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Yield or Tensile strength for calculating parts
8

Yield or Tensile strength for calculating parts

Yield or Tensile strength for calculating parts

(OP)
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

RE: Yield or Tensile strength for calculating parts

Depends - what's the failure mode of your widget?

BTW - what do you mean when you say "safety factor"?  What does that mean for your widget?

RE: Yield or Tensile strength for calculating parts

(OP)
by safety factor i meant to say "Factors of safety"

From 1.5 to 4 depending on the application.

RE: Yield or Tensile strength for calculating parts

RBX,

   What will happen if your widget fails?  Will it be annoying, or will people die?

   I would use a safety factor with any type of strength.  I would consider a factor of 1.5 appropriate when you have exact, reliable knowledge of how your part is loaded, and you have done fatigue analysis.   

               JHG

RE: Yield or Tensile strength for calculating parts

(OP)
Well lets say look at both scenarios

If the widget fails there will lives in danger..

And the opposite..
 

RE: Yield or Tensile strength for calculating parts

Again - what's the failure mode that you're talking about?

How well do you know the loads?  How well do you know the material?

In the designs that I do, I have multiple design margins (I abhor the terms "safety factor" or "factors of safety"), and each design margin is for a different failure mode.  I usually deal with about 4-5 different failure modes, plus fatigue, if that's also a concern.

RE: Yield or Tensile strength for calculating parts

(OP)
Well gather you can use which ever force with the factor of safety, just depends on the
-situation(lives in danger)
-material(brittle or ductile etc)
-How well you know the working conditions

RE: Yield or Tensile strength for calculating parts

Be an engineer (or get an engineer) and determine what your failure criteria are.  Yield strength and tensile strength only tell you when a material deforms or breaks.  It doesn't tell us anything about your application.

What is failure?  Failure to operate?  Operates too slowly?  Breaks in half?  Don't forget fatigue!  These are questions that need to be answered by an engineer, not picked out of a materials textbook.

RE: Yield or Tensile strength for calculating parts

I think it would more helpful if you can review Failure Criteria in your Shigley book, because the way you were describing how you were using it I believe is incorrect.

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

RE: Yield or Tensile strength for calculating parts

RBX,

   Have you taken mechanics of materials in school?

   If you did, you would have been told that factor of safety actually is a factor of ignorance.  You are admitting and accounting for the fact that you do not know exactly what the strength of your materials are, you do not know what the loads are going to be, and your analysis is not absolutely precise.  That last point is especially true if you are not a graduate structural engineer, sitting in front of the latest FEA package on your computer.  

   My machine design textbook (V.M.Faires) recommends safety factors of between 1.5 for dead loads based on the yield strength of ductile steel, to 20, for shock loads based on the ultimate strength of cast iron or timber.   

               JHG

RE: Yield or Tensile strength for calculating parts

RBX, even in some of the aerospace stuff I used to work on where mass was a major issue, we still used some kind 'safety factor'.  Figures of 1.5 on ultimate and I think it was 1.2 on yield spring to mind as typical but it did vary by circumstance.

For some applications, such as lifting equipment, the design factor was typically a lot more than 1.5.  Something like 8 or 10 springs to mind though it was a while back so I may be off.  It will depend on the 'codes' or other regulation applicable to the particular application & location.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Yield or Tensile strength for calculating parts

3
I don't support the assertion that "factor of safety" is a "factor of ignorance".  Judging by the tone of several replies here, the conversation has drifted off the point.

The "factor of safety" is just that, a margin for error based on uncertainties in several factors beyond the control of the designer.  There are uncertainties with strength of materials, assumptions made in modeling, and yes, perhaps rounding/truncation errors associated with the mathematics.

Often this quantity is understood to be a margin for error, but the intent is to have the design capable of withstanding an external influence beyond that stated as inputs to the paradigm.

I disagree with "ignorance" as being one of those quantities.  Else the computation would be been referred to as "factor of arrogance".

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

RE: Yield or Tensile strength for calculating parts

The ignorance comes mostly in the materials behavior (less so now than 50+ years ago)and the loads.  If you KNOW your loads to within the nth decimal place, then there is little ignorance, and hence less of a need for a higher design margin (recall from above that I abhor the term factor of safety).

Note to Cockroach - we don't use the term "factor of ignorance" because it makes us look bad to the public.  We sugar-coat it with a "safety" moniker.  But it's uncertainty (ignorance, if you will) in loads, materials, analyses, assumptions, failure modes, etc.

That also depends on your failure mode and evaluation method.  For example, if you are interested in buckling, and you do an eigenvalue buckling (Euler buckling) analysis without considering imperfections, then you ought to have a high design margin, as opposed to if you performed an elastic-plastic buckling analysis including imperfections.

