Polymer matrix composites functionalized with micrometer-sized ferromagnetic particles are widely used to introduce electromagnetic absorption capabilities in the microwave domain. While their electromagnetic performance is one field of active research [1], the associated changes in mechanical properties present another challenge due to strong stiffness contrast between matrix and particle, complex interface behavior, heterogeneity in particle distribution, and time-dependent matrix response. The homogenization concepts of such composites are often based on a representative volume element (RVE) to link the microstructure to the macroscopic response of the material. Although several models incorporate debonding [2] and viscoelastic/viscoplastic effects [3], coupling particle microstructures with progressive interface damage and rate effects remains comparatively challenging and is not yet routine in micromechanical RVE practice for particle-filled polymer matrices. In this study, tests are performed under uniaxial loading and compared to RVE models with and without particle debonding through cohesive zone contacts and matrix nonlinearity. This allows the isolated impact of each mechanism on the macroscopic stress–strain response and on local strain concentrations to be assessed. Including debonding leads to pronounced strain localization at the particle–matrix interface. The numerical results are compared with miniaturized tensile tests in which strain fields and effect localization are captured using in-situ digital image correlation inside a scanning electron microscope (SEM). Pattern generation is achieved through low pressure oxygen plasma treatment of the specimens. [4] The presented observations provide a link between microscale damage nucleation and the onset of macroscopic failure, and allow an estimate of the failure properties to be derived from the modeled response. Although the finite element model is based on idealized RVEs with spherical particles, deviations between simulations and experiments are discussed in relation to the real composite microstructural heterogeneity. The influence of shape variations and local clustering is considered. Overall, the results show that interface degradation and matrix viscosity are key drivers of the nonlinear mechanical response of particle-filled epoxies and must be accounted for in multiscale modeling frameworks intended for material design and performance optimization beyond the elastic regime.