|Ph.D Student||Li Jian|
|Subject||Instabilities and Elastic Waves in Microstructured Soft|
|Department||Department of Aerospace Engineering||Supervisors||Professor Itzchak Frankel|
|Dr. Stephan Rudykh|
This thesis presents a study of elastic instabilities and small-amplitude elastic waves in microstructured soft composites undergoing large deformations. Macroscopic and microscopic instabilities in soft composites with periodic microstructures are detected through numerical Bloch-Floquet analysis, and experiments on 3D-printed samples. In this thesis, I investigated the instabilities in periodic microstructured soft systems including
(i) Layered composites: I examined the role of phase compressibility on the onset of instability in compressible layered composites. I found that compressible layered composites require larger strains to trigger mechanical instabilities.
(ii) 3D fiber composite: I studied the elastic instabilities in 3D fiber composites with various fiber distributions. In periodically distributed fiber composites with the square in-plane periodicity, I experimentally observed that an increase in fiber volume fraction can result in a transition of the instability-induced patterns from small wavelength wavy pattern to the long-wave mode. I found that the composites with rectangular fiber periodicity exhibit cooperative buckling mode developing in the direction, where the fibers are closer to each other. Moreover, I derived a closed-form expression to predict the dependence of buckled wavelength on shear modulus contrast for single fiber composite.
(iii) Particulate composite: I investigated instability-induced domain formations and pattern transitions in particulate composites with stiff inclusions periodically embedded in a soft elastomeric matrix. I experimentally observed that the formation of microstructures with antisymmetric domains, and their geometrically tailored evolution into cooperative patterns of inclusions rearranged in wavy chains. I found that the domain patterns are realized in the composites for which macroscopic instabilities are predicted. I showed that these switchable patterns can be tailored by tuning composite microstructures.
(iv) Auxetic multiphase composite: I considered the instability phenomena in multiphase composites consisting of circular voids and stiff inclusions periodically distributed in a soft elastomer. I experimentally realized instability-induced pattern transformations in 3D-printed composites. I observed that composite microstructures rearrange into new morphologies, resulting in the closure of voids and giving rise to auxetic behaviors. I showed that distinct new patterns and auxetic behaviors can be tailored through altering the distribution of inclusions and loading direction.
Furthermore, I illustrated an application of employing instability-induced pattern transformations to manipulate small-amplitude elastic wave propagation. I showed that the buckled patterns in multiphase composites open new band gaps in remarkable low-frequency ranges. I found that the instability-induced wavy patterns give rise to the tunability of the widths and locations of shear wave band gaps in neo-Hookean laminates. Finally, I examined the oblique shear wave propagation in the finitely deformed layered composites. I observed the closure of band gaps in layered composites when the propagation direction deviates - even slightly - from the normal (i.e., perpendicular to the layer) direction.