August 22, 2023 at 5:58 am

Md_Salem

Subscriber

Hello Erik,

I need to extract the mode shape vectors of some cantilever plate to make a correlation between some of them analytically.

I used the workbench to simulate the problem and added the next APDL command to extract both mass and stiffness matrices in MMF:

/AUX2

COMBINE, FULL

/POST1

*SMAT, MatKS, D, IMPORT, FULL, file.full, STIFF

*SMAT, MatMS, D, IMPORT, FULL, file.full, MASS

*Export, MatKS, MMF, matK_MMF.txt

*Export, MatMS, MMF, matM_MMF.txt

Then I did modal analysis , and I got the two files of mass and stiffness matrices in MMF.

I used the next Matlab code to solve the eigen problem in order to extract mode shapes.

clc;

clear all;

format shortG;

format loose;

load matK_MMF.txt;

K = zeros(462,462);

for r = 2:5515

K(matK_MMF(r,1), matK_MMF(r,2)) = matK_MMF(r,3);

end

disp (K)

load matM_MMF.txt;

M = zeros(462,462);

for r = 2:1999

M(matM_MMF(r,1), matM_MMF(r,2)) = matM_MMF(r,3);

end

disp(M)

cheq=linsolve(M,K)

[Mode,Lamda]=eig(cheq);

lamda=diag(sort(diag(Lamda),'ascend')); % make diagonal matrix out of sorted diagonal values of input 'Lamda'

[c, ind]=sort(diag(Lamda),'ascend'); % store the indices of which columns the sorted eigenvalues come from 'lamda'

omegarad=sqrt(lamda);

omegaHz=omegarad/pi/2

mode=Mode(:,ind)

This code ran syntactically without any errors .

I checked for the first natural frequency, omegaHz(1,1), which is supposed to be 208.53 Hz as shown in the workbench analysis , but unfortunately it was 64023 Hz .

Would you please show me what is wrong with that problem?

Or is there any possible way to extract mode shapes vectors or modal matrix directly from ANSYS ?

Regards