sole factor that determines which material should be used.
Other factors, such as corrosion resistance and formability,
may determine the choice of the material. In addition, the final
characteristics may differ from those of the original material
after the processing and assembling.
Some examples are described below:
(i) Bend forming, press forming and weld joining are used with
steel materials for structures, machinery, and automobiles. It
is important that these processes should not introduce defects
such as cracks. In general, the formability and weldability of
steel materials tend to decrease with increasing strength.
(ii) The strength values obtained by the uniaxial tensile test
are not the only criteria used to evaluate the breakage of steel
materials. Under some conditions, materials will fracture under
stresses which are considerably lower than the static strength
value. Well-known examples include the fatigue failure under
repeated loading, creep rupture under constant stress at high
temperatures, brittle fracture at low temperatures, and delayed
failure due to hydrogen embrittlement.
(iii) During manufacture, the characteristics of parts subjected
to cold processing and welding undergo a change from those of
the original material and, in general, tend to deteriorate.
(iv) Although the design thickness of a plate can be reduced
by using high strength materials, the stiffness of the resulting
structure may sometimes become insufficient. In this case, stiffeners
need to be used and the total weight of the structure is not
significantly reduced.
In choosing the material to be used for a structure, it is necessary
to take the above-mentioned factors into consideration. |
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