ASME div2 Fatique Screening 1

[somebodyfromworld]

Hi
I've been asked to prepare a fatigue screening for ALLOY 718 / UNS N07718. I usually use ASME Part 2 to calculate allowable stress for fatigue screening but a client wants me to use API 17TR8, where allowable stress is found by 2/3 Sy. ASME gives me a value of 206MPa for N07718 but API 17TR8 gives me 518MPa for the same material.

if I use ASME (206MPa), I get 2572 in STEP 3, Maximum design number of full range pressure cycles including start up and shut down
if I use API 17TR (518MPa), I get 140 in STEP 3, Maximum design number of full range pressure cycles including start up and shut down

My concern is why higher value in allowable stress gives less number in maximum design pressure cycles. Should it be vise versa?

Kind Regards, Sergey

 

[TGS4]

Nope - that's right. Fatigue doesn't care what your "allowable" is - just the magnitude of your stresses. And higher stresses result in lower life. So, if you are designing to the higher allowable stress (i.e. going up to the full magnitude of the allowable stress), then your fatigue life will be lower.

 

[somebodyfromworld]

Thanks for a clarification, now I got it

 

[rickfischer51]

Is it a good practice to use a different allowable? Typically the allowable is derived with the method used to determine the stress measure in mind. Seems like calculating a stress deliverable with a formula from one code and comparing it to an allowable from another code is dangerous.

Rick Fischer
Principal Engineer
Argonne National Laboratory