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[FAQ]

“Expected Frequency” is commonly used in PSD fatigue calculation in aerospace applications. The Expected Frequency is provided as an approximation to zero crossing frequency (ZCF) commonly used in narrow band PSD fatigue calculations. It is calculated as the centroid of the area under a response PSD curve. Depending on the shape of response PSD curves (caused by the dominating modes), the calculated Expected Frequency can be conservative or unconservative comparing to the ZCF. Therefore, the Expected Frequency solely provides a “ballpark” value for users to make adjustment for either conservative or non-conservative estimate of fatigue life calculation. As a side remark, the ZCF is calculated as sqrt(m2/m0), in which m2 is second moment of area of response PSD while m0 is zeroth moment of area of response PSD. By virtue of random process theory, it is also equal to (Stress Velocity RMS / Stress RMS). Since Mechanical doesn’t provide it as a standard result, the Expected Frequency is just a quick solution alternative that the users requested above. I agree that the statement “Statistically, it is the mean frequency that is often used in the random vibration fatigue life calculation.” is misleading to users in terms of definition of ZCF. The statement was actually removed from ANSYS Help 17.1.