def _calc_damping_min(self):
        """minimum damping [decimal]"""
        return ((0.8005 + 0.0129 * self._plas_index * self._ocr ** -0.1069) *
                (self._stress_mean * KPA_TO_ATM)
                ** -0.2889 * (1 + 0.2919 * np.log(self._freq))) / 100
minimum damping [decimal]