Plotting stress tensor in MATLAB, Parametric Surface

Xaspire88

First off, I'm not sure if matlab questions go here or not, or if they are even dealt with on this forum. If not more or remove I guess.

I am no matlab wiz so this is mind-boggling to me. I am given a state of stress at a critical point of a component and told to plot a 3D parametric surface of the normal stress, but in spherical coordinates.*see attached figure*




I tried plotting the equation given for sigma_nn for the indicated values of theta(0:2pi) and phi(0:pi), and it output a surface but I was expecting to get a spherical shape, and this was no where near spherical.

Some of my matlab code:

>> sigma=[200 100 120;100 350 100;120 100 300];
>> theta=sym('theta');
>> phi=sym('phi');
>> n=[cos(theta)*sin(phi);sin(theta)*sin(phi);cos(phi)];
>> sigma_normal=sigma*n*n';
>> theta=0:0.2:2*pi;
>> phi=0:0.1:pi;
>> Z=eval(sigma_normal);
>> Z=abs(Z);
>> surf(Z)

any direction or advice as how to correctly plot a parametric surface in matlab such as this would be much appreciated.
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution