How to fix symbolic errors

Question

Randy Chen 2021-11-18

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1590214-how-to-fix-symbolic-errors

回答： Star Strider 2021-11-18

I have the following function defined that will be used in a main script:

function ke = Ke2Diso(E, nu, Xe, Ye)
E_matrix = E/(1-nu^2)*[1 nu 0;nu 1 0;0 0 (1-nu)/2];
syms zeta eta ;
N1 = ((1-zeta)*(1+eta))/4;
N2 = ((1+zeta)*(1+eta))/4; 
N3 = ((1-zeta)*(1-eta))/4; 
N4 = ((1+zeta)*(1-eta))/4;
N = [N1 N2 N3 N4];
dxdzeta = 0;
dxdeta = 0 ;
dydzeta = 0;
dydeta = 0;
for i = 1:4
    dxdzeta =dxdzeta+ Xe(i)*diff(N(i),zeta);
    dxdeta =dxdeta+ Xe(i)*diff(N(i),eta);
    dydzeta =dydzeta+ Ye(i)*diff(N(i),zeta);
    dydeta =dydeta+ Ye(i)*diff(N(i),eta);
end
J = [dxdzeta dydzeta;dxdeta dydeta];
B_xy = zeros(2,4)
for i =1:4
    B_xy(1,i) = diff(N(i),zeta);
    B_xy(2,i) = diff(N(i),eta);
     
end
dNdx_dNdy = inv(J)* B_xy
%2 by 4 matrix
B_e =  [dNdx_dNdy(1,1) 0 dNdx_dNdy(1,2) 0 dNdx_dNdy(1,3) 0 dNdx_dNdy(1,4) 0;...
        0 dNdx_dNdy(2,1) 0 dNdx_dNdy(2,2) 0 dNdx_dNdy(2,3) 0 dNdx_dNdy(2,4) ;...
        dNdx_dNdy(2,1) dNdx_dNdy(1,1) dNdx_dNdy(2,2) dNdx_dNdy(1,2) dNdx_dNdy(2,3) dNdx_dNdy(1,3) dNdx_dNdy(2,4) dNdx_dNdy(1,4)];
%Gauss quadrature
B_e_new = subs(B_e)
%B_e_new = subs(B_e,[zeta,eta],[1/sqrt(3),1/sqrt(3)]) + subs(B_e,[zeta,eta],[1/sqrt(3),-1/sqrt(3)])+...
%            subs(B_e,[zeta,eta],[-1/sqrt(3),1/sqrt(3)])+subs(B_e,[zeta,eta],[-1/sqrt(3),-1/sqrt(3)])
B = B_e_new(1/sqrt(3),1/sqrt(3))+B_e_new(1/sqrt(3),-1/sqrt(3))+B_e_new(-1/sqrt(3),-1/sqrt(3))+B_e_new(-1/sqrt(3),1/sqrt(3))
E_matrix_new = subs(E_matrix)
J_new = subs(J,[zeta,eta],[1/sqrt(3),1/sqrt(3)])+subs(J,[zeta,eta],[-1/sqrt(3),1/sqrt(3)])+subs(J,[zeta,eta],[-1/sqrt(3),-1/sqrt(3)])+subs(J,[zeta,eta],[1/sqrt(3),-1/sqrt(3)])
ke = det(J_new)*(transpose(B)*E_matrix_new*B)

This script has the following error:

Unable to perform assignment because value of type 'sym' is not convertible to 'double'.
Error in Ke2Diso (line 31)
    B_xy(1,i) = diff(N(i),zeta);
    
 Error in K_assembly (line 15)
    ke = Ke2Diso(E, nu, Xe, Ye); % obtain element stiffness matrix
Error in Main2DElastic (line 24)
K = K_assembly(ne, nnodes, X, Y, E, nu, nodemap);
Caused by:
    Error using symengine
    Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for variables.
    