How does the AVPRIN command affect principal and equivalent stress (SEQV) calculations in a random vibration (PSD) analysis?
In ANSYS, Release 11.0, if the AVPRIN command is not issued, the SegalmanReese method is used to calculate SEQV in a PSD analysis. The SegalmanReese method does not calculate principal stresses. It directly calculates SEQV from the component stresses. Thus, the output will have zero values for the principal stresses. If either "AVPRIN,0" or "AVPRIN,1" is issued, the principal stresses are calculated using the specified averaging method. For "AVPRIN,0", the component stresses are averaged at common nodes and the principal stresses are calculated using the averaged components values. For "AVPRIN,1" the elemental principal stresses are calculated and then averaged at common nodes. For either setting, the calculated principal stresses are used to determine SEQV. Thus, if either "AVPRIN,0" or "AVPRIN,1" are issued, SEQV is not calculated using the SegalmanReese method, and the output will have nonzero values for the principal stresses. After "AVPRIN,0" or "AVPRIN,1" is issued, you need to exit /POST1 to activate the SegalmanReese method. 

