% model to describe altering spring stiffness
function F = e_aging(x2d,x1d,t)
  F = zeros(length(t),1);
  for ii = 1:length(t)  
    if t(ii)>=2
      F(ii) = x2d(1)*exp(x2d(2)*t(ii)^x2d(3));
    else
      F(ii) = x2d(1)*exp(x2d(2)*t(ii)^x2d(3)) + x1d(1)*(t(ii)-2)^3;
    end
  end
end