% model to describe aging dashpot viscosity
function F = eta_aging(xeta2d,xeta1d,t)
  F = zeros(length(t),1);
  for ii = 1:length(t)
    if t(ii)>=2
      F(ii) = xeta2d(1)*t(ii)^xeta2d(2);
    else
      F(ii) = xeta2d(1)*t(ii)^xeta2d(2) + xeta1d(1)*(t(ii)-2)^3;
    end
  end
end

