Variable-Stiffness and Shape-Morphing Structured Media

Citation

Lu, Tracy (2025) Variable-Stiffness and Shape-Morphing Structured Media. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7wvv-cs52. https://resolver.caltech.edu/CaltechTHESIS:04022025-010011484

Abstract

Advancements in additive manufacturing and material synthesis with highly controlled geometries have enabled the creation of structured media, engineered materials with patterned micro- and meso-scale geometries that impart unique mechanical properties. By fine-tuning these architectures, structured materials can achieve properties beyond those of their base materials. A subcategory, structured fabrics, consists of discrete granular particles rather than continuous fibers. Their mechanical behavior is governed by jamming, a transition driven by geometric constraints, allowing them to switch between flexible and rigid states. By leveraging the interactions of the building blocks, structured fabrics enable tunable stiffness, global shape change, and adaptive functionalities, making them ideal for wearable, deployable, and morphing structures.

The first structured fabric study explores a topologically interlocking material (TIM) system with adjustable bending stiffness controlled by external pre-stress. The system consists of truncated tetrahedral particles connected by tensioned nylon wires, allowing stiffness to be tuned by varying wire tension. Experiments examine the effects of surface friction and interlocking angle on bending response, guided by Level Set Discrete Element Method (LS-DEM) simulations. The second design presents deployable 3D structures that fold without rigid mechanisms, offering compact storage and stable deployment. The design consists of computationally generated rigid tiles adhered to a pre-stretched elastic sheet, which transforms from a flat state and jams into a predetermined 3D shape when released. Although the designs exhibited unique mechanical properties, experimentally understanding their internal mechanics was challenging due to limited visibility of the concealed membrane upon jamming. To optimize future designs, simulations were conducted to analyze the effects of various pattern designs and folding on membrane behavior.

Thesis Files