Ultimate Stresses in Design Codes

[SW13]

This may be a novice question, but are ultimate stresses (tensile, compressive, etc...) that are listed for materials in design codes (such as the Aluminum Design Manual, Timber Design Manual, etc...) engineering stresses? I have always assumed that they are engineering stress because that is generally how we calculate stresses on members to compare to the design values, as opposed to using true or "actual" stresses. Thank you for any help.

 

[DaveAtkins]

Generally, the actual stress on a member (under SERVICE loads) must be some fraction of the ultimate stress. And be careful what you are referring to--ultimate tensile stress is not the same as yield stress.

DaveAtkins

 

[JAE]

I'm not sure what you mean by "engineering stress". Can you elaborate? Never heard that term before.





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[hokie66]

There is 'working stress' and 'ultimate strength', and never the twain shall meet.

 

[wannabeSE]

The only time I have heard "engineering" stress was in my strength of materials class that discussed engineering stress versus true stress. For instance when tensile testing steel specimens, the true stress is affected by factors such as the reduction of area and the stress-strain curve is also affected by strain hardening.

 

[SW13]

Sorry, I guess I didn't explain my question very well. I was referring to how design codes treat listed Ultimate Stresses--Ftu, for instance, say for a sample of 6063-T6 aluminum, which is 35 ksi. Is this 35 ksi an engineering stress value (as in, force divided by original cross-sectional area) or TRUE stress (as in, force divided by instantaneous area).

As a sample is tested, in regards to a stress-strain curve (think of a simple tensile test specimen), usually all we are interested in is the engineering stress--even though the material deforms throughout its load cycle until failure. We do this by taking the force divided by the original cross-sectional area, and likewise, the strain is a measure of some change in length divided by original length. However, true stress is a measure of force over actual area of the necking part and true strain is the rate of increase in length divided by the instantaneous length. I have attached an example curve (I found this on academic.uprm.edu)

I think I have answered my own question, but had a moment of doubt concerning whether or not design codes use TRUE STRESS values for ultimate strengths or ENGINEERING stress values. I believe they use engineering stress values.

I am sorry for the confusion and thanks for the interest.

 

[SW13]

I should also state why I believe they list engineering stress. I think it is because when we calculate stresses due to loads (what we compare to Ftu, etc...) we are using section properties of the original cross-section. It think it becomes a reference issue--we could also calculate stresses on a part using neck-down values to obtain a true stress, but would have to compare those to true stress ultimate values. Again, I think I have answered my own question. Sometimes if you just think on things a little, I find there is no need to ask a question! I jumped the gun here and asked my question without thinking it through. Thanks--

 

[BAretired]

I agree that you have answered your own question. The stress used is P/A where P is the load and A is the original area before necking or, as you call it, engineering stress.

BA

 

[leojones]

I also think what it assumed that they are engineering stress.But how the true stress is affected by the reduction of area?

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[BAretired]

The true stress is P/A_r where A_r is the reduced area.

BA

 

[msquared48]

There are three stress terms:

1. The Actual stress the member sees under a specified load,
2. The Allowable design stress for the material, and
3. The Ultimate yield stress for the material.

And JAE... the OP was probably referring to the stress engineers feel in their jobs at times.

Mike McCann, PE, SE (WA)

 

[BUGGAR]

It ain't the job stress, it's the required necking down under the stress that I don't like.

I think the OP meant why don't we use the reduction in area for calculating ultimate stress. That would introduce second statistical variation, one from the statistical variation of the force required to break the object and one from the statistical variation of the area at breakage. For practical purposes we can ignore the reduction of area and base everything on the original area. One less statistical variation.

In seismic design of refueling wharves in California (governed by MOTEMS), concrete piles which are the primary lateral force resisting system are now designed using computer programs based on actual force-elongation to failure diagrams provided with mill certs for reinforcing and prestressing steel so we do effectively allow for reduction in area in our ultimate strength design.