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# Linear perturbation modal analysis

## Linear perturbation modal analysis

(OP)
Hello,

I'm trying to perform modal analysis of laminated thick beam-like structure (SOLID186 2 or 3 elements per layer height, 7 layers) with aid of linear perturbation. Problem is, I'm not exactly sure whether I understand Ansys manual correctly. I'm trying to include prestress by applying temperature (tunif) in static analysis, which will cause stress due to mismatch of thermal expansion coefficients.

!---------------------------------------------------
! Analysis settings:
! Static analysis: simply supported beam, temperature, small deformations...
! Perturbation:
antype,static,restart,,,perturb
perturb,modal,nokeep ! free-free bc
solve,elform
antype,modal,....
...
solve
finish

/post1...
!---------------------------------------------------

Due to theory (zero total external force), there should be almost none change in natural frequency of let's say 1st (bending) and second (bending perpendicular to 1st) modes, (change between prestressed and non-prestressed modal analysis). Which is what I also get from Ansys results. Manual states:

„Thermal loads from the base analysis are ignored in a linear perturbation modal or full harmonic analysis. In addition, you cannot apply thermal loads in the second phase of a linear perturbation modal or full harmonic analysis.“

When I try clamped beam (not laminate) and use temperature to increase tensile stress, there is increase in frequency as it should be.

So my understanding is that it ignores thermal loads (in F_perturbed), but it creates tangent stiffness matrix (K_t) with stress from temperature included in both analyses (simply supported + free free and clamped). Is it right? I also get same results by using pstres,on only without perturbation.

Sorry for maybe stupid question, by the way, and delete it if you wish.

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