Fatigue life calculations
Altair EmployeeI came across in the documentation that the life is calculated using the formula.
Sr = SRI1(Nf)b
But, even after maintaining the co-efficients, TRTMENT, FINISH and [KF] as 1, the results are off (when compared with the manual calculation using the above mentioned formula) by a random factor (approximately 0.5), for each unique component I take for fatigue analysis (Refer image attached). Why is it so?
What is the calculation happening at the back end of the software?
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Why is the factor 0.5 coming into play?
Answers
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Is it possible to share the model file with us so that we can have a deeper look into the model?
Please share file using File Transfer link available in my signature.
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Altair Forum User said:
Is it possible to share the model file with us so that we can have a deeper look into the model?
Please share file using File Transfer link available in my signature.
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Hi Pranshanth,
Thank you, Can you let me know what is the maximum and minimum loads or Stress amplitude and mean stress?
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The maximum and minimum loads are 2000 N and -2000 N respectively. (defined by sine curve)
Stress amplitude is 198 Mpa
Mean Stress is 0 MPa (Because of fully reversed loading)
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Prashanth,
Thank you.
Since you mentioned as maximum stress, I wanted to confirm with you about the Sa and Sm.
I will check and update to you soon.
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Altair Forum User said:
Prashanth,
Thank you.
Since you mentioned as maximum stress, I wanted to confirm with you about the Sa and Sm.
I will check and update to you soon.
In the meanwhile, I tried the exact same problem in Ansys Workbench; where I got exactly the same result as that from the formula.
What am I missing out in Radioss?
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Hi Prashanth,
I am checking with experts. I will get back to you soon.
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Hi,
This reply is on behalf of Prakash & Expert.
SN curve in OS is based on the stress range. Stress range = 2*amplitude.
Looks like Ansys is using amplitude instead of stress range.In future version we will have option to choose between amplitude and range.
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Thank You, @Rahul R and @Prakash Pagadala for your efforts.
Based on your answer, I doubled the stress amplitude value and then substituted the resulting stress range in the formula Sr = SRI1(Nf)b .
When I used this new SRI1 value(It is double the old value), I'm getting the results as expected.
One last question. Does the formula Srange = 2×Samplitude hold good even for load curves that are not sinusoidal?
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Yes the relation range = 2 amplitude is valid in any case, even on random loads. The rainflow count will be the same.
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Thanks for the clarification.
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@Rahul R Do we also have to multiply the fatigue strength value by 2 while defining FL in MATFAT like we are doing for SRI1 value?
e.g if the fatigue strength of a material is 100 MPA then do i have to put 200 for FL in MATFAT?
TIA
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HI,
No, since Sr= 2*Sa we used this in the equation but for Sfl or FL it is not required to double the value.
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@Prakash Pagadala What if i have Stress at 1000 cycles and not at 1 cycle. In that case I have to use Point, 1 segment method for SN curve in Fatigue Process Manager. Do i have to multiply this stress@ 1000 cycles by 2 for SR1000 input?
Kindly revert back at your earliest convenience.
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