returns the G_r shear modulus

def g_reuss(self):
        """
        returns the G_r shear modulus
        """
        return 15. / (8. * self.compliance_tensor.voigt[:3, :3].trace() -
                      4. * np.triu(self.compliance_tensor.voigt[:3, :3]).sum() +
                      3. * self.compliance_tensor.voigt[3:, 3:].trace())