The investigation of the mechanical behavior of nonlinear composite metamaterials is essential for optimizing their performance across different engineering applications, in order to ensure structural integrity and prevent premature collapse. To this end, it is crucial to develop advanced theoretical and numerical models capable of reproducing the structural performance of such materials under failure conditions, with reference to both their static and dynamic nonlinear response. In addition, the opportunity to manufacture innovative materials with multifunctional properties as a result of new manufacturing technologies, makes it necessary to pay special attention to the design and the optimization of their microstructural geometry, particularly when instability phenomena are involved. To this end, the present work proposes a theoretical and numerical study for the investigation of the microscopic failure behavior of composite metamaterials along combined shear-compression loading path, assessing the influence of interfacial debonding and contact mechanisms on the onset of micro instability and bifurcations. The investigations are carried out via an advanced nonlinear homogenization approach that accounts for progressive irreversible decohesion and interface contact within a finite strain continuum mechanics framework [1]. Numerical results, carried out on a representative volume element assuming a Neo-Hookean hyperelastic behavior, assess the critical load factors related to primary instability and bifurcation revealing how the interplay between contact, decohesion and instability affects the performance of periodic microstructured materials. REFERENCES [1] F. Greco, D. Gaetano, R. Luciano, A. Pranno, G. Sgambitterra, A theoretical analysis of instability and bifurcation failure phenomena in periodic microstructured nonlinear composite solids embedding discontinuity interfaces, Meccanica, 60 (10-11), pp. 3169-3199, 2025.