Adventitious Bioinspiration: Understudied Loading Modes of Wood and Hierarchical Structures in Soft Robot Materials
Adventitious bioinspiration describes the observation of a biological phenomenon and work to understand and replicate it. The dissertation examines how this philosophy can be applied to familiar model organisms in understudied loading modes, or familiar bioinspired concepts to understudied applications. That wood achieves high strength and toughness through its cellular solid structure is well known, but how this macrostructure interacts with its mesostructural anatomy of distinct cell types and cell wall behavior is less understood, especially in non-quasistatic, compressive loading modes. We propose that by studying wood in impact and torsion, novel bioinspiration may be obtained from perhaps the oldest known and most widely used biological material.
First, impact testing was performed to understand why only certain species of wood were used to craft striking weapons and fortifications across eras and civilizations. It was hypothesized that these tree species combined unique anatomical features that allowed high energy absorption in low velocity dynamic loading. Scanning electron microscopy (SEM) and micro-computed tomography (μ-CT) revealed that trees with the greatest energy absorption per unit density combined uniform vessel distribution, intermittent rays, interlocking grain growth, and optimal fiber adhesion. In particular, a new hierarchical progressive delamination was observed in which not only tracheids ruptured and tore out, but their helically wound cell walls also unraveled to absorb energy. This contrasted from wood quasi-static behavior in which density was a reliable predictor of mechanical behavior without accounting for these mesostructural features.
Next, torsional testing on cholla cacti were conducted to understand their adaptations to twisting induced by high winds. Though tall trees are typically bent by wind loading, shorter, branching plants with high skewness (like cholla) are twisted instead. It was hypothesized that the helical macroporosity winding around their hollow wooden stalk was an adaptation to maximize torsional stiffness while minimizing mass in the resource-poor desert. Novel mesostructural characterization methods of laser- scanning and photogrammetry were used alongside traditional optical microscopy, SEM, and μ-CT to identify mechanisms responsible for torsional resistance. These methods, in combination with finite element analysis (FEA) revealed how cholla meso and macro-porosity and fibril orientation contributed to highly density-efficient mechanical behavior. Selective lignification and macroscopic tubercle pore geometry contributed to density-efficient shear stiffness, while mesoscopic wood fiber straightening, delamination, pore collapse, and fiber pullout provided extrinsic toughening mechanisms. These energy absorbing mechanisms were enabled by the hydrated material level properties. Together, these hierarchical behaviors allowed the cholla to far exceed bamboo and trabecular bone in its ability to combine specific torsional stiffness, strength, and toughness.
This dissertation also explores how familiar bioinspired concepts can be applied to understudied applications by designing and testing hierarchically structured jamming devices. In the soft-robotics field, bio-inspiration is often cited, pointing to the animal-like forms created—however, the concept of hierarchical architecture common to biological materials has yet to be applied effectively. It was shown how that by considering the hierarchical structure of the medium (primary level), the organization of jamming media (secondary level), and the organization of jammers (tertiary level) new functionalities not possible with conventional jamming technology could be obtained. At the primary level, optimal compositions of fibrous flakes and grains were identified to improve stiffness and strength per unit weight; fish-inspired ganoid scales were used to create flexible armors. At the secondary level, layers and grains were combined in the tensile and compressive faces of beams to maximize mechanical properties, while ganoid scales of different compositions were layered to create mechanical gradients, among other combinations of jamming media. Finally, at the tertiary level, the isotropy of triaxially woven jammers was demonstrated relative to traditional biaxial jammers; a cylindrical “finger-trap” weave with adjustable radius was shown. The improved mechanical weight-efficiency, anisotropy control, mechanical property gradients, and other features enabled by considering hierarchical design in jamming promise new application spaces for an established field, such as reactive wearable armors.
Finally, wood-templated silicon carbides were infiltrated with epoxy to investigate a scalable method of fabricating easily shapable ceramic-polymer composites with both flexural strength (σ Flexure) and fracture toughness (KIC). In situ SEM and μ-CT of fracture toughness samples indicated a lack of interaction between the ceramic and epoxy phases, leading to only marginal improvement over the rule of mixtures for KIC and underperformance for σ Flexure. Preliminary tests of surface oxidized wood-templated SiC showed qualitatively improved crack stability in in situ SEM.