Highly Nonlinear Granular Crystals

Crystalline materials are known to be characterized by a regular lattice structure, with atoms occupying various positions in the lattice. The geometrical arrangements of the atoms and their fundamental properties are what convey to the different materials their constitutive properties.
Granular crystals (or acoustic metamaterials) can be pictured in a similar way: imagine creating a geometrical lattice, or a periodic pattern, replacing the fundamental atoms with granular element spherical or of different shapes, made of different materials, in contact with each other. Their beauty stems from the combination of its apparent simplicity and the hidden self-organized complexity, leading to a revolutionary type of materials behavior.

The study of the wave propagation in a one dimensional chain of beads (much like the well known Newton’s cradle toy) has led in the past to the discovery of an all new area of wave dynamic determined by the contact interaction potential between the particles and the zero tensile strength between them. For example, uniform one dimensional chains excited by an impulse support the propagation of a completely new type of solitary waves, characterized by a finite length, an extremely fast decomposition of the initial impulse on very short distances and on the fine tunability of the wave properties and band gaps by an applied prestress (i.e. by magnetically induced precompression).

We study new 1-, 2- and 3-dimensional strongly nonlinear granular crystals to contribute to the experimental, numerical and theoretical investigations of the basic phenomena guiding the wave propagation in these systems. The work aims at building fundamental understanding of the behavior of these systems, studying phenomena related to the interaction between discreteness and continuum responses, order and periodicity, interface scattering and localization.  

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• Tunable Acoustic BandGap Materials

We are interested in exploring various potential applications of the newly designed granular crystals toward the development of new engineering applications. Our work on tunable phononic crystals (acoustic band gap materials) aims at studying the presence of forbidden frequency of sound propagation in new periodic structures. We study the fundamental interplay between the dispersion relation of periodic (layered) materials and the presence of nonlinearity. Band gaps in materials can be tailored by the variation of the materials properties composing the system, their geometry, but also the amount of static precompression (prestress) applied to such systems. As part of this work we want to explore new designs for light weight phononic crystals and their response in the presence of defects and inclusions. At a more fundamental level, we seek for the presence of gap solitons and other localization phenomena, which might open the door for new applications in the future.  Such systems can be applied to absorb vibration and sound in many engineering systems, currently we are particularly focusing on aerospace related damping.

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• Nonlinear Systems for Impact Energy Trapping

This project aims at studying the dynamic response of a novel category of one- and three-dimensional highly nonlinear systems for shock energy trapping, redirection and dissipation. Their design benefits from the highly nonlinear dynamic response of their fundamental components (granular particles) and is built on experimental and numerical reports on negative reflections, impulse trapping and redirection. The final objective of the work will be the experimental creation and testing of viable three-dimensional crystals alternative to the current state-of-the-art engineering materials generally available and of particular interest for military and defense applications. The final goal of creating such structures will be achieved by assembling ordered arrays of multiple 1-D chains (uniform and/or heterogeneous) embedded in a matrix. The design of these metamaterials will seek a balance between striving to maximize their performance in shock/vibration absorption and mitigation and devising practical methods for their assembly. From this perspective, the quest for particle’s geometries alternative to the classically approached spherical beads is justified on the basis of 1) seeking an additional level of tunability from the nonlinearity of the system and 2) easing the experimental realization and testing of such structures.

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• Highly Nonlinear Waves for a New NDE/SHM Paradigm

