Spontaneous fracture in shape memory polymers: Temperature-activated and constraint-induced fracture from programming

Shape memory polymers (SMPs), distinguished by their unique properties such as variable stiffness and shape memory effect, have exhibited extensive application potential. However, despite the promising applications of SMPs, their viscoelastic fracture behavior and mechanism, particularly under various loading conditions and thermal stimuli, remain insufficiently explored. This study investigates the thermo-activated viscoelastic fracture behavior of SMPs under constrained recovery and the variations in fracture toughness under different loading conditions. Thermally responsive SMPs were fabricated, and pure shear fracture tests were conducted, revealing that fracture toughness decreases significantly with increasing temperature and decreasing loading rate, attributed to reduced viscoelastic dissipation. Notably, spontaneous crack propagation is observed in stretch-programmed SMPs under thermal activation, with higher programmed stretch ratios leading to lower self-rupture temperatures. To explain this phenomenon, a viscoelastic fracture framework is established incorporating temperature-dependent relaxation and recovery strain, validated by experiments and finite element simulations. The proposed energy release rate model accurately predicts critical rupture temperatures, providing a reliable framework for failure prediction in programmable smart materials. This work bridges thermomechanical behavior and fracture mechanics, offering design insights for SMP-based structures in soft robotics and biomedical applications.