Fatigue Limit Calculation 1

[atvhead]

how do you guys calculate fatigue limit? i have been told to use 0.5 or 0.4 of the ultimate tensile strength but ive also be shown to use 0.4 of the ultimate or 0.75 the yield strength, whichever is lowest. what do you guys think? and could you direct me to somewhere that says which method to use?
Thanks a bunch!

Bob
_innov8_

 

[satchmo]

.5 of the Sut is just the starting point. Then all of the fatigue factors come into play. Mechanical Engineering Design by Shigley and Mischke is my bible for this stuff. Depending on stress concentration and other factors, the fatigue stress can be quite low.

 

[atvhead]

satchmo - thanks for the quick answer. i think i am just putting in too much thought here. i have done the endurance calculation using 0.5 * Sult, and then multiplied all my correction factors, notch sen, temp, reliability etc. with the endurance limit. and yes, it is much much lower. like around 7000 psi compared to a Sult of 93000 psi.

Bob
_innov8_

 

[atvhead]

and yes, i opened right up to my college book "mech eng design" by shigley once you said it. thanks for the tip. good book.

Bob
_innov8_

 

[Tmoose]

Are you including the difference in properties in many rolled materials' longitudinal and transverse directions?
That one usually ruins my day.

 

[tbuelna]

Evaluate the loading condition (ie. torsion, bending, contact, combined loads, etc.), add in knock-down factors for stress concentrations (Kt), add in knock-down factors for reversing loads (R), figure out the number of load cycles, and then finally look up the allowables for your particular material and heat treat in the S-N diagram of an accepted source like MIL-HDBK-5.

Most importantly though, consult your local, friendly stress analyst. He's a qualified professional, and I'm not!