Quantifying hinge torque in self-folding, shape memory polymer origami under varying thermal loads

Self-folding origami actuated by thermally responsive shape memory polymers (SMPs) presents a promising mechanism for the deployment of space structures. Pre-strained SMP sheets shrink when heated. The SMP sheets are patterned with black ink hinges that heat locally via a surface heat flux when exposed to infrared (IR) light. The localized heating induces gradients in shrinking, or shape recovery, which in turn produces folding. However, the thermal and mechanical loads applied to the structure during deployment pose unique challenges to the application of SMP as actuators. Herein, we investigate the effects of thermal loads (variable IR flux and convection) and folding resistance (mass moment of inertia) on self-folding behavior of polystyrene (PS) SMP sheets. We analyze digital video recordings to determine the transient bending angle (the angle traversed by the folding face of the SMP sheet) and hinge torque. It is observed that higher heat fluxes and reduced convection lead to higher maximum bending angles, lower fold start times, and higher hinge torques. We found that the measured hinge torque increases with increasing folding resistance. In addition, we highlight the significance of convective heat transfer by demonstrating self-folding under vacuum at IR flux intensities as low as 2,622 W/m2 at a wavelength of 980 nm and identify a minimum surface heat flux required to induce self-folding in the presence of convection. Comparisons to dynamic mechanical analysis (DMA) and a thermal finite element analysis (FEA) offer insight to the initiation of folding under varying thermal loads. These results provide a necessary understanding of the effects of varying thermal loads on the behavior of self-folding origami for deployable space structures.