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Name: Kirk N.
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: 2002


Question:
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?



Replies:
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 compressive strength.

Andy Johnson



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