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

 

[TGS4]

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?

 

[RBX]

by safety factor i meant to say "Factors of safety"

From 1.5 to 4 depending on the application.

 

[drawoh]

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

 

[RBX]

Well lets say look at both scenarios

If the widget fails there will lives in danger..

And the opposite..

 

[TGS4]

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.

 

[RBX]

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

 

[TheTick]

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.

 

[Twoballcane]

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."

 

[drawoh]

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

 

[KENAT]

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

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Cockroach]

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

 

[TGS4]

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?

 

[tomwalz]

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.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[GregLocock]

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

http://eng-tips.com/market.cfm?

 

[Turbo20V]

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.

 

[KENAT]

Turbo20V, as I alluded to above, sometimes you are required to assess both ultimate & yield though with different factors.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[TGS4]

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.

 

[Turbo20V]

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.

 

[TGS4]

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

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.