Name: Kirk N.
I have a question about steel properties. Steel is
generally defined by tensile strength. Is the compressive capacity the
same as the tensile capacity because steel is a homogeneous substance?
Steel is more or less a linear elastic material. Unlike concrete, which is
much weaker in tension than in compression, steel theoretically responds the
same way in either tension or compression. However, with enough applied
force, steel and other metals will cease to behave elastically and begin to
behave plastically. When a material is linearly elastic, its deformation,
or strain, will be directly proportional to the applied force and it will
return to its original shape when the force is removed. A plastic material,
on the other hand, will permanently deform without breaking (think of taffy
or perhaps the stringiness of melted mozzarella cheese on a pizza).
In real life, of course, there is no such thing as a perfectly elastic or
plastic material. In the case of steel, structural engineers are concerned
about the tensile strength in terms of both the ultimate strength and the
yield strength. When a specimen reaches its yield strength, it will begin
to stretch and transition from elastic to plastic behavior. As more force
is applied, the steel will reach its ultimate tensile strength and break.
Structural engineers take advantage of this property in their designs. In
an extreme event, such as an earthquake or major structural failure, this
plastic phase is useful because it allows the structure to sag and absorb
extra loads. If steel is too brittle, it will shatter instead of
stretching, possibly causing the structure to come crashing down. In either
case, the structure is ruined, but the plastic behavior of the steel allows
time for escape.
In compression, steel can still behave plastically. Think about how a coin
looks when it is run over by a train. This property allows metals such as
steel to be pressed into things like car bodies. However, in some
compressive situations, shear forces can also develop inside the specimen.
While steel behaves equally under tension and compression, depending on the
type of steel, it can be substantially weaker under shearing loads. In this
case, the steel may suffer a shear failure before reaching an ultimate
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Update: June 2012