Spectral band-structure identification of amplitude-dependent viscoelastic mechanical metamaterials

Investigators: J. Shim, A. Aref, W. Alnahhal (Qatar U)

Funding Source: QNRF-NPRP (Feb. 2016 - Feb. 2019)

Abstract: The demand for engineered materials with customized energy absorption, vibration control, and acoustic shielding is driven by a host of extreme applications in construction, automotive, aerospace, and consumer products. Requirements for exceptional material performance offer new opportunities and challenges. In such applications of significant societal benefits, mechanical metamaterials (MMs) incorporating viscoelastic constituents hold a great promise to solve some persisting problems associated with extreme loadings/environments.  The impetus for the incorporation of viscoelastic constituents in MMs mainly stems from their strong energy absorption property, and possession of a low elastic modulus which allows reaching practical low-frequency band-gaps (<10 kHz) that human body is highly affected by. However, a recent study on phononic crystals (PCs) reports that the nonlinearity driven by finite-displacement excitations allows some higher harmonic waves to penetrate into band-gaps, which can adversely affect the attenuation properties of PCs subjected to finite-amplitude excitations. Thus, there is a gap in knowledge and a need to investigate the amplitude-dependent attenuation properties of viscoelastic MMs for practical applications, whose loading conditions are typically represented by broadband frequency content and multitude of finite-amplitude excitations. 

The objectives of this research proposal are (1) to identify the amplitude-dependent attenuation characteristics of viscoelastic MMs, (2) to assess the effectiveness of amplitude-dependent viscoelastic MMs subjected to a wide range of excitation frequencies and amplitudes, and identify potential key factors by which their band-gap performance can be effective, (3) to identify vital considerations that can be decisive for numerically predicting the amplitude- and frequency-dependent behavior of viscoelastic MMs. In order to achieve these objectives, the PIs propose to employ split Hopkinson pressure bar (SHPB) apparatuses with viscoelastic bars as the core experimental tool for the identification of attenuation characteristics of MMs. The central hypothesis is that the attenuation characteristics of bars used in SHPB will not hinder the identification of the band-gaps of MMs for the frequency range of interest (<10 kHz). In order to test the central hypothesis, the PIs have a plan to perform numerical analysis as well as a series of experiments on elastic/viscoelastic MMs.

The successful completion of this research will meet the strong need to assess the effectiveness of nonlinear viscoelastic MMs subjected to practical and wide-range loading conditions. The proposed work provides a transformative approach that combines both numerical and experimental techniques by developing robust SHPB-centric experimental protocols to befit the investigation of viscoelastic MMs subjected to a wide range of excitation frequencies and amplitudes, which have not been possible by conventional experimental techniques such as ultrasound technique, impedance tubes, and electrodynamic shakers. More importantly, the novelty of the planned research stems from (a) realizing the opportunity to study viscoelastic MMs that have hardly been investigated within the framework of nonlinear behavior either numerically or experimentally, and (b) transforming knowledge from a traditional discipline of SHPB strain-rate testing to an emerging and multi-dimensional utility for the evaluation of MMs.

SHPB apparatus deployed at UB.

Snapshot of the SHPB apparatus deployed at the University at Buffalo.