Principal Stresses calculation problem 3

[NEOSOULUS]

Dear All,
I have a structure for which i have carried out frequency response analysis using Hypermesh as pre-processor and NASTRAN as solver. I have the following queries:
1) In the .f06 output file, the principal stresses are not printed.
2) I have used hex-hedral meshing for the entire structure. After carrying out the meshing and frequency response analysis only Normal shear stress are printed. When i use the formula(see attachment) to calculate 3d principal stresses, at some points, i am unable to calculate the angle due to incorrect values(of the terms inside bracket of cos inverse-see attachment). Please tell me how can i resolve this issue? Moreover i have not given any damping value for the material during frequency response.

Please advise.

 

[hezhj2000]

The f06 file will not have principal stress output for Sol 111. You can use a post-processor to get the principal stress with output files like *.xdb, *.op2 or *.DBALL/MASTER.

 

[NEOSOULUS]

Thank you very much your single line helped me out in getting out of the confusion i had for the last two weeks. I have extracted the principal stresses from the program.
But now the problem is that what does "negative" values of principal stresses signify? Can you help me out in understanding that?

 

[nlgyro]

compression