A constitutive model for the impact behavior of FFF 3D-printed Tough PLA

Advancements in Fused Filament Fabrication (FFF) have enabled the production of polymers with enhanced properties. However, their complex mechanical behavior, especially under impact, remains difficult to model. This study introduces a constitutive model for FFF-printed Tough PLA that accounts for anisotropy, strain-rate sensitivity, strain softening, and tension–compression asymmetry. Quasi-static and dynamic impact tests were conducted to calibrate and validate the model. A Three-Network Viscoplastic (TNV) framework was developed to capture the material’s viscoelastic, viscoplastic, and hyperelastic responses. The model incorporates two networks based on the Yeoh hyperelastic formulation with a power-law description of elastic and viscoplastic anisotropy, while a third network employs the anisotropic Holzapfel–Gasser–Ogden–Bergstrom (HGOB) model. The TNV model predicted the uniaxial stress–strain response with an average fitting error of 9.91%, successfully capturing the nonlinear response, strain-rate dependence, and strain-softening behavior observed in tensile experiments. In addition, stress relaxation tests revealed a reduction in stress from approximately 20 MPa to 18.7–19.0 MPa, corresponding to a normalized relaxation of about 5–7% across the different printing orientations. Tension–compression asymmetry was also quantified, with minor deviations observed under compressive loading. Overall, the proposed TNV model provides a robust predictive framework for the mechanical response of FFF-printed Tough PLA across loading conditions.