1,720,994 research outputs found
Non-equilibrium Strain and Elastic Hysteresis in Static and Dynamic Experiments in Sandstones
Full text linkThe physical origin of hysteretic elasticity in consolidated granular media is still debated. We show that hysteresis in quasi-static experiments (slow loading/unloading cycles) and dynamic acoustoelastic testing (fast perturbation of the sample obtained by a propagating or standing wave) can be seen as a consequence of slow dynamics, which also induces elastic anisotropy due to nonlinearity. Both conditioning (i.e. the evolution of the elastic properties towards a non-equilibrium steady state when a load is applied) and relaxation (i.e. the slow recovery of the original elastic properties when the strain is reduced) are described as the consequence of a non-equilibrium strain generated in the material by the applied load. Experiments performed span over five orders of magnitude in strain (from 10-7 to 10-2) and we demonstrate the proposed model captures well the observed phenomenology in the full strain range considered
Separation of Damping and Velocity Strain Dependencies using an Ultrasonic Monochromatic Excitation
Full text linkPrecise knowledge of the dependence of elastic modulus and Q-factor on amplitude of excitation is a prerequisite for the development and validation of models to explain the hysteresis observed in quasi-static experiments for various media, i.e. the different deformations at the same applied stress observed when stress change rate is positive or negative. Separation of different contributions to dynamic nonlinearity (e.g those due to non equilibrium effects, often termed conditioning) and independent estimation of nonlinearities originated by the strain dependence of velocity and damping factor are required, which is often not possible with standard approaches. Here we propose and validate a method which, measuring the response of a sample to a monochromatic excitation at different amplitudes, allows fast, continuous and quasi real-time monitoring of the dependence of the material elastic properties on amplitude: dynamic elastic modulus (related with velocity through density) and Q-factor of the mechanical resonances (related with wave amplitude attenuation parameter
Effects of a dc bias on electrical impedance spectroscopy in electrolytic cells
No full textCharacterization of electrolytic cells is often inferred from measurements of their electrical properties as a function of frequency by applying a sinusoidal voltage. In some cases, a dc bias is intrinsically present in addition to the ac stimulus and its effects are often neglected although they may become relevant in certain situations. As a consequence, the interpretation of the observed results might be misleading. Our aim is to estimate the influence of the bias solving numerically the Poisson-Nernst-Planck model for an electrolytic cell submitted to an ac external voltage superimposed to a bias. According to our analysis, even in the case of a small bias, the estimated Debye frequency and the low-frequency resistance of the cell are significantly influenced by it
Hybrid experimental/computational approach to Time Reversal source localization in thin plates using image source method
No full textStandard approaches to source localization usually rely on trilateration, using signals detected at multiple receiving points, with further research focused on improving localization robustness and reducing the number of sensors required. Time Reversal (TR) is a valid alternative, however it is often difficult to define a reliable hybrid (computational/experimental) approach, particularly in the case of plates where dispersive properties of the propagating waves play a crucial role to obtain focusing. In this paper, we present an analytical method that describes wave propagation in thin plates, accounting for edge reflections and dispersive properties, and validate it by comparison to experimental data. The Image Source Method (ISM), employed and extended in this study, provides an analytical means of Green's function computation through multiple edge-reflected propagation paths and proves to be reliable and fast to study propagation of Lamb waves in thin plates. A one-channel hybrid TR approach based on ISM is also proposed, utilizing experimental signals transferred to the model for backpropagation computation. In particular, the determination of the optimal signal duration to be time-reversed is discussed
Adhesion and plasticity in the dynamic response of rough surfaces in contact
No full textSeveral phenomenological models have been proposed in the last decades to understand and describe the phenomenology of elastic hysteresis observed in dynamic experiments, i.e. the combination of nonlinear phenomena, usually referred to as “fast” and “slow” dynamics, including harmonic generation, resonance frequency shift, time variation of elastic properties when large conditioning strain is applied. These models correctly reproduce various experimental observations for repeated loading–unloading cycles in quasi-static conditions, but they lack a convincing interpretation in terms of possible physical mechanisms. The aim of this work is to provide a model for the description of the dynamic behavior of a material with an internal microcrack, which attempts to link measurable macroscopic observables to physical crack features at the microscale (e.g. crack concentration, roughness, adhesion, elasticity and plasticity). The existence of adhesive/continuous phases, of crack activation thresholds, of harmonic generation and of the presence of metastable equilibrium states are emerging features of the model and are proposed herein as a possible source of slow dynamics effects. The proposed model can serve as a basis for the extension to a more macroscopic view, i.e. a material containing a network of microcracks
Relaxation of Viscoelastic Properties of Sandstones: Hysteresis and Anisotropy
No full textElastic nonlinearity in sandstones is given by the combination of slow and fast effects, leading to hysteresis and relaxation of the viscoelastic properties after the sample is perturbed (conditioning). While a lot of experimental evidences of slow dynamics are available, quantification of nonclassical nonlinearity is still difficult: nonlinear indicators must be introduced which are independent of both experimental configuration and on conditioning strain amplitude, which is often difficult to quantify in an accurate way. In this contribution, we introduce and analyze some possible quantities describing nonlinear viscoelasticity in sandstones, derived from a non-equilibrium strain model. We will show that they identify real material properties, in the sense that they are not dependent on experimental details (such as experiment duration, conditioning protocol, frequency, ...) and can be measured easily with good repeatability.We quantify the temporal relaxation of viscoelastic properties of sandstones;We introduce measurable indicators allowing to quantify hysteresis, anisotropy, and relaxation in sandstones;We propose a non-equilibrium strain model to describe observed hysteresis and anisotropy in nonlinear hysteretic viscoelastic properties;We show a linear correlation exists between modulus and damping evolution during relaxation in sandstones
Conditioning and elastic nonlinearity in concrete: Separation of damping and phase contributions
No full textElastic properties of concrete are affected, already at low strains, by its nonlinear properties, which are very sensitive to the presence of microcracks and hence to progression of damage. Conditioning and memory effects, which are both nonlinear effects are also manifested in intact samples and understanding their role in the definition of the propagation of elastic waves is crucial for the development of techniques aiming to quantify the nonlinear response and extract information about the microstructure of concrete specimens or concrete-based structures. Results are presented here to make evident the possibility to experimentally detecting and comparing the nonlinearity and conditioning induced by elastic modulus and damping nonlinearities in different concrete samples
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