Yield in Material

[RMXByker]

Good Morning Everyone,

I am working on some beam deflection problems for work. I am calculating not just if the outer surface of the material is going to yield or not but exactly how deep the yield penetrates the material. Is there a standard practice percentage that yield can go into the material before the percentage of material not in yield can no longer snap the other back?

Thank you,
RMX

 

[boo1]

Elastic limit (yield strength)
Permanent deformation will occur when the elastic limit is exceeded. It is the lowest stress at which permanent deformation can be measured. Most metals have a linear stress-strain relationship up to the yield point.

http://www.eduresourcecollection.com/civil_sm_Elasticity.php

 

[RMXByker]

I understand the elastic limit or yield strength but when calculating a beam and determining if it will yield or not, this calculation can be figured throughout the thickness of the material. If a certain percentage of the material is not in yield the overall beam will not yield as the material that isn't in yield will overcome the material in yield and "pull" it back to its original shape. Obviously this isn't something that would be great for a fatigue situation but for a one time loaded case it would be beneficial to be capable of knowing that percentage limit. So what I'm looking for is a percentage of how deep the yield can penetrate the material and have the non-yielded portion pull it back into shape.

 

[boo1]

Beam bending is greatest in the outer fiber and zero at the NA. Just consider the beam stress at the outer fiber.

 

[RMXByker]

But you can't simply consider the stress at the outer fiber. It is possible that the very outer fiber is the fiber that is in yield therefore you wouldn't ever notice it until you began to fatigue the material.

 

[Twoballcane]

I think that is what the "I" is for. It takes in account the "stiffness" of the beam. Once the outer surface yields, it over came I. Also, once the outer surface yields, micro tears start to occur and propagate towards the center. That is why we have factor of safety to make sure the part does not yield.

Tobalcane
"If you avoid failure, you also avoid success."

 

[sdra2]

Doesn't answer your question directly, but might help...

Also, look at the same link with a "15" at then end. I don't think there is a "standard percentage" like you are after.

 

[IRstuff]

Seems to me that hypothetically, even if only part of object has yielded, the entire object is compromised, so it's irrelevant whether the whole thing has broken or not.

The part that has yielded may lead to crack formation or corrosion intrusion.


TTFN

FAQ731-376

 

[RMXByker]

Twoballcane - You must not be aware that compression springs can see 10x the ultimate strength on the outer surface and still meet over a million cycles.

ITstuff - The part I am currently dealing with is a simple plastic component that will flex around a snap feature and snap in behind it to create a positive snap. Therefore, cracks or compromised material is non-important as this material will only see a compressive load. That is why I have been asking about an industry standard but it seems there really isn't one. I can see that if this was a metal component that compromisation would be much more critical but in my specific case it is not all that important.

Thank you all for your inputs. Let me know if any of you have any more thoughts!

 

[TVP]

If you are designing a snap fit using thermoplastic (polymer) materials, there are a number of good resources for this. Have you reviewed any of the design guides from DuPont, BASF, etc.? Alternatively, there are two books published by SPE that cover this subject:

Designing Plastic Parts for Assembly, 6th Edition

http://www.4spe.org/online-store/designing-plastic-parts-assembly-6th-edition

Joining of Plastics: Handbook for Designers and Engineers, 3rd edition

http://www.4spe.org/online-store/joining-plastics-handbook-designers-and-engineers-3rd-edition

 

[DanStro]

If this is a simple bending problem wouldn't you be able to figure this out by using:

sigma(bending)=Mc/I where you vary c from 0 to the outer surface and find where sigma(bending)>= the yield stress?

Dan

 

[EngineerTex]

This is something that would require FEA and an EXACT knowledge of the yield strength of the material. To complicate it further, no matter what material you use, it's not going to be isotropic. This is why when you have tensile specimens cut from plates, bars or beams, they are cut from the interior of the item.

However, let's say that you could magically determine that every part of the material is exactly the same. The next problem is that it would be shape-dependent.

Next, if you say that you only have a certain part of the material in yield, you have to remember that once you yield the material, if you're talking about steel or aluminum, you have made the material stronger, per strain hardening. So now you have a deformed portion of the beam, which is stronger than the remainder of the beam in a location where the rest of the beam does not have much ability to "spring" it back to where it was originally.

You're looking at something that is quite similar to welding distortion. The bad news is that if you ran a weld bead on the outside fiber of your beam, perpendicular to its axis, you will see exactly what will happen, which is that it is going to distort permanently. If you are talking about trying to figure how little over yield you can go without permanent distortion, the answer is going to be "not very much."

Engineering is not the science behind building. It is the science behind not building.

 

[IRstuff]

re: springs, but the design and application would take that 10x load into account, and the user would be aware that it's only good for 1 million cycles. But if you used such a spring in an application that required 10 million cycles, that would be a big issue, and you would not be allowed to design in any level of yielding.

TTFN

FAQ731-376

 

[RMXByker]

TVP - I have already reviewed a lot of these materials but no one speaks of the ability to "pull" the material back from yield on the outer surface by using a specified percent of unyielding material around the neutral axis.

DanStro - Yes, that is one of the ways that you determine the maximum depth that the yield has penetrated the material. But my question is how much depth by percentage is allowed before the beam has been permanetely deformed.

 

[RMXByker]

IRstuff - That is correct. But in my case as I stated before I am only flexing this part a single time. There will not be a fatigue load in that direction. You are certainly correct though!

 

[Twoballcane]

Ummm...are we talking beams or springs, because the two applications will use different materials and will see different forces. So you can not compare one with the other. Also the springs will depend on the S/N curves. So if you do have yield, you will eventually fail. If you don't have yield, the spring can go on virtually forever.


Tobalcane
"If you avoid failure, you also avoid success."

 

[dhengr]

I have a thought..... You should get a couple of good Mechanics of Materials books and study them. Get yourself a good mentor or two who will help you understand basic engineering concepts. Take these kinds of questions to your supervisor so he/she knows what you don’t know and can help keep you out of trouble. You probably shouldn’t be designing things of much importance given your experience and knowledge level, without considerable supervision. And, that should come from within your company, so you don’t produce a product that fails or hurts someone. If it needs design attention, is it really “not all the important?”

 

[boo1]

dhengr, agreed

 

[rb1957]

usually the extreme fiber stress due to bending would be limited by yield.

if you want to go beyond that cozzone is a good method for determining the plastic bending strength of a section.

 

[RMXByker]

Interesting thought there dhengr...let me think about it. I find it interesting that everyone is still under this amazing determination that this is about a steel or metallic product. Again, it isn't therefore a lot of the experiences that are being talked about above are useless. By the way dhengr, I have these books at my desk and have a true understanding of how these things work. Intersetingly enough I believe it is quite obvious that I have a decent amount of experience considering that I have recognized how these formulas are utilized in the context of a classroom in comparision to how they are truly utilzed in the field. For anyone to be ignorant of the thought that you can't take a product into a yielding point and bring it back based on the depth of the yield shows your lack of experience in how these things truly work in the real world. I would suggest getting you head out of the books and taking a true look into the real world application of the problems and not just the theory.