טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLi Jian
SubjectInstabilities and Elastic Waves in Microstructured Soft
Composites
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Itzchak Frankel
Dr. Stephan Rudykh


Abstract

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.