Experimental Validation of the Attenuation Properties in the Sonic Range of Metaconcrete Containing Two Types of Resonant Inclusions

Abstract

Background Metaconcrete is a new concept of concrete, showing marked attenuation properties under impact and blast loading, where traditional aggregates are partially replaced by resonant bi-material inclusions. In a departure from conventional mechanical metamaterials, the inclusions are dispersed randomly as cast in the material. The behavior of metaconcrete at supersonic frequencies has been investigated theoretically and numerically and confirmed experimentally. Objective The feasibility of metaconcrete to achieve wave attenuation at low frequencies demands further experimental validation. The present study is directed at characterizing dynamically, in the range of the low sonic frequencies, the—possibly synergistic—effect of combinations of different types of inclusions on the attenuation properties of metaconcrete. Methods Dynamic tests are conducted on cylindrical metaconcrete specimens cast with two types of spherical inclusions, made of a steel core and a polymeric coating. The two inclusions differ in terms of size and coating material: type 1 inclusions are 22 mm diameter with 1.35 mm PDMS coating; type 2 inclusions are 24 mm diameter with 2 mm layer natural rubber coating. Linear frequency sweeps in the low sonic range (< 10 kHz), tuned to numerically estimated inclusion eigenfrequencies, are applied to the specimens through a mechanical actuator. The transmitted waves are recorded by transducers and Fast-Fourier transformed (FFT) to bring the attenuation spectrum of the material into full display. Results Amplitude reductions of transmitted signals are markedly visible for any metaconcrete specimens in the range of the inclusion resonant frequencies, namely, 3,400-3,500 Hz for the PDMS coating inclusions and near 3,200 Hz for the natural rubber coating inclusions. Specimens with mixed inclusions provide a rather uniform attenuation in a limited range of frequencies, independently of the inclusion density, while specimens with a single inclusion type are effective over larger frequency ranges. With respect to conventional concrete, metaconcrete reduces up to 90% the amplitude of the transmitted signal within the attenuation bands. Conclusions Relative to conventional concrete, metaconcrete strongly attenuates waves over frequency bands determined by the resonant frequencies of the inclusions. The present dynamical tests conducted in the sonic range of frequencies quantify the attenuation properties of the metaconcrete cast with two types inclusions, providing location, range and intensity of the attenuation bands, which are dependent on the physical-geometric features of the inclusions.

Analysis on the Dynamic Wave Attenuation Properties of Metaconcrete Considering a Quasi-Random Arrangement of Inclusions

“…The important side effect of the energy trapping is the relief of mechanical strains and stresses acting on the matrix. The stress attenuation has been predicted theoretically and numerically in Mitchell et al (2014), Mitchell et al (2015), Mitchell et al (2016); the attenuation of the energy carried by the waves has been verified experimentally in Briccola et al (2017), Briccola et al (2019), Briccola et al (2020).…”

“…Modal analysis reveals the mechanisms that reduce the mechanical energy on the matrix: the rotation and the translation of the core, and the periodic deformation of the coating. A compressive planar wave, as the one used in the experimental validations of metaconcrete Briccola et al (2017), Briccola et al (2019), Briccola et al (2020), activates the translational modes of the core, resulting in a core motion in opposition of phase with respect to the matrix. Yet, rotational modes of the core cannot be activated by a planar wave, and the low content of energy carried by the signals used in experiments cannot excite the deformation modes of the coating.…”

“…The translation of the core is the mechanism associated to the sole natural frequency f T I of a 1-D mass spring model, easily computed to provide a simple estimation of the resonant oscillation frequency of the inclusion Mitchell et al (2014); Briccola et al (2020). The difference in the value of f T I obtained from the 1-D model and the modal analysis of the unit cell resides in the lack of thee-dimensionality, and it is markedly affected by the value assigned to the Poisson's coefficient of the coating.…”

“…For the specificity of metaconcrete, the attainment of multiple attenuation band gaps can be targeted easily by combining bimaterial inclusions of different size and materials rather than introducing multilayered inclusions. A recent experimental campaign, conducted on ten metaconcrete specimens with two types of aggregated randomly cast, provided some design hints on the possibility to enhance the performance of metaconcrete in the view of applications addressing vibration and shock mitigation Briccola et al (2020).…”

Split Hopkinson bar tests on metaconcrete: modeling and numerical simulations

“…Different frequencies can be targeted and thus filtered out from the matrix by properly tuning the geometrical and mechanical parameters of the aggregate: as a result, the vulnerability of standard concrete to damage due to a propagating shock wave is greatly reduced [2,3]. So far, a discrete number of experimental investigations have been performed to evaluate the attenuation properties of metaconcrete within the sonic and ultrasonic range [4][5][6][7], but none of these studies consider the impulsive nature of blast and impact loading. Despite plate impact experiments have been performed to study the attenuation properties of a phantom made by an epoxy matrix with embedded resonant inclusions [9], experimental campaigns on locally resonant metamaterials with brittle matrix considering soft or hard impact have not been performed yet.…”