Hello,
trying to calculate how optistruct gives results about fatigue.
I've tried with an easy loading before, symmetric cycle with R=-1 and TABFAT 1.0 -1.0 and it gave me the correct sigma_range. I used 415 as UTS on Hypermesh to generate the wohler curve, giving me SR1=1769,15 b=-0.125. Using the formula Nf = (2*SGVON / 1769) ^ (1/-0,125) I found the cycle number obtained by Optistruct. Where SGVON is the value of the sigma obtained on Hyperview.
Ok then I've tried an asymmetric cycle with TABFAT 1.0 -.5. I've read the stress and using Goodman formula I should have 1.6M cycles...giving a wrong result. Instead Optistruct tells me that the element survives 18224 cycles.
I read for example sigma max = 286MPa. Sigma Min 143 (it's the half of the max..)
Sigma alternate=71
Sigma mean=214,5
sigma ultimate = (415MPa)
Goodman = sigma alternate / (1-sigma mean / sigma ultimate) = 148MPa
Sigma (range) = 2* Sigma (alternate) = 296MPa -> giving 1.6 million cycles??
How do I calculate this value in case of asymmetric cycle? (-1<R<0) ? What am I missing?
Thanks in advance,
Antonio
