Yield or Tensile strength for calculating parts 8

[RBX]

I have always used the "ultimate strength" of a material when i know the safety factor and the "yield strength" in other situations.(when i dont have the safety factor)

Is this accurate?(just having doubts after a debate i had with a fellow machine designer)

Thank you in advance

 

[mfgenggear]

I just wanted to say the OP brought out interesting questions and answers.

I just wanted to say "very good advice & disscussions"
very very good engineers on this site.

MfgEngGear

 

[jte]

LOL.

Hey - if it was easy to do this stuff, even Philosophy Majors could do it.

I dunno. I'm aware of one Philosopher who was working on a PhD thesis which was essentially "when is a circle a circle". Kind of an engineering - FEA type approach: If you have a polygon which has X number of sides, and you increase X, when is it essentially no longer a polygon but a circle. Imagine the fun the Philospy Dept. folks had with that one.

I'd hate to see a true Philosophy discussion on this issue. The only thing two philosophers can agree on is that the third philosopher is wrong!

jt

 

[drawoh]

I'm not sure why no one has provided the simple answer to this question.

Stress>Ultimate = part broken.
Stress>Yield = permanent deformation.

The OP never answered my question about his mechanics of materials training. Your point is correct. It also is obvious to anyone trained in structural analysis. Hence my reference to factor of ignorance.

JHG

 

[dhengr]

RBX:
It seems to me that the OP suggests some serious lack of understanding of the entire engineering design process. There are some good points made in some of the above posts, but you must be smart enough to read btwn. the lines a bit to get them. You must have had that mechanics of materials course and several other related courses (maybe a college degree) to really be proficient at what you say you are doing. And, then you would know that we have many uncertainties in our materials, loadings, manufacturing methods and our analysis methods. The F.S. (factor of safety), M.S. (margin of safety), R.F. (resistance factors) and L.F. (load factors) are all means of adjusting our analysis and design to account for these uncertainties, and these are not only dependant on yield or ultimate stress. You shouldn’t be using a F.S. to covering you’re a$$ for lack of knowledge of what you are doing (ignorance?). That’s not good design or engineering, and you shouldn’t be calling it engineering if that’s what you are doing.

The yield stress and the ultimate stress and their relationship for a given mat’l., are important mat’l. properties in the design and analysis process. Mild steel has a well defined yield point, a linear stress/strain curve to that point, and allows much strain beyond that point, plastic range and strain hardening, before it approaches its ultimate strength. Many engineering mat’ls. do not exhibit well defined yield points, for some high strength steels we use a .2% strain offset to define a yield strength, and some strain remains, but not a significant additional increase in stress before ultimate. Many brittle or hard materials don’t have much of a linear region prior to yield, or a definable yield strength, they just go to ultimate strength, in a gradual curve, but without much warning before failure. There are other interrelated material properties which must be considered in a good design, such as elastic and shear modulus, Poisson’s ratio, etc. Then you have the actual mat’l. specs. which call a for min. yield and ultimate and % elongation (ductility), which are usually exceeded in the mat’ls. delivered. These are mechanical properties of the material. Should you design for the min. or the actual values? The above covers only a few of the many mat’ls. we might use in a design.

Then you have the deflection, elongation, shortening, crushing, etc. etc. within the elastic range and beyond; until failure, which you define as part of your design process. Can your design tolerate these? Designing to ultimate usually means a sudden failure, can you tolerate that and what are the consequences? Fatigue and fracture have many fathers other than yield or ultimate strength. And, dealing with them involves considerable design and engineering finesse. Some buckling problems give some forewarning, others are sudden and catastrophic, and don’t forget secondary effects here. While the above items (and others) are a function of the material properties, they are as much, or more, a function of your particular design (widget). And, thus the many questions about ‘your mode of loading and failure; and does the machine just slowing down or does it hurting someone.’ You didn’t respond very well to these questions. Who does set your F.S.? Our design factors of safety or margins of safety are very important part of our work, and Fy or Fu, however they are actually set, are only a small part of determining what the F.S. or M.S. should be. Then, don’t forget such things as residual stresses due to your manufacturing processes, remember toughness, resilience, ductility, temperature effects, strain rate, mat’l. flaws, etc. Which materials are isotropic and which are anisotropic, you better know that. How are they affected by various processes, or their environment? Fact is, most codes give you a F.S. and tell you it is wrt to Fy or Fu, you don’t get to pick. That’s a short course in strength of materials, where each of my sentences equals a few chapters in the books you should be studying.

