Hi Novak,
The following code works for me. I made the following changes.
1) Used for loop
2) Removed sigma_theta from the last line. This is because plot3 command accepts only three inputs.
3) Used linspace command to include the radius range.
Thanks
Sijo George
%%%%----Updated code
sigma_inf=100;
a=10;
theta=[0:0.1:pi/2];
r = linspace(0,100,16);
for i = 1:length(r)
sigma_rr = (((sigma_inf/2)*(1-(a/r(i))^2))+(sigma_inf/2)*(1-4*(a/(i))^2+3*(a/r(i))^4))*cos(2*theta);
end
for i = 1:length(r)
sigma_theta =(((sigma_inf/2)*(1+(a/r(i))^2))-(sigma_inf/2)*(1+3*(a/r(i))^4))*cos(2*theta);
end
plot3(theta,r,sigma_rr);