function y = fcn(nu_r,D)

D_nl = [D(1,1)*abs(nu_r(1)) zeros(1,5);
        0 D(2,2)*abs(nu_r(2)) zeros(1,4);
        0 0 D(3,3)*abs(nu_r(3)) 0 0 0;
        0 0 0 D(4,4)*abs(nu_r(4)) 0 0;
        0 0 0 0 D(5,5)*abs(nu_r(5)) 0;
        zeros(1,5) D(6,6)*abs(nu_r(6))];

y = D_nl;