%The function calculates the matrix for rotating the stress tensor around z-axis.
%Inputs:
    %alpha- rotation angle.
%Outputs:
    %N- the matrix used to rotate the stress tensor around z-axis.
function N=StrRMatrixZ(alpha)
N=[cos(alpha) sin(alpha) 0;
   -sin(alpha) cos(alpha) 0;
   0 0 1];