In the field of engineering, a major challenge is reducing the weight of structures to improve their performance and functionality for specific applications. To achieve this, researchers are focusing on both optimizing the structure and developing new materials that have superior thermo-mechanical properties but are lightweight. One such material is polymer nanocomposites, which combine the desirable properties of polymers with the unique features of nanoparticles. Studies have shown that boehmite nanoparticle (BNP)/epoxy nanocomposites are among the most promising composites for lightweight structures [1]. To advance material innovation, this study aims to develop a finite deformation phase-field formulation for BNP/epoxy nanocomposites and investigate the effect of hygrothermal conditions on their viscoelastic-viscoplastic fracture behavior under cyclic loading [2, 3]. The formulation incorporates a definition of the Helmholtz free energy, which considers the effect of nanoparticles, moisture content, and temperature. The free energy is additively decomposed into a deviatoric equilibrium, a deviatoric non-equilibrium, and a volumetric contribution, with distinct definitions for tension and compression. The proposed derivation offers a realistic modeling of damage and viscoplasticity mechanisms in the nanocomposites by coupling the phase-field damage model with a modified crack driving force and a viscoelastic-viscoplastic model. Numerical simulations are conducted to study the cyclic force-displacement response of both dry and saturated nanocomposite samples, considering BNP contents and temperature. [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.