fatigue sizing

[Ing.Vedder]

Hello everyone,
I come in search of help here from you because I have come across a field of which I know little or nothing... the fearful dimensioning to fatigue.
First of all I have already searched on the forum but I have not found much, I put the problem to you.
through a static analysis using fem software on a plate of a bolted flanged joint I found the tension of von mises in an area of this plate. the joint during his life will be urged to fatigue button r>0 for numerous cycles (>6 million)
This voltage of vm (precisely 185 n/mm2) is close to the admissible sigma (190 n/mm2) for a common carbon steel (s275jr or fe420 more or less same thing).. and the peak point is right next to a drilling for a bolt of the plate (point of voltage concentration close to a drilling on a plate).
turning online I saw that for the above material the curves of wholer give an infinite sigma to life of 210 n/mm2 (see attached image).
Is that enough to feel calm from the point of view of fatigue?
do you have examples or guidelines to suggest? I sail a little in the dark. .
Many thanks to all

e.v.

 

[bisogna_rimanere_umili]

taking for good results of the fem analysis (on which you can write philosophical treatises), for the fatigue limit you must surely take into account also the roughness of the piece.. .

 

[Ing.Vedder]

hi humble (bel nick :finger, the observation you make on the data obtained from the fem is correct, but I admit they are right, I would like to know how to behave on a fatigue analysis. .

Thank you.

e.v.

 

[bisogna_rimanere_umili]

the fatigue limit is an additionally reduced rupture limit with appropriate coefficients b2 b3 and kf (memory wash). . .
kf is the carving that if you do fem analysis you have already taken into account, others are the surface finish and the size of the component. . .

the load of what type is: constant, floating?
If not, a load story must be hypothesized. . .

I think fatigue is one of the most complicated things to deal with as it needs, to do something quite precise, of many data or to hypothesize them with criterion. . .

 

[Ing.Vedder]

I agree with you that the carving is already included in the fem analysis, so would the coefficients for surface finish and component size remain? Can I just get away with it?
the load is of type button to r>0 . in practice the bolted flanged joint is always under load, even in rest conditions.poi cyclically it is further loaded with a peak load that stresses it and then returns in rest conditions (but not zero). .

That's hard work!

Thanks again

e.v.

 

[Fulvio Romano]

I did not understand... if 190 mpa is the admissible sigma, how does 210 mpa to be the limit of fatigue?
in other words, if you load it statically to 190 breaks, but it resists to 210 indefinitely? is it made of antimatiera?

 

[Onda]

s275jr has a yielding sigma of 275 (then depends on the thicknesses) and breaking of about 420 (like the old fe420 name).
the limit of 210 is reported as limit of life to infinite fatigue. so compared to the voltage of the fem there is a ratio of 1.13.

I sincerely believe that it is necessary to consider a safety coefficient (ignorance) between the working voltage and the maximum permissible tension to fatigue.
I mean, that in performing an analysis we make a series of approximations that then lead us to get approximate values:
approximation on mesh and geometry
approximation of constraints
approximation on applied loads (maybe for you are perfectly known)
besides the coefficients that have already exposed you due to the surface finish and the size of the piece.
all these coefficients multiplied between them give a safety coefficient not to be exceeded.
but not knowing the analysis and assumptions on which the model stands, I believe that a factor of 1.13 is reduced to take into account all the necessary approximations

 

[Ing.Vedder]

@ fulvio
hi fulvio, ahaha no, no antimatter, I explained badly..with 190 n/mm2 I meant the usual admissible sigma that is adopted for verification to the admissible tensions of the s275jr. At the same time though, looking on the internet I saw that for infinite life verification 210 n/mm2 is adopted..(deduced by experimental tests)..so?! my deductive reasoning is that if through fem analysis (admitting the goodness of analysis) I get 185 n/mm2 and the limit is 210 I am in the right... Do I make it too simple?

@
I agree perfectly with what you say, the fact is that the problem at the moment is purely "burocratic" (the piece in question will never come into operation because replaced) and I should do a technical analysis on this. so not entering into the merits that the is too little as margin (which I share) I just wanted an opinion on my way of reasoning, whether I make it simple or am in the right.

thanks again to both

e.v.

