eft ( \dot{\delta}_{n}^{0}R_{eff}E_{eff}^{2}\sqrt{m_{eff}} \right )^{2/5} \left ( 1+\beta^{-2} \right )$$
%
% * *Equivalent time*:
%
% $$K_{n} = 1.198 \left ( \dot{\delta}_{n}^{0}R_{eff}E_{eff}^{2}\sqrt{m_{eff}} \right )^{2/5} \left ( exp \left ( -\frac{tg^{-1}(\beta)}{\beta} \right ) \right )^{2}$$
%
% The unloading stiffness coefficient $K_{n}^{U}$ can be computed by 2
% different formulas:
%
% * *Constant*:
%
% $$K_{n}^{U} = \frac{K_{n}^{L}}{e^{2}}$$
%
% * *Variable*:
%
% $$K_{n}^{U} = K_{n}^{L} + SF_{n}