Reducing structural weight is a critical challenge in engineering, driving the need for optimized designs and lightweight materials with superior thermo-mechanical properties. Polymer nanocomposites, which merge the favorable characteristics of polymers with the unique features of nanoparticles, have emerged as promising candidates for such applications. In particular, boehmite nanoparticle (BNP)/epoxy nanocomposites have shown great potential for lightweight structures [1]. However, nanoparticle agglomeration within the polymer matrix can diminish reinforcement efficiency. To address this, we develop a molecular simulation-informed phase-field formulation for BNP/epoxy nanocomposites that investigates the effects of nanoparticle agglomerate size and content on fracture behavior [2, 3]. Coarse-grained molecular simulations of agglomerated BNP/epoxy systems are performed to study nanoparticle/polymer interactions, which in turn guide the calibration of an amplification factor for the phase-field model. The phase-field formulation incorporates a Helmholtz free energy definition that accounts for nanoparticle influence and temperature, decomposing the energy additively into deviatoric equilibrium, deviatoric non-equilibrium, and volumetric components—with distinct treatments for tension and compression. By coupling a phase-field damage model featuring a modified crack-driving force with a viscoelastic-viscoplastic framework, the model realistically simulates damage and viscoplasticity mechanisms. Numerical simulations further evaluate the force-displacement response of nanocomposite samples as a function of BNP size and content. REFERENCES [1] B. Arash, W. Exner and R. Rolfes. A viscoelastic damage model for nanoparticle/epoxy nanocomposites at finite strain: A multiscale approach. Journal of the Mechanics and Physics of Solids, 128:162–180, 2019. [2] B. Arash, W. Exner and R. Rolfes. A finite deformation phase-field fracture model for the thermo-viscoelastic analysis of polymer nanocomposites. Computer Methods in Applied Mechanics and Engineering, 381:113821, 2021. [3] B. Arash, W. Exner and R. Rolfes. Effect of moisture on the nonlinear viscoelastic fracture behavior of polymer nanocompsites: a finite deformation phase-field model. Engineering with Computers, 39:773-790, 2023.