But, back to the OP - what failure mode(s) are you examining?

RE: Yield or Tensile strength for calculating parts

Personally, I agree that you have to look at the failure mode to determine what strengths have to be considered.  What follows is an excerpt from a speech I give to saw filers.   It is aimed at middle aged and older people who have a high school diploma with maybe one or two science classes.   It is not designed for the audience here but it will give an idea of how I analyze what strengths are needed when I design a tool for a specific application.  

How carbide saw tips wear out and become dull.

1. Wear – the grains and the binder just plain wear down
2. Macrofracture – big chunks break off or the whole part breaks
3. Microfracture – edge chipping
4. Crack Initiation – How hard it is to start a crack
5. Crack propagation - how fast and how far the crack runs once started
6. Individual grains breaking
7. Individual grains pulling out
8. Chemical leaching that will dissolve the binder and let the grains fall out
9. Rubbing can also generate an electrical potential that will accelerate grain loss 10. Part deformation - If there is too much binder the part can deform
11. Friction Welding between the carbide and the material being cut
12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.
13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc
14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal
15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets hotter faster than the inside. As the outside grows rapidly with the heat the inside doesn't grow as fast and this creates stress that tends to cause flaking (spalling) on the outside.
16. Compression / Tension Cycling - in interrupted cuts the carbide rapidly goes though this cycle. There is good evidence that most damage is done as the carbide tip leaves the cut and pressure is released.
17. Tribology – as the tip moves though the material it is an acid environment and the heat and friction of the cutting create a combination of forces.
 

Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.    

RE: Yield or Tensile strength for calculating parts

I have worked on a project with a factor of ignorance of 0.5, where we had measured the important loads, and measured the material properties, and if it failed nobody got hurt and we tested each production part against the main failure load.

But note that despite all that the system was designed for 150% of the known measured loads it would see in service.
 

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

RE: Yield or Tensile strength for calculating parts

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

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

You would apply your "factor of whatever" to the yield strength in most cases because your design most likely would be changed by plastic deformation of the part. (yielding)Very few solid mechanical designs involve plastic deformation.

Ultimate strength is obviously the failure strength where your part will suddenly become two parts, which is generally not good, and should be avoided.

This is not to be confused with some engineering calculations and formulas that specify the use of ultimate strength in the calculations. It would obviously not be appropriate to substitute the yield strength here when the ultimate is specified.  

RE: Yield or Tensile strength for calculating parts

Turbo20V, for the failure mode of a bar in tension, you would be correct.  And for those specific situations where that happens, I would agree with you.  What about the other failure modes?  What about a plate in bending - would you limit the stress to yield, or some fraction of ultimate?  For bending, there are different failure modes.  What about ratcheting?  For a secondary stress, you could have a limit of 2*Yield that would still ensure no plastic collapse, but also preclude ratcheting.

It's NEVER that simple.

RE: Yield or Tensile strength for calculating parts

I don't even know what ratcheting IS. Nor do I know if this persons widget is a plate or a platypus. Anyone can add complexity to an analysis, it is quite easy to do. What about corrosion? thermal cycling? Stress concentration? Hydrogen embrittlement? What if the sample is not uniformly heat treated, then what? before you know it you wont feel confident in anything.

I simply provided the most basic explanation of ultimate and yield stress. I didn't put a PE stamp on my post or anything.

I would like to know about ratcheting however.   

RE: Yield or Tensile strength for calculating parts

Ratcheting is a mode of failure caused by repetitive application of a load.  Under the right circumstances, you could get progressive plastic deformation that would, eventually, exhaust your material's ductility (or ruin your part's usability).

I appreciate your contribution, but even the definition of a stress is not as straight-forward as one might think.  Bending vs membrane vs peak stresses, etc

Quote (Turbo20V):

Anyone can add complexity to an analysis, it is quite easy to do.
That's kinda the point here, isn't it?  Your list is far from exhaustive, yet are very important considerations.  Of course, all of this in lock-step with the various failure modes, and (back to the original post), the "factor".

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

RE: Yield or Tensile strength for calculating parts

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

RE: Yield or Tensile strength for calculating parts

LOL.

Quote (TGS4):

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

RE: Yield or Tensile strength for calculating parts

Quote (Turbo20V):

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

RE: Yield or Tensile strength for calculating parts

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.

 

RE: Yield or Tensile strength for calculating parts

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.
 

RE: Yield or Tensile strength for calculating parts

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.

RE: Yield or Tensile strength for calculating parts

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.
 

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