why is not equal

sigma_0 = 2;
theta_deg = [0, 90, 180, 270]; 
r = 1:0.1:2;
shear_Q = zeros(length(r), length(theta_deg));
shear_Q_rad = shear_Q;
for i = 1:length(theta_deg)
       theta = theta_deg(i);
       thetaR = deg2rad(theta);
       ksi = 1./r;
       shear_Q(:, i) = (-1/2.*sigma_0.*(1-(3.*(ksi.^4))+(2.*(ksi.^2))).*sind(2.*theta));
       shear_Q_rad(:, i) = (-1/2.*sigma_0.*(1-(3.*(ksi.^4))+(2.*(ksi.^2))).*sin(2.*thetaR));
end
shear_Q
shear_Q = 11x4
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0

Of course, 2 times each of the angles in theta_deg makes the angle a multiple of 180 degrees, and the sine of an integer multiple of 180 degrees gives us 0.

shear_Q_rad
shear_Q_rad = 11x4
1.0e-15 *

         0         0         0         0
         0   -0.0740    0.1479   -0.2219
         0   -0.1154    0.2308   -0.3461
         0   -0.1388    0.2775   -0.4163
         0   -0.1518    0.3036   -0.4554
         0   -0.1588    0.3175   -0.4763
         0   -0.1621    0.3242   -0.4862
         0   -0.1632    0.3265   -0.4897
         0   -0.1631    0.3261   -0.4892
         0   -0.1621    0.3242   -0.4864

Now why are the elements of shear_Q_rad not all zero? Go ahead and type out the exact transcendental value of the constant π. Go ahead, I'll wait for you to get to the last digit. Roundoff error creeps in when you convert the angle in degrees into the angle in radians.

Alternately, you may find the sinpi function useful.