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freefree beam natural frequency
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December 10, 2009 3:07am UTC
freefree beam natural frequency
hallo friend
I first conducted a simulation to calculate the natural frequency of the first mode of a beam of 4m length, 40cm height and 70cm in width. the theoretical calculation gives me f1 = 128 Hz but Comsol gives me 0 Hz
do you have an idea about the problem?
thank you in advance
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December 14, 2009 2:29pm UTC in response to Carol Platon
Re: freefree beam natural frequency
Hi
a "0" for a structural mode means a (or several) DoF that is not blocked, if you have 6 of them at about "0" Hz, its typically that you are looking at the first 6 (x,y,z, Rx,Ry,Rz) freefree modes, and your 128 Hz will show up as mode no 7 (increase the number of mods in the solve tab to see this)
This was assuming a 3D model, if you are in 2D its 3 modes x,y,Rz that would be at "0".
If you have only 1 or 2 modes at "0" Hz then typically you have not constrained enough DoF to block one face.
I did a quick check in 3D default values (steel) I get 125.65, 206.30, Hz for mode "7 & 8". But if I fixee one of the small faces I get 20.49, 35.17 and 123.2, 161.72 Hz for the first 4 modes respectively in Rz and Ry . First, is the meshing fine enough ? by doing an overkill going from 1'794 to 273'873 Dof the frequencies changes to respectively 125.39 and 206.14 in freefree and 20.41, 35.05, 122.26, 156.20 Hz in clamped free mode, so default values are about OK
Furthermore, one must be carefull to observe the mode shapes, in freefree and in clampedfree they differ, the two first in clampedfree cannot show up in freefree because of symemtry reasons. In fact a free free mode beam behaves as a symmetric L/2 length beam hence there is a factor 4 in frequency difference for this mode between clampedfree and freefree for this specific mode, try to set a symmetry boundary codition on the small edge instead of fixed, and compare to the freefree values.
Analytically in a simplified way the first eigenmodes are typically for clampedfree f=gamma/2/pi*sqrt(K/M) with k=3*E*I/L^3 with L the beam length, E the young modulus, M the total mass (mostly expressed as m*L=M the mass per length or M=rho*h*b*L with rho the density) I=h*b^3/12 the inertia with h width, b thickness (in the normal direction of rotation) amd gamma is a (mass) partcipation factor depending on the way the beam is clamped (sqrt(gamma)=(1.875, 4.694, 7.855) for the first clampedfree modes, while sqrt(gamma)=(4.730, 7.8532) for the first freefree modes (note: these are for the same given flexure mode, while the 2 cases from COMSOL are the first modes in two perpendicuzlar directions). This gives typically for E=200[GPa] and rho= 7850[kg/m^3] the value of steel: f1 = gamma*b/4/pi/L^2*sqrt(E/rho) = 5.8[Hz]*1.875^2 = 20.38[Hz] in clampedfree and 5.8[Hz]*4.73^2=129.71[Hz] in freefree
All above in simplified nonshear theory, ignoring "nu" which is an acceptable first approcimation.
Furthermore as the heigt to width ratio is only 0.7/0.4=1.75 we should see a mode difference of this ratio (sqrt(h*b^3/b*h^3)=b/h = 1.75), indeed 35.17/20.49=1.716 and 206.3/125.65=1.64, good enough
Well as you have not precised your boundary conditions, neither the material, I might be wrong with some of the numerical values, but the rest I believe is more or less OK, no? There is quite a lot to say about mode shapes...
For the theor, have a look at "Formulas for Natural Frequency and mode shape" R.D. Blevin, Kruger, 1979, reprint 2001, ISBN 1575241846
Hope this helps
Ivar
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December 15, 2009 12:49am UTC in response to Ivar Kjelberg
Re: freefree beam natural frequency
hallo;
you wrote: a "0" for a structural mode means a (or several) DoF that is not blocked, if you have 6 of them at about "0" Hz, its typically that you are looking at the first 6 (x,y,z, Rx,Ry,Rz) freefree modes, and your 128 Hz will show up as mode no 7 (increase the number of mods in the solve tab to see this)
end.
that mean the first natural frequency occurs at the 7 mode in the case of freefree beam.
thank you very much
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April 16, 2010 9:18am UTC in response to Carol Platon
Re: freefree beam natural frequency & stress concentration structural
Hi
sorry for the late reply, but you are perfectly right, in a 3D representation of a freefreestructure it is the 7th mode that is the first interesting (nontrivial one), as the first 6 represent the free motion: 3 displacements and 3 rotations in a 3D space.
For you second question about stress concetration, I need a little more info.
Are you studying stress concetrnation at a "sharp edge" ?
in which case FEM will tend to give you "infinity" as you have a singularity at the edge. These singularities on "sharp edge effects" are typical for structural or even ACDC electric or magnetic fields.
one must "soften" the edges, to a reasonnable "engineering" level (note that: if you put a radius, the FEM meshing will simplify it to a polygonal shamfer).
If you need some litterature, I can only advise to study through:
"Peterson's Stress Concetration Factors" by W.D. Pilkey, Wiley&Sons, 1997.
For Eigenmodes take a look at:
"Formulas for natural frequencies and mode shapes" R.D. Blevins Krieger, 2001
These are classical approaches, but are the best exercices you might get to fit to FEM. It would be a nice project that someone went systematically through these two books and ported the examples to a FEM tool like COMSOL and reported the differences
Have fun Comsoling
Ivar
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April 16, 2010 1:34pm UTC in response to Ivar Kjelberg
Re: freefree beam natural frequency & stress concentration structural
this what i'm studying:
www.comsol.fr/showroom/gallery/988/
Single Edge Crack: the KI parametre converge just for 1000 elements
but the maximum value of Von Mises stress increase all time and do not converge
thank you in advance
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September 29, 2011 6:51am UTC in response to Ivar Kjelberg
Re: freefree beam natural frequency
Dear Sir do you have the soft copy of that book? i have searched it but havenot found it in library and also on net. Sir i want to find the resonance frequency of longitudinal free free microbeam. I have used the formula f=(1/2L)*sqrt(E/p) but when i simulated the beam the resonant frequency is not correct. So if you any formula for finding the resonant frequency in longitudinal beam than please tell it to me.
Regards
Abid
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September 29, 2011 11:39am UTC in response to Abid Iqbal
Re: freefree beam natural frequency
Hi
unfortunately i do only have paper books (yet ;), but if you try on Wiki or on the web I'm sure you can find the relevant analytical formulas

Good luck
Ivar
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