Don’t kid yourself, many engineers with the latest FEA package on their computers, aren’t very precise either. They are just kidding themselves if they tell you that they are. And, in many instances it allows them to pretend they know what they are doing, without having a very good intuitive understanding of the part or structure they are dealing with or how it really reacts to forces and constraints. They don’t need common sense or a feel for how the structure really acts, they have a computer program which actually inhibits this understanding. And, in some instances this yields (I mean results in) crap, and it’s still crap even when printed out to eight decimal places.

Ignorance wouldn’t have been my choice of words, but drawoh’s use of it may not have been out of line either in his context. It isn’t a dirty word the way it’s been used here, we might just as well admit something less than absolute intelligence, knowledge or certainty. As long as we understand that is our meaning, in this discussion and in this context. But, because of the potential for misinterpretation by judges and juries (the gen. public) and the intentional misinterpretation by lawyers, I would not use that word outside of this discussion. On the witness stand I might substitute the term ‘some degree of uncertainty,’ and explain some of the reasons for our inability to be exact and certain.

Many industries dictate design stresses, factors of safety, load factors or resistance factors and the like, and these are not always only related to Fy or Fu, nor do we get a constant F.S. for various failure modes or design considerations. Examples: ASME for pressure vessels and other products; AISC, ACI, NDS, and IBC for buildings and the like. In many instances we have to come up with our own design criterial, guided by some of these other codes and design guidelines, and we need to set our own F.S. or M.S. and you better do this in an intelligent way. In the machine design business, maybe you want to do some reverse engineering; what has worked in similar situations, for what expected part life, what types of failure and the reasons for same; maybe you should increase or can decrease your F.S.

 

[dhengr]

TGS4:
Do you have another name for “Ratcheting?” Do you mean strain-hardening or strain aging? They can tend to increase the yield and tensile strengths, while decreasing ductility. Thus, moving the part closer to its ultimate strength or failure, upon reloading to the same or a greater load level. In some instances, involving fatigue, there is a benefit to loading a structure or a part of the structure into the plastic range, above its normal operating stress. Because, upon unloading, the larger volume of surrounding material, still working in the elastic range, well push that small over stressed area into compression. Then, upon reloading into the normal operating stress range, that part which had been a potential fatigue sensitive area is now working at a better stress range or at a better mean stress; thus, improving the part or structure’s fatigue life.

 

[TGS4]

Sorry - no different name for it. I agree that strain hardening tends to benefit many cyclically-loaded parts. However, as I mentioned, in certain situations with cyclic plasticity, the over-all dimensions (and the cumulative strain) do not asymptote to a final value, but rather continue increasing until the ductility of the part is exhausted, or the part is no longer functional. If it would be helpful, I can post an except from a paper that better describes this failure mode.

 

[dhengr]

TGS4:
Yes, Please! It would be interesting to read the article you are referring to. I’m not sure how far into the strain hardening range of a material you would go in working a cyclically loaded part. That stress level is most likely too high for any significant cyclic endurance. I think one of us has plasticity and strain hardening turned around in their effects. As I understand it, you can work a part or structure part/area into the plastic range, unload it, and reload it to the same level, and the effect is to just move the beginning of the stress-strain curve to the right (on the whole curve) with essentially the same slope or the same modulus of elasticity. If you load the part to a higher level, you cause more plastic deformation, if you can tolerate that; i.e., you move the straight sloped part of the stress-strain curve even further to the right on the whole curve, again with about the same slope. When you get into the strain hardening part of the curve, that’s when things can get strange. Then you do start stepping up the further yielding and tensile stress higher than normal at that strain point in the curve, and this also tends to reduce the remaining ductility. That’s called strain aging, and brings you pretty close to ultimate failure. The deformation or endurance of your part might have you declaring, or actually seeing, failure at a much lower stress level. Many of the higher strength and more exotic steels and materials behave differently than described above, and do not have a well defined plastic range or plateau.

What I was talking about in my previous post is that in some instances, in a FEA output, you will see a relatively small area/material volume which has a very high stress level (Von Mises stresses or whatever), surrounded by a larger volume stressed, below yield, at a more moderate (acceptable) level. And, the larger surrounding material, when unloaded, can have the affect of forcing the smaller yielded volume to actually shift into compression when unloaded, and into a better stress range when working under normal load conditions. This may involve intentionally overloading the part to get this effect. If you stay within the strain limits of the first complete loading cycle you will get a hysteresis loop, for the stress/strain, which the part will tend to follow on repeated load cycles. If you increase the strain on future load cycles, the shape of the loop may not change appreciably, but the entire loop will step up, with the max. stress approaching ultimate again.