Notes on constitutive modeling of 3D-printed PLA materials.

Materials based on polylactic acid (PLA) are now extensively utilized across various fields, including biomedical engineering, where they are increasingly favored for the development of implantable devices and tissue replacements. Due to its ease of melting, PLA is also among the most commonly used materials in 3D printing. This makes PLA-based materials highly attractive for the production of customized implants and personalized medical devices. To support the design of such components, the computer simulations involved must rely on reliable constitutive models. The current approaches for PLA-based 3D printed material models include linear elasticity, linear viscoelasticity or elastoplasticity, hyperelasticity, and advanced nonlinear theories dealing with irreversible processes. While linear theories are undoubtedly simplistic, nonlinear models frequently result in complex descriptions which are dependent on too many material parameters. In this context, our study aims to show the ability of the Quasi-Linear Theory of Viscoelasticity to accurately reproduce the tensile test results for PLA-based materials produced using Fused Deposition Modeling. We will demonstrate this with tensile test results performed on PLA-PHB strips with TAC added as a plasticizer. The nonlinear stress-strain curves obtained from the experiments were successfully fitted by using a model compatible with finite strain theory, comprising only three material parameters. One of these parameters relates to equilibrium elasticity, while the other two correspond to the Maxwell element, which describes dissipative behavior. These results highlight the potential of this modeling approach to capture the essential aspects of the mechanical response using a minimal parameter set, which offers a promising balance between simplicity and predictive power for applications in simulation-based design.