Past Research
1. Microstructural design studies for locally dissipative acoustics metamaterials
Stress wave attenuation performance of locally dissipative acoustic metamaterials with various damped oscillator microstructures is studied using mechanical lattice models. The presence of damping is represented by a complex effective mass. Analytical solutions and numerical verifications of transmissibility are obtained for Kelvin-Voigt-type (KVO), Maxwell-type (MO) and Zener-type (ZO) oscillators with viscous damping. Although peak attenuation at resonance is diminished, dissipative microstructures can provide broad spectrum attenuation without increasing mass ratio, obviating the bandgap width limitation of locally resonant microstructures. KVO gives the best broad spectrum performance akin to a low-pass filter for excitation frequencies above a cut-off value for a prescribed transmissibility criterion. For ZO, by tailoring the damping and stiffness, the frequency range of appreciable attenuation can at least span the bandgap widths of two limiting locally resonant cases. Static and frequency-dependent measures of optimal damping that maximize the attenuation characteristics are proposed. A transitional value for the excitation frequency is identified within the locally resonant bandgap, above which there always exists an optimal amount of damping that renders the attenuation for the dissipative metamaterial greater than that for the locally resonant case. Further, microstructures with hysteretic damping depict a frequency-independent scaling in the effective mass and attenuation factor, but the bandgap width remains nearly the same as that for the corresponding viscously damped microstructures. This study demonstrates that dissipative microstructures, which are to some extent unavoidable in practical designs, when tuned, provide a means to substantially enhance the attenuation bandwidth of locally resonant acoustic metamaterials while also enabling the removal of the energy sequestered by the oscillators from the system.
2. Dynamic load mitigation using negative effective mass structures
Structures created by incorporating resonating endo-structures within a load bearing exo-structure forbid dynamic disturbances within a specific frequency range from propagating into them without attenuations. Their dynamic behavior can be characterized using a negative effective mass (NM) density. The suitability of such negative effective mass structures (NMS) as infrastructural building-blocks implicitly less susceptible to both harmonic and broadband impact-type loadings is demonstrated. For harmonic loading, an apparent damping coefficient is derived to compare the degree of attenuation achieved in the wholly elastic NMS to an “equivalent” conventionally damped structure. Parametric studies were used to design and construct a low-frequency vibration isolator with tip-loaded cantilever beam resonators that evinced 98% payload isolation at resonance. Under impact loading, using a numerical optimization procedure, it was established that resonator frequencies towards the higher end of the incoming spectrum gave the most reduction in transmitted peak stress. Compact and efficient resonators were constructed using plate springs with chemically etched reentrant patterns and high density resonator mass material. Tests performed using an impact pendulum on a resonator stack attached to a transmission bar, substantiated a peak stress reduction of about 60% and filtering of the resonator frequencies in the transmitted spectrum. Further, drop tower tests performed on a NM infrastructural building-block to gauge its ability to mitigate shocks transmitted to a target structure relative to a mass-equivalent block of steel demonstrated a 25% reduction in peak transmitted acceleration.
3. Multifunctional applications of acoustic metamaterials as an energy harvester, vibration isolator and transducer
Energy harvesting from structural vibrations using an array of multifunctional resonators based on the theory of locally resonant acoustic metamaterials (LRAM) was demonstrated. Such LRAM exhibit a stop band for elastic wave propagation. The band gap frequency range depends on the local resonance frequency of the microstructure. One method to realize this is through the use of an array of embedded resonators where the external work done is stored as kinetic energy of the internal mass when the forcing frequency is close to the local resonance frequency. This mechanism can be used to harvest energy by converting the kinetic energy into electrical energy, thus bestowing a multifunctional utility to the structure. A spring-loaded magnet enclosed in a capped PMMA tube equipped with copper coils was used to create a proof-of-concept unit cell that acts both as a resonator and as a linear generator. Experiments on a serial array of seven unit cells exhibit a band gap between 146.5 (local resonance frequency) and 171 Hz with a peak voltage generation of 3.03 V at steady state. The continuous effective power generated by a single unit cell across a 1-Ohm load resistor was 36 mW, indicating the feasibility of constructing vibration isolation structures that can power simple electronic devices. The energy harvesting circuit can also be used as a transducer to measure the LRF of a unit cell and the first global natural frequency of the structure can also be estimated from the beat frequency in the voltage signal.
4. Mechanical behavior and failure of neat and silica nanoparticle impregnated Kevlar fabrics under transverse indentation
Silica nano-particle impregnated Kevlar fabrics exhibit significantly enhanced ballistic performance while retaining flexibility. Although individual yarns exhibit rate dependent, nonlinear elastic behavior in tension, the behavior of the fabric itself is complicated because of the interaction of the two distinct media of yarns. It was found that fabrics impregnated with nanoparticles exhibit significant improvement in shear stiffness and a slight increase in tensile stiffness along the yarn directions over their neat counterparts. A homogenized continuum constitutive model was developed to characterize the nonlinear anisotropic properties of nanoparticle-impregnated fabrics undergoing large shear deformation using a procedure similar to the classical laminated plate theory. The parameters for the model were determined based on uniaxial and 45o off-axis tension tests for different fabric styles. Numerical simulations based on this model yielded good agreement with experimental results.
The response and failure of neat and nano-particle impregnated Kevlar fabric under transverse indentation was further investigated. Static indentation tests were performed on neat and treated Kevlar fabric. The combined deformation behavior of fabric with a deformable backing material was also examined. Compression tests were done to determine a suitable foundation material. The failure mechanisms were found to depend on multiple factors. Based on these observations, three failure modes – yarn breakage, yarn sliding and indentor slip-through – were identified. A stress based criterion to predict the failure for the uniaxial case was introduced. This criterion was implemented in numerical simulations in conjunction with the constitutive model via a UMAT subroutine in ABAQUS. A comparison of the experimental and simulated load-displacement curves show good agreement until failure initiation. Prediction of failure is achieved with good accuracy for the uniaxial case whereas an estimation of onset of failure based on yarn locking is possible for the off-axis case. Comparison to the fabric material model directly available in ABAQUS shows that the model yields better results and is also computationally much more efficient.
5. Numerical Study of Stress Wave Attenuation in a Silica-Rubber-Epoxy Metacomposite
The stress wave attenuation characteristics of a metacomposite composed of spherical rubber-encased silica particle inclusions in an epoxy matrix is studied using numerical simulations. This metacomposite is manufactured using a new microfluidics and colloidal crystal growth process. Firstly, a representative unit-cell for the metacomposite is discretized into its equivalent mass-spring model using static FEM simulations. Steady state dynamic simulations are performed to obtain the transmissibility over a broad band frequency range that encompasses the band gap. The results display nearly complete attenuation of harmonic waves in the vicinity of the resonance frequency of the inclusion. This metacomposite is ideally suited to mitigate waves with a dominant content in the range of a few kilohertz.