Can a fatigue analysis be done based on results from a PSD analysis?
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There is no automated way, but there is information from the PSD analysis that could be used to do a fatigue analysis. We introduced the Segalman-Reese SEQV value in ANSYS 8.1 which is a commonly-used stress quantity for doing fatigue analysis. The procedure would be: (1) Identify the highest value of SEQV and its location on load step 3 (one sigma values). (2) Find the corresponding value of SEQV on load step 4 for this location. (3) This load step 4 value divided by the load step 3 value is the statistical average frequency in radians/sec. (4) Knowing the average frequency and the duration of the psd excitation produces a total number of cycles. Cycles = Duration x Statistical Avg Freq / 2pi . (5) Then the number of cycles associated with different stress ranges can be computed from the assumption that the 1 sigma value of SEQV is not exceeded 68% percent of the time, the 2 sigma value is not exceeded 95 percent of the time, etc... The ASME Code calculates fatigue by summing n1/N1 + n2/N2 + ... The lower case n values are the number of cycles computed above. The upper case N values are the maximum number that could be sustained for that stress range based on the S/N curve. |
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