If I understood well, you are looking for vibration modes restricted to the plate plane. They are not usually the first frequencies and will appear between flexural modes. So, to make it easy just constrain all other displacements for all the nodes, e.g. with a plate aligned with Oxy plane just put UZ, ROTX, ROTY equal to zero and then your analysis will give only the in-plane modes. Hope it helps.

Thanks Miguel, yes its is the case in fact. I could do it using the plane 182 element which do not have out of plane displacement i.e. z displacement.

I should mention that I'm a new user and dont know much about ANSYS. Now I'm trying to get the nodal displacement data so that I can do curve fitting. i.e. for u(x,y) and v(x,y). Please if you can guide me to export the nodal displacements it will be highly appreciable.

Dear Shahid,

I have this old Ansys input file that maybe can help you:

!*************************************************************

! example: composite 0/90.

! 176 nodes

! in-plane modes

!*************************************************************

/PREP7

!************** input *************

altura=0.450 !m height

largura=0.350 !m width

esp=0.0030 !m thickness

EXX=21e10 !modulus of elasticity

EYY=21e10 !modulus of elasticity

GXY=8.08e10 !shear modulus

vXY=0.3 !poisson coefficient

vYX=(EYY/EXX)*vXY !poisson coefficient

dens=7.8e3 !material density

!******************* element *************

ET,1,SHELL93 ! shell with 8 nodes

!ET,1,SHELL63! shell with 4 nodes

! keyopt for shell63

!KEYOPT,1,1,1

!KEYOPT,1,2,0

!KEYOPT,1,3,0

!KEYOPT,1,5,0

!KEYOPT,1,6,0

!KEYOPT,1,7,0

!KEYOPT,1,8,0

!KEYOPT,1,9,0

! **************************************************************

R,1,esp ! real constant

MPTEMP,1,0

MPDATA,EX,1,,EXX

MPDATA,Ey,1,,EYY

MPDATA,PRXY,1,,vXY

MPDATA,GXY,1,,GXY

MPDATA,DENS,1,,dens

BLC4,0,0,altura,largura

!****************************************************************

! rectangular grid with 0.035 m in y and 0.030 m in x

FLST,5,2,4,ORDE,2

FITEM,5,2

FITEM,5,4

CM,_Y,LINE

LSEL, , , ,P51X

CM,_Y1,LINE

CMSEL,,_Y

!*

LESIZE,_Y1,0.035, , , , , , ,1

!*

FLST,5,2,4,ORDE,2

FITEM,5,1

FITEM,5,3

CM,_Y,LINE

LSEL, , , ,P51X

CM,_Y1,LINE

CMSEL,,_Y

!*

LESIZE,_Y1,0.030, , , , , , ,1

!*

MSHKEY,0

CM,_Y,AREA

ASEL, , , , 1

CM,_Y1,AREA

CHKMSH,'AREA'

CMSEL,S,_Y

!*

AMESH,_Y1

!*

CMDELE,_Y

CMDELE,_Y1

CMDELE,_Y2

!***************************************************************

!opcao 1- restriction in z displacement (for shell63 and shell93)

!***************************************************************

n_nos=501 !nodes number for shell93-elem with 8 nodes

FLST,2,n_nos,1,ORDE,2

FITEM,2,1

FITEM,2,-n_nos

/GO

D,P51X, , , , , ,UZ, , , , ,

!******************************************************************

FINISH

!******************************************************************

/SOLU ! solution

ANTYPE,2 ! modal analysis

MSAVE,0

! option 2

MODOPT,LANB,20 ! output: 198 first modes

! discards 176 and plus the rigid body movements

EQSLV,SPAR

MXPAND,20, , ,0

LUMPM,0

PSTRES,0

MODOPT,LANB,20,0,0, ,OFF

SOLVE

And, I have this article that maybe help you with the experimental in-plane mode analysis from a rectangular plate.

Thanks dear Fabiabo Batisa, I'll check into it.