Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item DEFORMATION MECHANICS OF SOFT MATTER UNDER EXTERNAL STIMULI(2019) Cheng, Jian; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Artificial soft matters are a class of materials which can be easily deformed by external stress, typical examples include foams, colloids, elastomers, and hydrogels. Due to their unprecedented and unique properties, such as large deformability, high resemblance to biological systems, versatile response to multi-physical stimuli, and biological compatibility, soft matters have found applications in fields like soft actuators and robots, soft sensors, bio-mimicking material systems, micro-fluidic system control, biomedical engineering, etc. In these applications, the large deformability of soft matters has taken an enabling role. The deformation theory of polymeric soft matters can date back to 1940s in the early infancy of the statistical mechanics sketch of rubbery materials, with a fast growth in the most recent decade concurring the latest progress in soft matters. However, the mechanical modeling of soft matter leaves many open questions. This doctorate research is devoted to advance the understanding of the deformation mechanics of soft matter, specifically, from the following aspects: (1) how the chemo-mechanical interaction between the solvent molecules and the polymeric network invokes anomalous behaviors of a thin-walled hydrogel structure under internal pressure, in contrast to its polymer counterpart; (2) the application of the dielectric elastomer as sensing medium in soft sensor technology; (3) the development of a novel light-responsive hydrogel material system with the application in bio-mimicking shape transform; (4) and enriching the existing theory to facilitate the mechanistic understanding of the deformational behaviors of a type of fiber-reinforce anisotropic hydrogels. For that, this dissertation (1) reveals the delayed burst of hydrogel thin-shell structures as a new failure mechanism, which is dissimilar from the instantaneous burst of a rubber shell: at a subcritical applied pressure the burst occurs with a delay in time; (2) presents a facile design of capacitive tactile force sensor using a dielectric elastomer subjected to a modest voltage and a pre-stretch; (3) develops a theoretical framework to simulate the light-responsive deformation of the proposed hybrid hydrogel system; and (4) from the perspective of micromechanics, constructs a constitutive model suitable for the microfiber-reinforced anisotropic hydrogel, with large deformation, mass transportation, and the origin of anisotropy are intrinsically captured.Item Dynamic Control of Fiber Orientation for Additive Manufacturing via a Soft-Actuating Nozzle(2019) Armstrong, Connor; Bigio, David I; Sochol, Ryan D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recently, additive manufacturing of fiber-reinforced composite hydrogels has been used to create self-assembling and self-folding structures through hydration-triggered shape change. Additive manufacturing of shape-changing structures has applications in spatially-limited environments such as in-vivo biological implants and components for space travel. Fiber orientation in composite hydrogels dictates the degree of anisotropic swelling deformation of hydrated structures. This thesis explores the impact of extrusion channel geometry on fiber orientation as well as the relationship between fiber orientation and swelling deformation of composite hydrogels. To study the impact of fiber orientation on swelling deformation, fiber orientation in composite hydrogels was varied using diverging extrusion dies of increasing divergence angles. It was found that increasing channel divergence angle reduced the number of fibers oriented in the direction of flow, which led to increasingly isotropic swelling deformations. To create a gradient of fiber orientations in extruded structures, an extrusion nozzle utilizing soft actuators to alter its divergence angle in real-time was developed. Hydrogels extruded through the soft-actuated dynamic nozzle exhibited similar fiber orientation and swelling behavior to those extruded through the fixed divergence angles. Spatially-varied swelling deformation characteristics promise to improve additive manufacturing of self-assembling and self-folding structures by increasing the complexity of controllable shape change geometries achievable in extruded composite polymer structures.