COMBINE SHELL AND BEAM ELEMENTS

[Chicca96]

Hi everyone!

I have a doubt and I hope there is someone who can give me an advice.

I am modeling a MEMS gyroscope for my Master thesis and I am starting with a very basic structural design. My gyroscope is composed by a central mass suspended by for beams. Since the central mass thickness is smaller than other dimensions, I modeled it with shell181 elements, while beam are modeled using beam4 elements. Since these elelements have the same DOFs, can I connect them simply sharing the nodes at interface? I ask this because I have read on the 2.3 section of ANSYS APDL 2019 R3 guide ("Limitations on Joining different elements") that tt is inconsistent to connect only one node of a 3-D beam element to a 3-D shell element such that a rotational DOF of the beam element corresponds to the ROTZ of the shell element.

Moreover, I performed a modal analysis in order to obtain the eigenfrequencies of my structure. I noticed that, if I mesh the central mass with a coarser mesh, my result is closer than the analytical one. I can't explain this and I'm scared that it is linked to the previous concept.

I share an image of the model and my APDL code (I hope it will be useful)

Thanks for the help.

Kindest,

Francesca

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
finish
/clear
/PREP7

!GEOMETRY

E=166e3
densi=2332e-18
A1=500 ! Weidth mass 1 (micron)
B1=500 ! Hight mass 1 (micron)
T1=15 ! Thickness mass 1 (micron)

!Suspension mass 1
L1X=200
L1Y=200
w1=4
! Spring
et,1,beam4
! "r", Set number, Area, Izz, Iyy, thick-z,thick-y,theta,
r,1,w1*t1,t1*w1**3/12,w1*t1**3/12,t1,w1
mp,ex,1,E
mp,dens,1,densi
mp,prxy,1,0.28


! First suspension
K,1,A1/2,B1/2,0
K,2,A1/2,B1/2+L1x,0
K,3,A1/2+L1y,B1/2+L1x,0
L,1,2
L,2,3
esize,10
lmesh,all


! Second suspension
K,4,-A1/2,B1/2,0
K,5,-A1/2,B1/2+L1x,0
K,6,-A1/2-L1y,B1/2+L1x,0
L,4,5
L,5,6
esize,10
lmesh,all

! Third suspension
K,7,-A1/2,-B1/2,0
K,8,-A1/2,-B1/2-L1x,0
K,9,-A1/2-L1y,-B1/2-L1x,0
L,7,8
L,8,9
esize,10
lmesh,all

! Fourth suspension
K,10,A1/2,-B1/2,0
K,11,A1/2,-B1/2-L1x,0
K,12,A1/2+L1y,-B1/2-L1x,0
L,10,11
L,11,12
esize,10
lmesh,all

LSEL,U, , , ALL

! Mesh mass 1
ET,2,shell181,,,2
SECTYPE,,SHELL
SECDATA,T1,
mp,ex,2,E
mp,dens,2,densi
mp,prxy,2,0.28

LSTR,1,4
LSTR,7,10
LSTR,1,10
LSTR,4,7
AL,ALL ! Creo area

ESIZE,150 ! Element length

amesh,all

Nummrg,node,0 !Merges coincident or equivalently defined nodes.

! Boundary condition

Ksel,s,,,3
nslk,s
d,all,all

Ksel,s,,,6
nslk,s
d,all,all

Ksel,s,,,9
nslk,s
d,all,all

Ksel,s,,,12
nslk,s
d,all,all


/SOLU
nsel,all
antype,modal
modopt,subsp,4
mxpand,4,,,yes
nsel,all
solve

/POST1 ! List solutions
SET,LIST
SET,FIRST