Anisotropic Mechanical Responses and Enhanced Recovery Force of Knitted Sandwich-Structured Shape Memory Polymer Composites

Abstract

Knitted-fabric reinforced shape memory polymer composites (SMPCs) offer exceptional structural designability; however, the intricate coupling between their hierarchical architecture and synergistic thermo-mechanical performance remains insufficiently explored. In this study, a series of sandwich-structured SMPCs were fabricated using a rib-knitted fabric core and plain-woven face sheets, specifically investigating the effects of layer count, loop orientation (0°/90°), and core configurations on their mechanical and shape memory behaviors. The results reveal that the mechanical properties are dominated by the knitted architecture, where the 90° orientation exhibits significantly higher flexural strength than the 0° orientation due to the interlocking mechanism of loop pillars. While the introduction of fabric reinforcement slightly constrains the shape recovery ratio (R_r), it dramatically elevates the recovery force. Notably, the two-layers core SMPC in 90° direction achieves a peak recovery force of 15.6 N at 2% strain, representing a 6.0-fold increase over the neat shape memory epoxy polymer (SMEP). Furthermore, both shape recovery ratio and recovery force exhibited a non-monotonic parabolic trend with increasing bending strain, peaking at a critical threshold of 8%. This phenomenon is attributed to the energy-damage competition between increased elastic energy storage and irreversible structural degradation, such as loop over-stretching and interfacial debonding. Fractographic analysis confirms that failure is primarily driven by the mismatch in deformation capacity between the surface woven layers and the knitted core. This work provides a useful evidence for the synergistic optimization of load-bearing and actuation functions in next-generation intelligent sandwich-structured SMPCs.