Coherent Structures In Granular Crystals PDF Download

Elastic waves and vibrations play central roles in numerous engineering fields and technologies such as impact mitigation, vibration isolation, ultrasonic imaging, and even electronic filtering components. Enhanced control over such dynamics has the potential to enhance existing applications or create entirely new technologies, with one avenue of such control being waveform manipulation using dispersion and nonlinearity. An attractive approach that has seen significant advancement recently is the field of phononic crystals and acoustic metamaterials, which use structure to tailor existing, and enable new, effective material properties. One such structure is the granular crystal, defined as ordered arrays of discrete elastic particles in contact, which supports dispersion tailoring in addition to nonlinearity resulting from the contact mechanics between the particles. This interplay between dispersion and nonlinearity has produced a large amount of research recently, however, it has mostly been limited to macroscale systems with millimeter to centimeter-sized spheres. Such macroscale systems have limited applicability to many engineering solutions with size and weight constraints. Using microscale spheres to create architectured material with enhanced functionality is a promising idea, however, the dynamic behavior can not be assumed to be identical to macroscale systems because different physics are expected to become important at small scales, the most consequential being adhesive forces. This work experimentally investigates whether three-dimensional microgranular crystals support nonlinear dynamics analogous to their macroscale counterparts, which is currently an open question. Key elements known to allow wave tailoring in macroscale systems are studied individually before building up to direct analysis of nonlinear dynamics in three-dimensional microgranular crystals. First, a two-dimensional microgranular crystal monolayer adhered to a substrate is utilized to investigate, within linear dynamical regimes, interparticle vibrational modes and horizontal-rotational degrees of freedom, both known to affect propagation in three-dimensional systems. Using this experimental design, horizontal-rotational interparticle modes were observed, and described by a recently developed unified theory, for the first time in a microgranular crystal, with the key takeaway being that adhesive forces enhance the role of rotations and form interparticle networks that drastically alter the mode frequency. Next, the contact mechanics of a microsphere monolayer was studied with three different methods of estimating the adhesive force and compared by assuming an elastic contact mechanics model. This unique comparison found the measurements varied widely, suggesting adhesion-induced plasticity may play a major role for polymer microspheres. Subsequently, the behavior of a disordered three-dimensional assembly of microspheres was explored by controlling static and dynamic loading amplitudes, which directly reveal the nonlinear nature of the contact and the weakly nonlinear dynamics that result from it. It was discovered that the nonlinear behavior was drastically different from macroscale counterparts initially, however, the behavior was approximately similar after mechanical conditioning. Lastly, strongly nonlinear dynamics of an ordered three-dimensional microscale granular crystal was investigated in a preliminary study by characterizing the dependence of sound speed on acoustic wave amplitude and found to behave approximately similar to macroscale systems, though additional data is needed for a rigorous analysis. This finding bodes well for translating the promise of macroscale granular crystals to the microscale. The work contained in this thesis lays out a path to exploring even more complex microgranular crystal dynamics.