Fy and Fu from a tensile test.

[StrEng007]

Can anyone remind me why the Fy derived from tensile tests (ie, not a compression test) is used in the design of compression members for mild/high strength steels? Does this go back to the material being isotropic and sharing a yield value regardless of tension or compression stresses? Recall that the design of hot rolled steel compression members is governed by Fy and E.


Thanks

 

[jayrod12]

This isn't a rule or anything, but to me intuitively the results are more predictable. If you tested in compression you could have premature failures from buckling. Items in pure tension never buckle.

 

[StrEng007]

What if your specimen is arranged such that the cross section and length don't lend themselves to buckling? I think that's how most compression tests are done.

 

[ChadV]

I would think that making the specimen such that it won't buckle would require either more complicated conditions at the ends, or a specimen with a much larger cross-sectional area... either way leads to a more complicated testing machine, and if you need to yield a larger area the machine needs to supply more force.

I'd say it's basically what you said in your first post - since we know tensile and compressive yield are the same, we can test either one, so we do whatever test is easiest.

 

[BAretired]

When a bar is tested in tension, it necks down to a smaller area. The ultimate strength is always given in terms of its initial cross sectional area. If the same bar is tested in compression, due to Poisson's Ratio, the area actually increases, giving a higher apparent value for Fu. This means that the tension test is actually conservative for a compression failure where buckling is prevented.

BA

 

[RPMG]

Why tensile stress: Steel is isotropic. Tensile Fy = Compressive Fy. Poisson's Ratio is too small to take effect. Tension is easier to test, and conservative as pointed out.

Compressive Stress: Fy is not used for elastic buckling. Fy is used for inelastic buckling. I believe this is the cause of your question. It's because compression members should never behave as plastic members. Plastic hinges in columns would make them unstable.

The necking behavior which baretired pointed out is interesting. Those stress-strain curves that we studies which fall downward as necking occurs are based on the original cross-section area. If you factor in the reduced cross-section area, the curve continues upward. This is beyond the values used for design though.

 

[MacGruber22]

Engineering strain as BAretired points out.

"It is imperative Cunth doesn't get his hands on those codes."

 

[gtaw]

I was under the impression that hot rolled (wrought) materials were anisotropic. That is, the mechanical properties parallel to the direction of roll were superior to those in the transverse or through directions.

Best regards - Al