Constitutive modeling and numerical simulation of multi-responsive light-thermal shape memory polymers

A constitutive framework is developed to describe the mechanical behavior of multi-responsive light-thermal shape memory polymers (SMPs) that undergo shape-fixation and recovery cycles driven by the combined effects of thermally induced crystallization and light-induced photochemical network formation. The model is based on the theory of multiple natural configurations, which enables the simultaneous representation of multiple coexisting networks, each with its own stress-free state. Analytical solutions are presented for uniaxial deformation under different programming conditions and varying sequences of thermal and photochemical activation. Results demonstrate that both the order of network formation and conditions during programming strongly influence the shape-fixation and recovery behavior. The model is further implemented as a user material subroutine (UMAT) in the finite element package ABAQUS. To demonstrate the applicability of the developed constitutive model, we have numerically simulated a three-dimensional geometry undergoing complex shape-fixation and recovery cycles.