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Need Help with Fundamentals
7

Need Help with Fundamentals

Need Help with Fundamentals

(OP)
When we carryout design calculation of any element, should we consider yield strength or ultimate strength as its basis for calculation ?
Kindly also do guide me if the question itself is wrong.

RE: Need Help with Fundamentals

Are you a student?

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

RE: Need Help with Fundamentals

Too broad a question.

You use both depending on what you're trying to find.

Sometimes it is different safety factors on those two numbers and you choose the lowest.

Can you allow the item to yield or not?
Is the item only going to see high load once?

This process of thinking is called design.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Need Help with Fundamentals

2
You'll answer your own question once you define what you consider to be a failure of a particular component you're designing.

If you've got a structural member of some sort, then most often failure is when the member starts to deform from it's original shape (plastically deforms). That happens when stress reaches the yield strength.

If you're designing a part that is supposed to break at a certain load (for example, to protect other more expensive elements of the assembly from overload), then you'd be interested in what it's ultimate strength is.

If you're designing a an element for a precision CNC machine, then you're more interested in deflections rather than yielding, i.e. you define failure as a point when elastic deformation reaches a certain value, which will happen much before the material reaches its yield strength.

If you're designing a column, it might buckle, so failure will be the function of the geometry of your column and elasticity of the material.

RE: Need Help with Fundamentals

What has not been mentioned above is the use of design codes and standards...

Competent engineers design devices in accordance with established rules

ASME Boiler, Piping and pressure vessel codes have "Allowable Stress Levels" that have been developed by wise elders using both yield strength and ultimate strength levels at various temperatures

Loading cases and safety factors are already built into the analysis equations...

MJCronin
Sr. Process Engineer

RE: Need Help with Fundamentals

Competent engineers make the rules.

Ted

RE: Need Help with Fundamentals

2
Ask about a specific element and you may get more useful responses.
There are design codes that cover structural steel, pressure vessels, piping, etc., and in those cases, you use the criteria from the code, which may be based on yield, ultimate, buckling of various types, deflection, etc. Quite often, the stress and the method of calculating that stress are specified.
Designs may be limited by fatigue properties or corrosion concerns.
In absence of any other criteria, there are various theories of failure to predict when a part will fail in certain ways.

RE: Need Help with Fundamentals

The approach taken for shaft sizing is that fatigue is related to fluctuating stresses, typically reversed bending for rotating shafts, while torque is steadily applied. Thus, ASME B106.1 (withdrawn, but still relevant to the material handling industry), uses both ultimate tensile strength (for calculating fatigue endurance limit) and yield strength (relates to the transmitted, non-fluctuating torque). So if your component will fail by fatigue, ultimate strength may be important, and if yielding is a criteria, then yield strength is important.

RE: Need Help with Fundamentals

And it depends on the characteristics of the material. Plastic, wood, glass, cast iron, steel, cement, earth, reinforced concrete, composite materials, each of which have different failure characteristics, sometimes also depending on design loading, as many will fail differently under various types of loading conditions, type, or direction of loads etc. even the geometry of the item can also play a significant part in its failure mode, buckling, or stress concentration, fatigue, uniaxial, biaxial, or triaxial loading etc.may influence different type of failure modes. Wood loaded perpendicular to grain is a completely different material than if loaded parallel to grain.

RE: Need Help with Fundamentals

(OP)
"A single conversation across the table with a wise man is better than ten years mere study of books."

Respected All,

Thank you very much for your precious guidance.

@John R. Baker > No sir, I was jr maintenance planning engineer before corona.
@Littleinch > Sorry for such a broad one, I'm still a novice in the field to give a concrete scenario. It was just doubt raised in my mind while re-reading topics. btw liked your quote.
@Sucka > Really your answer made me a lot wiser.
@MJCronin > Definitely Sir, agree with your idea. Standards include a lot, empirical relations are bread & butter of industry. I was just curious with the broad idea.
@hydtools > Sure Sir, But until & unless I know the reason for specific rules. Using those rules make me feel heavy on heart.
@Tmoose > Sir, Added book to my reading list.
@dvd > Sir, nice example, it made easier to grasp.
@-fifty three > Sir, The material based aspect was a gem to the collection.

RE: Need Help with Fundamentals

V Thanki - very respectful response on your part, although one never quite knows who is sir and who is madam. Good luck with your endeavors.

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