Accessing entries of symfun

10 visualizzazioni (ultimi 30 giorni)
Ali Baig
Ali Baig il 17 Nov 2020
Risposto: Ameer Hamza il 17 Nov 2020
I am trying to solve a structural mechanics problem for which I have defined
clear all
close all
clc
syms psi(x_1, x_2, x_3)
syms E nu
u_1 = diff(psi, x_1, 1);
u_2 = diff(psi, x_2, 1);
u_3 = diff(psi, x_3, 1);
epsilon_1 = diff(u_1, x_1, 1);
epsilon_2 = diff(u_2, x_2, 1);
epsilon_3 = diff(u_3, x_3, 1);
gamma_12 = diff(u_1, x_2, 1) + diff(u_2, x_1, 1);
gamma_13 = diff(u_1, x_3, 1) + diff(u_3, x_1, 1);
gamma_23 = diff(u_2, x_3, 1) + diff(u_3, x_2, 1);
epsilon = [epsilon_1; epsilon_2; epsilon_3; gamma_23; gamma_13; gamma_12];
C = E / ((1 + nu) * (1 - 2 * nu)) * [1 - nu nu nu 0 0 0; ...
nu 1 - nu nu 0 0 0; ...
nu nu 1 - nu 0 0 0; ...
0 0 0 (1 - 2 * nu) / 2 0 0; ...
0 0 0 0 (1 - 2 * nu) / 2 0; ...
0 0 0 0 0 (1 - 2 * nu) / 2];
sigma = C * epsilon
Since the size of C is 6 x 6 and the size of epsilon is 6 x 1; the order of sigma should be 6 x 1. However when I use
size(sigma)
I get 1 x 1, which does not make sense to me. Also, I want to access individual entries of sigma. How can I do that?

Risposta accettata

Setsuna Yuuki.
Setsuna Yuuki. il 17 Nov 2020
Modificato: Setsuna Yuuki. il 17 Nov 2020
you can try with formula():
clear all
close all
clc
syms psi(x_1, x_2, x_3)
syms E nu
u_1 = diff(psi, x_1, 1);
u_2 = diff(psi, x_2, 1);
u_3 = diff(psi, x_3, 1);
epsilon_1 = diff(u_1, x_1, 1);
epsilon_2 = diff(u_2, x_2, 1);
epsilon_3 = diff(u_3, x_3, 1);
gamma_12 = diff(u_1, x_2, 1) + diff(u_2, x_1, 1);
gamma_13 = diff(u_1, x_3, 1) + diff(u_3, x_1, 1);
gamma_23 = diff(u_2, x_3, 1) + diff(u_3, x_2, 1);
epsilon = [epsilon_1; epsilon_2; epsilon_3; gamma_23; gamma_13; gamma_12];
C = E / ((1 + nu) * (1 - 2 * nu)) * [1 - nu nu nu 0 0 0; ...
nu 1 - nu nu 0 0 0; ...
nu nu 1 - nu 0 0 0; ...
0 0 0 (1 - 2 * nu) / 2 0 0; ...
0 0 0 0 (1 - 2 * nu) / 2 0; ...
0 0 0 0 0 (1 - 2 * nu) / 2];
sigma = C * epsilon;
sigma = formula(sigma) % -------- %

Più risposte (2)

Walter Roberson
Walter Roberson il 17 Nov 2020
sigma is a function rather than an array. It is a 1x1 symfun object. It does not have individual entries: it is a formula.
You can invoke the function on symbolic arguments to get back the array that would be the result for those arguments.
Or you can use
feval(symengine, 'op', sigma)

Ameer Hamza
Ameer Hamza il 17 Nov 2020
sigma is a symbolic function. You need to gives inputs to get a 6x1 matrix. For example, check
size(sigma(1,1,1))
For a general case, you can write
sigma = C * epsilon
Sigma = sigma(x_1, x_2, x_3)

Prodotti


Release

R2019a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by