The interphase model applied to the analysis of masonry structures

Masonry material presents a mechanical response strongly dependent on the static and kinematic phenomena occurring in the constituents and at their joints. At the mesoscopic level the interaction between the units is simulated by means of specific mechanical devices such as the zero thickness interface model where the contact tractions and the displacement discontinuities are the primary static and kinematic variables respectively. In many cases the joint response depends also on internal stresses and strains within the interface layer adjacent to the joint interfaces. The introduction of internal stresses and strains leads to the formulation of the interphase model, a sort of enhanced zerothickness interface. With the term interphase we shall mean a layer separated by two physical interfaces from the bulk material or a multilayer structure with varying properties and several interfaces. Adopting the interphase concept, different failure conditions can be introduced for the physical interfaces and for the joint material. In the present work the interphase constitutive laws, taking into account the joint stiffness degradation and the onset of irreversible displacements, are derived in a thermodynamically consistent manner assuming an appropriate form of the Helmholtz free energy, function of the internal and contact joint strains and of other internal variables which regulate the evolution of the non-linear phenomena. The interphase model has been implemented in an open-source research-oriented finite element analysis program for 2D applications.