This project is aimed at developing an experimental research program to create a new paradigm for Non Destructive Evaluation and Structural Health Monitoring (NDE/SHM) of materials and structures based on Highly Nonlinear Solitary Waves (HNSWs). The proposed research leverages on the tunability provided by highly nonlinear systems to open up a new field of theoretical and experimental investigations aimed at: a) understanding the coupling between a highly nonlinear oscillators and linear structures; b) detecting defects across scales: from the micro- to the macro-scopic level; c) evaluating applied stress in a given system; d) characterizing the mechanical properties of materials tailoring the pulse properties during propagation (inverse approach) aided by numerical modeling, and e) designing new, and therefore patentable, actuators/sensors technology for stress wave generation and detection. In the last two decades researches and applications of elastic stress waves (both in the sonic and ultrasonic range) for NDE/SHM have thrived owing to their capability of assessing the elastic properties of materials and the presence of damage. The recent discovery and development of the highly nonlinear wave theory and its numerical and experimental validation offer a new tool to the NDE/SHM community. The soundness of engineering systems is essential to avoid catastrophic failures that may be accompanied by severe consequences for the environment, can lead to the loss of human life, and produce tonnage of demolition waste. It is therefore of paramount importance to the nation’s sustainability, economy growth and safety that NDE/SHM, able to accurately detect defects at early stages or to characterize the mechanical properties of a given structure, are used. With the proposed research we plan to delve in the fundamental understanding of highly nonlinear waves coupling with materials and structures, offering a direct opportunity to transfer the technology in viable commercial applications much improved over the state-of-the-art actuating/sensing technology for NDE/SHM. The work builds on complementary expertise at the University of Pittsburgh and the California Institute of Technology Caltech and is at the core of the development of a new start up venture called Solitonik, which aims at commercializing new actuator and sensors systems for a variety of different applications.

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• Bioinspired energy dispersive composites

The objective of this proposed work is to understand and subsequently mimic energy dissipation, impact absorption, and mechanical properties of biologically inspired materials subjected to quasi-static, high-impact, and lifecycle fatigue deformation. Emphasis will be given to the study of artificially assembled micro- and nano-structured composites for the creation of flexible and durable protectors. In addition, understanding the effects of different strain rates on the composites with sub-micron component dimensions is an emerging area of research offering broad opportunities for the development of fundamental discoveries in the mechanics of nano/micromaterials and engineering applications. Dynamic phenomena at the sub-micron scales have not been investigated in details and will be a significant part of the proposed project. Drawing the inspiration from hard biological systems will allow us to obtain new innovative materials with unprecedented properties dictated by the choice of the specific properties of their individual components in a truly across-scales “smart” composite. The multiscale nature of this problem will prompt future new theoretical and numerical research and will most likely lead to the discovery of new phenomena applicable to various engineering devices. The proposed research will concentrate on the following four unique aspects:
1) Fabrication of two types of novel energy-dispersive composite materials: nano-particle reinforced nanostructured foams and vertically-stacked granular nano-composites via top-down and bottom-up approaches.
2) Gaining a fundamental physical understanding of the underlying phenomena during quasi-static and dynamic mechanical testing of these energy-dispersive materials. This will be done by studying the shock-energy trapping and pulse disintegration of the selected nanocomposites in single- and multi-layered configurations.
3) Modification of the existing synthesis techniques for individual components to elicit particularly desired properties in the composite.
4) Assembly of multi-layered composites to create and study new artificial systems once the behavior of single layers is understood.
The project is an active collaboration between the Daraio and Greer groups at Caltech.

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• Periodic Systems of Nanostructures

 

Surface engineering for contact interaction control
To better tune the nonlinear dynamic properties of layered materials, we are interested in engineering the contact interactions between the elements of the system. The CVD growth of various nano-structures, carbon nanotubes, nanowires, etc. is desirable to tube the mechanical response of our metamaterials and to discover new wave phenomena for various applications in light weight heterogeneous structures for protecting layers and energy absorbing devices.

 

Advanced and in-situ characterization of nano- and biological materials
Advanced characterization techniques play a vital role on materials science. They represent the key factor for research advancements in all areas related to nano- and bio-materials.
Conventional, well known TEM, AFM and FIB microscopy techniques, combined with in-situ testing (stress-strain, temperature, optical and electrical behavior) and tomography are part of our lab’s efforts to characterize new materials and their mechanical behavior.

One other project now being developed is aimed at investigating the origin of life: can SEM and TEM help finding a signature of the oldest form of life? A structural study of the carbon forms found in the oldest sedimentary rocks on earth may help finding an answer to this question.

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