Strain-compatible complex shear modulus [kN/m²].

def comp_shear_mod(self):
        """Strain-compatible complex shear modulus [kN/m²].
        """
        damping = self.damping
        if COMP_MODULUS_MODEL == 'seed':
            # Frequency independent model (Seed et al., 1970)
            # Correct dissipated energy
            # Incorrect shear modulus: G * \sqrt{1 + 4 \beta^2 }
            comp_factor = 1 + 2j * damping
        elif COMP_MODULUS_MODEL == 'kramer':
            # Simplifed shear modulus (Kramer, 1996)
            # Correct dissipated energy
            # Incorrect shear modulus: G * \sqrt{1 + 2 \beta^2 + \beta^4 }
            comp_factor = 1 - damping ** 2 + 2j * damping
        elif COMP_MODULUS_MODEL == 'dormieux':
            # Dormieux and Canou (1990)
            # Correct dissipated energy
            # Correct shear modulus:
            comp_factor = np.sqrt(1 - 4 * damping ** 2) + 2j * damping
        else:
            raise NotImplementedError
        comp_shear_mod = self.shear_mod * comp_factor
        return comp_shear_mod