 

[Fulvio Romano]

@ fulvio
hi fulvio, ahaha no, no antimatter, I explained badly..with 190 n/mm2 I meant the usual admissible sigma that is adopted for verification to the admissible tensions of the s275jr. At the same time though, looking on the internet I saw that for infinite life verification 210 n/mm2 is adopted..(deduced by experimental tests)..so?! my deductive reasoning is that if through fem analysis (admitting the goodness of analysis) I get 185 n/mm2 and the limit is 210 I am in the right... Do I make it too simple?

@
I agree perfectly with what you say, the fact is that the problem at the moment is purely "burocratic" (the piece in question will never come into operation because replaced) and I should do a technical analysis on this. so not entering into the merits that the is too little as margin (which I share) I just wanted an opinion on my way of reasoning, whether I make it simple or am in the right.

thanks again to both

e.v.

If we're on the bureaucracy, then just make the right arguments, even if they don't always guarantee the functioning of the piece!

the reasoning you do is right. the fatigue limit usually refers to a "good" surface finish, in the sense, not mirrored, but not even a casting of foundry. are theoretical values, strongly indicative, variance is very wide. that value is an average value, meaning that with a roughness "good" more or less 210 is a fatigue limit, if you need more you can polish and/or ball.

 

[pollo]

but are you sure that 210 is the limit for infinite life?
to me it seems a little too high: I remembered values of about 25 mpa
probably is the one for breakup to 2 million cycles

 

[Ing.Vedder]

thanks to all for opinions, excellent and useful as always boys.

e.v.

 

[luigi.paiano]

attention:the fatigue limit should be calculated not only from the whoeler curve but also considering among factors cl cg cs,the first depends on the type of stress,the second from the size and the third from the surface treatment! ! ! ! !
is there only alternating tension??? ? Is there also average voltage?? ? ? ?

 

[max83m]

attention:the fatigue limit should be calculated not only from the whoeler curve but also considering among factors cl cg cs,the first depends on the type of stress,the second from the size and the third from the surface treatment! ! ! ! !
is there only alternating tension??? ? Is there also average voltage?? ? ? ?

and these three coefficients from where do I get them?? ? ?

 

[folle76]

and these three coefficients from where do I get them?? ? ?

mah, I suggest you ask one with a stratospheric curruculum. Try to contact this below.

personal data of max83m
profession
designer - structural analyst
sector
Mechanical engineering
region of residence
Piedmont
software used
catia v5 - ug - solidworks - hypermesh - patran - abaqus - nastran - optistruct
experience
design equipment for mechanical machining in solidworks.
design components and assembly car frame in caia v5.
structural analysis in the linear field with nastran and optistruct, and in the field not linear with abaqus (user interfaces used: hypermesh and patran)

 

[meccanicamg]

and these three coefficients from where do I get them?? ? ?

are the coefficients that in technical literature are identified by curves and equations.

I find that everything you need is in this pantry http://www.unibg.it/data/courses/8555/8346- verification resistance a fatica.pdf then you can deepen. the coefficients indicated here are b2 b3 as from Italian indications. those reported by luigi are those of American law astm.

 

[folle76]

are the coefficients that in technical literature are identified by curves and equations.

I find that everything you need is in this pantry http://www.unibg.it/data/courses/8555/8346- verification resistance a fatica.pdf then you can deepen. the coefficients indicated here are b2 b3 as from Italian indications. those reported by luigi are those of American law astm

If a structural analysis specialist (mechanical sector), however hard in the push buttons, you do not know the fundamentals of fatigue design is better than you immediately fold. otherwise take the three coefficients as they appear on the flop. and then: all in! and cross your fingers. the statistical dispersion of the experimental results of fatigue tests plays in his favor.

 

[meccanicamg]

If a structural analysis specialist (mechanical sector), however hard in the push buttons, you do not know the fundamentals of fatigue design is better than you immediately fold. otherwise take the three coefficients as they appear on the flop. and then: all in! and cross your fingers. the statistical dispersion of the experimental results of fatigue tests plays in his favor.

careful not to shave the controversy... Of course it also makes me a strange effect to see a superdvanced so and then fall on questions.

 

[pollo]

cv, like business cards, are like declarations of conformity of machines: are presumption of conformity. To prove that they're good, that's another thing.

 

[luigi.paiano]

I agree on every word of mechanicsmg

 

[meccanicamg]

:finger: very well