fatigue calculation

[max83m]

Bye to all,
I wanted to know from some fatigue expert if, starting from the results of a static that gives me the maximum tension on a component, it is possible from this given to make reasonings on the fatigue.
I had several opinions. some say to brutally compare the maximum static voltage with the sigma_d value = 0.5 * rm * 0.7 where rm clearly is the breaking voltage of the material and 0.7 a coefficient that takes into account various factors (form, scale, surface finish, etc.).
others say to compare the maximum voltage from the static with rs * 0.8 where rs is the yielding of the material.
in both cases, if my maximum tension is below that limit, infinite life is guaranteed.
I have made another reasoning, starting from the haigh diagram (which has the two limit straights "rs-rs" "rm-sigma_d", sign my point of work (sigma_a, sigma_m) and this must be within the area bound by the intersection of the two limit straights. but the doubt arises in defining sigma_a and sigma_m; I used sigma_a=sigma_m= max voltage/2. but certainly not the best way to proceed.
I know very well that the ideal would be to use a program that receiving in input the media force and the alternating force with which to press the piece, give as output the sigma_a and sigma_m, but I do not have this program.
and it requires me to estimate the fatigue life of a component simply knowing the maximum voltage of a piece subjected to a static load.

If anyone knows, please help me. help me.

Thank you! !

 

[meccanicamg]

Hello max83m,
I believe that your question is mainly general and not specific finite element analysis software.

we start from a concept to clarify: static analysis is used to evaluate static resistance to breakage or deformation. fatigue analysis is used to evaluate as a result of repeated loads over time, the behaviour of the material.

# Static fea with the values of von mises (as from hand calculation) comparison with the limit sigma defined by rm/3 or rs/1.5 or similar.

# fatigued fea, you need a wohler diagram that reflects the component in question, as detailed as possible, with all influence coefficients. set the load history to the piece and you will see the areas that will undergo an accumulation of damage in the worst time.

If you don't want to or can simulate fatigue with fea, you'd better build an excel sheet that makes verification (see my output in another post http://www.cad3d.it/forum1/attachment.php?attachmentid=15520&d=1277140685) or use the method of gough chickenard and compare its sigma with that limit.

attention to the formulas used, to the considerations of the alternating sigma and everything else.