Are PSD results correct if the following message is used during the solve. “One or more diagonal terms of the modal covariance matrix (i.e., when mode i = mode j) is negative. The 1 sigma results may be incorrect as a result of the input PSD curve being improperly represented.”


January 25, 2023 at 7:34 amFAQParticipant
No. Some recommendations are: 1. Check the PSD input curve fit with PSDGRAPH. PSD input frequencies should not increase by more than one order of magnitude per point. 2. Take the square root of the integral of the response PSD [RPSD in post26] and compare this quantity to the Post1 1 sigma value. They should be equal. 3. Do not input a curve with very low frequency values, far below the natural frequencies of the structure.

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