%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2021-02-19 21:28:20
 % @LastEditTime: 2021-02-19 21:54:44
%--------------------------
function F = deform_grad(dN, edofMat, U)

    I = eye(3);
    eleNum = size(edofMat, 1);
    F = cell(8, 1);
    
    for gp = 1:8
        B = dN{gp};
        Fe = zeros(9, eleNum);
        for ele = 1:eleNum
            edof = edofMat(ele, :);
            ue = U(edof); 
            Fe(:, ele) = I(:) + B * ue;
        end
        F{gp} = Fe;
    end
end