Maximum strain response not matching eigenfrequencies

[OscarPacheco]

Hello.

I am doing a modal response analysis to one component which is bolted in its upper part. I first do a pretension step to take into account the force of the bolt, then I do *frequency step to obtain the natural frequencies and then I apply a unit sine sweep to obtain the responses. I am running in two different problems:
1) The stresses from the pretension step are only present in the frame 0 of the rest of the steps, but in the following frames it seems to dissapear.
2) If I plot, for example, the strain with respect to the frequency, the peaks of strain are not produced in the frequencies corresponding to eigenvalues (see image attached)

Can anyone help me? Thank you so much

Here is an example of the code I am using:

*AMPLITUDE, NAME=A1
20, 1, 1500, 1
**
*NMAP, NSET=NALL, TYPE=SCALE
0,0,0
1e-3, 1e-3, 1e-3
*BOUNDARY, FIXED
BC, 1,3
BC_Bolt, 1,3
**
**=============================================================================
** Pretension step
**=============================================================================
*STEP, name=preTension_of_M8_screw, NLGEOM=YES
*STATIC
*CLOAD
SolidPretension1_Frc, 1, -15e3
*OUTPUT, FIELD
*ELEMENT OUTPUT,ELSET=membrane
S
**#****NODE OUTPUT
**#***U
*END STEP
**
**=============================================================================
** Eigenfrequency Check
**=============================================================================
*Step, name=eigFreq_extraction, perturbation
*Frequency, eigensolver=Lanczos, acoustic coupling=off, normalization=mass
50, 0, 2000., , ,
*BOUNDARY, FIXED
SolidPretension1_Frc, 1,3
*OUTPUT, FIELD
**ELEMENT OUTPUT, ELSET=PartBody
**NODE OUTPUT
*END STEP
**=============================================================================
** Skaka Z
**=============================================================================
*STEP, name=SinusoidalAccZ, perturbation
Frequency response Z
*STEADY STATE DYNAMICS, INTERVAL=EIGENFREQUENCY
20, 1500, 20, 5
*MODAL DAMPING,MODAL=DIRECT
1, 100, 0.011
*BASE MOTION, DOF=3 , TYPE=ACCELERATION, AMP=A1
*OUTPUT, FIELD
*ELEMENT OUTPUT, ELSET=membrane
S,E
*NODE OUTPUT
*END STEP

 

[JBlack68]

the peak stress occurs at the damped frequencies, are they not

 

[OscarPacheco]

Thanks for your reply. Do you mean then that this results are ok? The resonant frequency is for example 223,57 (from the .dat file, calculated in the second step) and the damped natural frequency is 220, 691 (from the graph, calculated in the third step). Damped frequency is a little bit lower than the resonant frequencies.
Thanks for your view

 

[Mustaine3]

Maybe that's the problem in your understanding...

Load magnitudes (including the magnitudes of prescribed boundary conditions) during a linear perturbation analysis step are defined as the magnitudes of the load perturbations only. Likewise, the value of any solution variable is output as the perturbation value only—the value of the variable in the base state is not included.

 

[JBlack68]

what is the damping used? 15.9% by my estimate !!!
so you need to make sure to output at the nmodes calculated