05/25/2022
By Joshua Morris

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a Doctoral Dissertation defense by Joshua Morris on “Design, Characterization, and Analysis Methods for Low Frequency Mechanical Metamaterials”.

Candidate Name: Joshua Morris
Defense Date: Monday June 6, 2022
Time: 10 a.m.-Noon
Location: Southwick 240 and virtual via Microsoft Teams

All interested students and faculty members are invited to attend the defense in person or via remote online access.

This will be a virtual defense via Microsoft Teams. Those interested in attending should contact the student (Joshua_Morris@student.uml.edu) and committee advisor (Alireza_Amirkhizi@uml.edu) at least 24 hours prior to the defense to request access to the meeting.

Committee:
Advisor, Alireza Amirkhizi, Associate Professor, Mechanical Engineering, UMass Lowell
Christopher Hansen, Associate Professor, Mechanical Engineering, UMass Lowell
Alessandro Sabato, Assistant Professor, Mechanical Engineering, UMass Lowell
Thomas Plaisted, Materials Engineer, CCDC Army Research Laboratory

Abstract:
Mechanical metamaterials interact passively with pressure waves by adopting dynamic material properties not ordinarily found in nature. Acoustic and ultrasonic energy excites a microstructure consisting of periodically repeating internal resonators which disrupt waves travelling within the material near their natural frequency. Manipulation of a material's apparent properties is useful in the development of sound insulation, cloaking, blast attenuation, and lenses. Advancement in the field of mechanical metamaterials requires cells that are smaller/lighter, easier to fabricate (additive manufacturing), and feature broadband response. There is a strong push for design, analysis, and characterization tools that bridge the gap between the simulated design space and practical applications.
For an increasing number of design variables, finite element models (FEM) quickly become prohibitive and reduced order models (ROM) become necessary. A ROM is utilized to construct design maps relating center frequency and stop band width to geometric parameters. An optimized cell geometry with the widest possible stop band at a target center frequency was located using the maps. Manufacturing limitations constrained the solution to maintain feasibility. Extension of the tool for multi-material 3D prints was performed, highlighting the potential to double the width of the stop band using two enhanced resin systems in one cell. The compactness of the ROM enables its use in optimization tools such as genetic algorithm (GA) for solving more complex design problems. The features of optimized solutions for an array with 6 cells (24 design variables) are quickly solved. Functionally graded systems with layered (or fully symmetric) cell arrangements provide carefully tailored stop band performance. The sensitivity of the array to manufacturing tolerances is also quantified by applying uniform and standard deviation errors to a large population of ROM solutions. For the first time, metamaterial performance is represented as a range rather than a discrete solution, more accurately reflecting the physical conditions of in-situ transmission loss.
Analysis of heterogeneous materials with no knowledge of their internal qualities requires making measurements outside of the material, typically within a known ambient domain. A multi-point modelling approach utilizing two measurement points before and after the metamaterial sample permits the separation of incident and reflected waveforms for a frequency domain excitation. Information for both amplitude and phase surrounding the media enables the extraction of its apparent dynamic properties and scattering coefficients. Interestingly, the apparent properties are partially dependent on the ambient media's properties through interactions with the edge (or boundary) cells in the metamaterial array. This, and other finite size effects, are studied as part of a transition from numerical predictions to practical performance. Following the observations made with FEM, a unique lab-scale characterization experiment is constructed capable of measuring metamaterial phenomena. Transmission loss, phase velocity stop bands, and negative apparent density are validated for 3D printed metamaterial samples. This dissertation contributes to the design, analysis, and characterization toolset for sub-wavelength, broadband, additively manufactured mechanical metamaterials with consideration to the physical constraints of real world applications.