COMPUTE_DFNLDX

function dfnl = compute_dfnldx(obj,x,xd)

This function computes the Jacobian of nonlinear internal force with respect to the displacements x in a second-order mechanical system. Currently, this function does not support velocity dependent nonlinearities.

assert(obj.order == 2, ' dfnldx can only be computed for second-order systems')

dfnl = sparse(obj.n,obj.n);
for j = 1:length(obj.fnl)
    dfnl = dfnl + expand_tensor_derivative(obj.fnl(j),x);
end