In the modeling of fastening and bonding applications, an accurate description of interphases is essential. Interphases are thin regions where deformation localizes and significant plastic dissipation may occur. In standard finite element formulations, resolving these regions requires very fine meshes, leading to high computational cost. To alleviate this difficulty, reduced-order interface models have been developed, in which the interphase behavior is projected onto a lower-dimensional interface representation [1, 2, 3, 4]. Most existing interface models are restricted to limiting cases of either soft or stiff interphases [2], often depend on a specific choice of coordinate system [2, 4], and may suffer from numerical instabilities when implemented in conventional finite element frameworks[1, 3]. These limitations reduce their applicabil- ity to more general interphase configurations commonly encountered in engineering practice. In this work, we propose an extended generalized interface model that overcomes these restrictions. The formulation is based on Taylor expansions with respect to a small parameter and employs separate limiting procedures for the interphase thickness and material properties. This strategy enables a unified treatment of soft, stiff, and intermediate interphases within a single framework. The resulting reduced- order model accurately captures the mechanical response of the interphase over a broad range of material parameters while remaining robust in standard finite element implementations. The proposed framework naturally incorporates viscoelastic effects, allowing the modeling of creep phe- nomena across different interphase stiffness regimes. Owing to its conceptual simplicity, the model can be readily extended to account for hardening or softening plasticity as well as large deformation kine- matics. Overall, the proposed formulation provides a versatile and computationally efficient tool for the analysis of complex interphase behavior in bonded and fastened structures. REFERENCES [1] Benveniste Y., & Miloh T., Imperfect soft and stiff interfaces in two-dimensional elasticity, Me- chanics of materials, 33(6), 309-323, 2001. [2] Dumont S., Rizzoni R., Lebon, F. & Sacco E., Soft and hard interface models for bonded elements, Composites Part B: Engineering, 153, 480-490, 2018. [3] Gu S. T., Liu J. T., & He Q. C., The strong and weak forms of a general imperfect interface model for linear coupled multifield phenomena, International