Goodman diagram for a component designed for Infinite life 1

[ninjaz]

Hello

I've done frequency analysis for a component and the stress values are slightly greater than allowaable stress limit.

Is it possible/necessary to check whether the stress values are fitting in goodman diagram.

My component should have infinite life.

Is there any difference between "goodman diagram" and "goodman modified diagram"

 

[rb1957]

? what allowable stress ... yield, ult, stress for a certain life ???

have you tried to google for "goodman diagram" ?

Quando Omni Flunkus Moritati

 

[ninjaz]

Hello rb1957,

It is allowable yield stress.. Yes i have tried google. But i couldnt get an clear idea for where goodman diagram should be used.

 

[rb1957]

if you have a fatigue stress > yield, your fatigue life is probably measured in minutes (rather than infinite) ...

i found this description for goodman diagrams ...

http://www.engineeringarchives.com/les_machdes_modifiedgoodmandiagram.html.

what fatigue info do you know for your material ??

Quando Omni Flunkus Moritati

 

[AeroStructAnalyst]

If your stresses are higher than your yield stress, and your plasticity is localized, surely you should be using a strain life analysis rather than stress life? I thought Goodman's diagram helps you to determine your "equivalent" stress amplitude to correct for your "mean stress" effect for non-symmetric loading?

Also, if your stresses are higher than your yield stress and your plasticity is not localized, i.e. you have bulk section yielding with high plasticity, then neither strain life nor stress life fatigue analysis methodology with give you a conservative life prediction and the reasonable thing to do is to optimize your component design to reduce the stress in the structure.

 

[izax1]

I would say that your stresses are too high if your requirements are infinite life and you have stresses above yield.

What Factor of Safety are used in the allowable stress? Are you operating at a resonance?
Achieving infine life would usually require stresses below fatigue limit. If it is steel. For Alu, you would often no have a fatigue limit.

 

[bxtsafe]

ninjaz, your question is confuse. Please, reformulate it.
Thanks,
Regards.

 

[jagad5]

If your material is ductile and you exceed yield with the highest load, you may still have infinite (or very long) life. There are materials which have an endurance strength that is higher than their yield strength. For example, annealed 304SS has Ftu = 70, Fty = 25 and Se = 35 ksi. The steel work hardens significantly and this has to be accounted for in the fatigue analysis of that material.

Goodman is a very conservative approach. After accounting for work hardening, residual stresses, etc., compute the new mean and alternating stresses and plot that point on your Goodman diagram to see if your scenario is in the safe zone or not.

Doug

 

[bxtsafe]

The endurance limit is always lower that the yield strenght. And this rule is valid to all materials. I think it is very difficult a metal be under plastic cyclic strain and doesn't failure by fatigue.

 

[jagad5]

bxtguard
The generalization is correct but there are exceptions. Perhaps most notably, annealed stainless steels can be work hardened significantly. The endurance limit for annealed 304L is higher than the yield strength. ASM Handbook, Volume 19, Fatigue and Fracture cites Fty = 32 and Se = 40. The Handbook of Stainless Steels says 25 and 35 ksi. The first cycle to 40 ksi resets the yield strength, and can be done an "infinite" number of times.

Cyclically hardening materials can have an endurance strength (load / original area) that is higher than the yield strength (load / original area). The strength increases compared to the original condition (I'm not a metallurgist so I can't explain the mechanism) and the cross section is reduced. The %RA for annealed 304SS is about 80%. There's an excellent discussion of the topic in Fatigue and Durability of Structural Materials by Manson & Halford.

Doug

 

[bxtsafe]

Doug,

This annealed 304L steel is a very interesting metal, a true "out of line" case.