Yield Strength and Young's Modulus

No.

And you are supposed to know why.

why? can you tell me the reason?

in ANSYS during preprocessing one need to give only E and value of poisson's ratio and after solution has come, one can determine that given solution is safe or not is decided by yield strength... then wht is actual process done in ANSYS based on E and poisson's ratio

I'm not familiar with ANYSYS, but it doesn't sound as if it's ANY good (geddit?)

I'd expect the program to prompt you for yield strength and/or UTS, as well as E and poisson's ratio.

You can't calculate yield strength from E and poisson's ratio. Various grades of steel have a wide range of yield strength hence allowable design stress, but E and poisson's ratio do not change. For this reason, when a design is limited by deflection rather than stress, as is often the case, high strength grade steel is a waste of money.

Cheers...........Codey

I too agree, If ANSYS does the stress analysis based on E and ν then I have to doubt the software (or it's authors).

Though I doubt it does, some where it must prompt for the other details.

Sorry gents, I misread that, I thought it said ANYSYS, hence my little joke.

Codey

That still stands because of the extra whY. .

In this context it has earned it's place. We should ask ANSYS to put it (or may be How) in it.

No.

The E value is used in the Stiffness Analysis.

The postprocessor then uses your input yield stress to perform the Code Check.

Hope this helps

Stewie

Most FEA programs, including ANSYS, report stress as if there is only linear (elastic) deformation. For that, you don't need to know the yield strength.

Inelastic deformation is found only in the top end FEA programs and is usually more expensive than it's worth since we try to avoid exceeding the elastic limit.

But there's a wide range between allowable design stress and elastic limit. Factor 2 - 3-ish? Shouldn't be designing between the two. I suppose if it tells you the actual stress (from strain and elastic modulus) you can compare it with allowable, but better if allowable entered and any problem flagged up.

Cheers..........Codey

PS hear-hear to Ed Weldon's #9

Back from the unreality of ANSYS and CAE in general and into a bit of the real world. In the real world a given material have widely varying yield strength based on small percentage of alloying elements as well as history of processing and service use. Exposure of materials to certain temperatures, stress, strain, cyclic loading, chemical environments can produce wide variations in yield strength without having any measurable effect on Young's Modulus or Poisson's ratio.

If you don't understand such basics of physics and engineering then you have no business offering your services in the world of engineering design no matter what is your level of mastery of computer based engineering tools. Am I being harsh here? You better believe it. The mistakes you might make in your design work may kill or maim my fellow humans.

Ed Weldon

Ed, I agree 3000%. It's dangerous.

ANSYS know what they are doing. The guy Pareesh doesn't, because he is not prepared to understand elasticity theory. Read Stewie's answer above. There is more going on after preprocessing.

NO!!! The yield strength of any material is competely independent of both young's modulus and poisson's ratio. There is no equation to relate these properties because they are not related-period.

Good answer, I'd say. Although, if one knows the allowable elongation, then you can work from modulus to "backwards" to yield strength.

I think that's going at it the wrong way round. You would normally know the yield strength and divide that by E to get the elongation (strain) at that stress.

Cheers........Codey

Blink,

Your second sentence is completely mystifying to me. Care to elucidate?

Young's modulus is stress/strain, so if you know the strain (maximum elongation) before yield or failure, then you can calculate the maximum stress. The only time I've had a need to do this is with unidirectional composites, for which I had the maximum elongation and modulus, but not the tensile strength.

Hello paresh,

Poisson's ratio has a positive number in most materials, apart from foam which has a negative value. That should answer part of your query.

I kindly suggest you get yourself a good Physics book or a good Engineers book. Young's Modulus and Poisson's ratio are all pretty basic ideas. I can understand your confusion, but fully understanding these two principles may help you realise where you are going wrong?

No insult here, but you cannot fully know anything without first getting some idea of its principle and then applying it to a suitable subject.I think a poke around in Hooks law may help you with Young's modulus. Though Hook's law may sound simple, and may appear to be stating the 'obvious', by extrapolation its principles can also be assigned to solid and fluid mechanics, and may point your errors out if you have not already sorted the problem out. By the way I do expect you to have sorted them out.......... You would not be doing what you are if you were not bright!

It can get frustrating when you cannot decide on how or why to use one or other principle, but a little thought after a break and a bottle of scotch does wonders!

I hope you have found the answer you were looking for now, but, if not, Google all these Principles and they should become perhaps more easily understood?

Good luck, and tell me/us what you are actually trying to discover, OK?

sure. i will tell what was actually i looking for... thanks for giving me right direction to answer. and thanks for your valuable time.

bb,

I was not aware that Poisson's ratio could be negative for any material. How could this be? This means that if you stretch it in the X direction, it expands in both the X and the Y direction. I don't understand. Can you point to a source for this statement?

Research "auxetic" materials.

Here is an example:

http://yfrog.com/epauxeticcell1550x412j

It is thought that there are no known natural/biological examples, except perhaps bone thissue.

Hi ba/ael,

I have to say it is some while since last talked about this, let alone used it. I believe I used it last to set a question in my brother in laws Golf Club. He is a Mechanical Engineer specialising in the design of large Trailers for Tanks, and armoured troop carriers. Starting at around around 3 million Dollars and upwards for the Prime mover and Trailer complete.

BTW I am saying this because I no longer have it in my memory to recall. Just recall the name of the Principle thats all.

But anyway, this is the page and the piece I read. I guess it is not everyday an Auxetic or a knowingly 'auxetic' item is used or bought.

http://en.wikipedia.org/wiki/Auxetics

Please note, I have not spell checked the pasted items.

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Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures which flex when stretched. Such materials are expected to have interesting mechanical properties such as high energy absorption and fracture resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

Auxetics can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic string straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume.

The term auxetic derives from the Greek word αὐξητικός (auxetikos) which means, "that which tends to increase," and has its root in the word αὔξησις, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.^[1] Auxetic materials can be single molecules or a particular structure of macroscopic matter.

Scientists have known about auxetic materials for over 100 years^[2] but have only recently given them special attention. The earliest published example of a synthetic auxetic material was in Science in 1987, "Foam structures with a negative Poisson's ratio" by R.S. Lakes from the University of Iowa. The use of the word auxetic to refer to this property probably began in 1991.^[3]

Typically, auxetic materials have low density, which is what allows the hinge-like areas of the auxetic microstructures to flex.^[4]

Examples of auxetic materials include:

• Certain rocks and minerals^[2]
• Living bone tissue (although this is only suspected)^[2]
• Specific variants of polytetrafluorethylene polymers such as Gore-Tex^[5]

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Good luck.

Interesting. Thanks, bb.

Hi ba/ael,

No problems my friend. Can I take this opportunity to wish you happy Xmas?

Good luck.

Thanks, bb and a very merry Christmas to you.

Please consider that the yield strength and hardness of many engineering metals are controlled by heat treatment. Yet with the large variation of yield strengths obtainable by heat treating, poisson's ratio and young's modulus remain constant. For example, heat treatment of carbon steel has a profound affect on its yield strength, but no affect on poisson's ratio or young's modulus. The same principle applies to work hardened metals, precipitation hardened metals, etc.