Non-classical nonlinear elasticity in micro-inhomogeneous materials such as rocks and cracked or granular materials leads to a number of phenomena ranging from hysteresis and memory to a transient response of elastic properties to perturbations in dynamic or quasi-static experiments. Dynamic acousto-elastic testing (DAET) provides very detailed observations of some of these phenomena that are still not fully understood in terms of their physical origin. We suggest that the observations of non-classical nonlinear elasticity can be related to the physics of friction. We propose a conceptual model for the nonlinear elasticity based on friction of internal interfaces and the process of contact aging that leads to an increase of friction with increasing contact time. The central element of the model is the continuous interplay between (1) softening that occurs as small-scale damage due to shear motion of internal contacts and (2) stiffening (healing) as a thermally activated process of establishing connections across the contact at the current strain state. Chemical bonds, mineral fibres or capillary bridges are the most likely candidates for the physical nature of these connections. Our model qualitatively describes dynamic softening, hysteresis, slow dynamics and the shape of DAET loops including the absence of cusps and the loop orientation that leads to a stiffening at both maxima and minima of the dynamic strain.
In physical acoustic laboratories, wave propagation experiments often suffer from unwanted reflections at the boundaries of the experimental setup. We propose using multidimensional deconvolution (MDD) to post-process recorded experimental data such that the scattering imprint related to the domain boundary is completely removed and only the Green's functions associated with a scattering object of interest are obtained. The application of the MDD method requires in/out wavefield separation of data recorded along a closed surface surrounding the object of interest, and we propose a decomposition method to separate such data for arbitrary curved surfaces. The MDD results consist of the Green's functions between any pair of points on the closed recording surface, fully sampling the scattered field. We apply the MDD algorithm to post-process laboratory data acquired in a two-dimensional acoustic waveguide to characterize the wavefield scattering related to a rigid steel block while removing the scattering imprint of the domain boundary. The experimental results are validated with synthetic simulations, corroborating that MDD is an effective and general method to obtain the experimentally desired Green's functions for arbitrary inhomogeneous scatterers.
Rendering objects invisible to impinging acoustic waves (cloaking) and creating acoustic illusions (holography) has been attempted using active and passive approaches. While most passive methods are inflexible and applicable only to narrow frequency bands, active approaches attempt to respond dynamically, interfering with broadband incident or scattered wavefields by emitting secondary waves. Without prior knowledge of the primary wavefield, the signals for the secondary sources need to be estimated and adapted in real time. This has thus far impeded active cloaking and holography for broadband wavefields. We present experimental results of active acoustic cloaking and holography without prior knowledge of the wavefield so that objects remain invisible and illusions intact even for broadband moving sources. This opens previously inaccessible research directions and facilitates practical applications including architectural acoustics, education, and stealth.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.