Viscoelastic damping in crystalline composites and alloys (original) (raw)

Viscoelastic damping in crystalline composites: A molecular dynamics study

Composites Part B: Engineering, 2016

Molecular dynamics (MD) simulations were used to study viscoelastic behavior of model Lennard-Jones (LJ) crystalline composites subject to an oscillatory shear deformation. The two crystals, namely a soft and a stiff phase, individually show highly elastic behavior and very small loss modulus. On the other hand, when the stiff phase is included within the soft matrix as a sphere, the composite exhibits significant viscous damping and a large phase shift between stress and strain. In fact, the maximum loss modulus in these model composites was found to be about 20 times greater than that given by the theoretical Hashin-Shtrikman upper bound. We attribute this behavior to the fact that in composites shear strain is highly inhomogeneous and mostly accommodated by the soft phase. This is corroborated by mode-dependent Grüneisen parameter analysis showing that in the low frequency regime, Grüneisen parameters, which measure degree of anharmonicity, are about twice greater for the composite than each individual homogenous crystal. Interestingly, the frequency at which the damping is greatest scales with the microstructural length scale of the composite, a feature we also observe for superlattice structures.

Damping characterization of viscoelastic composites using micromechanical approach

Behavior and Mechanics of Multifunctional Materials and Composites 2011, 2011

When studying composite material systems, mechanical properties, such as stiffness, strength, fracture toughness or damage resistance are the subjects of greatest interest and in most of the cases are considered in the context of simple static loading conditions. However, in almost all applications, composites, like most materials are subjected to dynamic loading which requires that the dynamic response of the composite be analyzed. For structural materials which are linear elastic, the stress-strain response is not dependent on strain rate, and there is no hysteresis or damping. However, this is not the case for viscoelastic materials for which both the stiffness and loss properties directly depend on strain rate and implicitly depend on temperature via time temperature superposition, which in case of harmonic loading leads to frequency dependent response. For viscoelastic composites in which at least one of the constituent materials is viscoelastic, there is great utility in the ability to predict the effective dynamic mechanical properties as a function of the constituent phase properties and geometry. In this paper micromechanical methods combined with the correspondence principle of viscoelasticity are used to obtain the effective damping properties of viscoelastic composites. When materials with different damping properties are present in a composite, the damping properties of the resulting composite are different than that of the constituents. The correspondence principle helps to consider all the frequency dependent properties of the constituent materials and conclude the effective damping vs. frequency. In this study the matrix phase is considered to be viscoelastic and spherical elastic/viscoelastic particles are dispersed into the matrix.

Soft elasticity and mechanical damping in liquid crystalline elastomers

Journal of Applied Physics, 2001

The dynamic soft response of polydomain liquid crystalline elastomers to simple shear is reported. Significantly, these materials also show extremely large loss behavior with tan ␦ exceeding 1 or even 1.5 over very wide temperature ranges, with clear implications for damping applications. By comparing materials that exhibit different types of liquid crystalline phases, we identify the nematic state as a better damping phase than that in materials with smectic phases. Additionally, we provide experimental evidence for directions which should be explored for further improvements in the damping behavior of liquid crystalline elastomers.

Perspectives in mechanics of heterogeneous solids

Acta Mechanica Solida Sinica, 2011

The Micro-and Nano-mechanics Working Group of the Chinese Society of Theoretical and Applied Mechanics organized a forum to discuss the perspectives, trends, and directions in mechanics of heterogeneous materials in January 2010. The international journal, Acta Mechanica Solida Sinica, is devoted to all fields of solid mechanics and relevant disciplines in science, technology, and engineering, with a balanced coverage on analytical, experimental, numerical and applied investigations. On the occasion of the 30 th anniversary of Acta Mechanica Solida Sinica, its editor-in-chief, Professor Q.S. Zheng invited some of the forum participants to review the state-of-the-art of mechanics of heterogeneous solids, with a particular emphasis on the recent research development results of Chinese scientists. Their reviews are organized into five research areas as reported in different sections of this paper. §I firstly brings in focus on micro-and nano-mechanics, with regards to several selective topics, including multiscale coupled models and computational methods, nanocrystal superlattices, surface effects, micromechanical damage mechanics, and microstructural evolution of metals and shape memory alloys. §II shows discussions on multifield coupled mechanical phenomena, e.g., multi-fields actuations of liquid crystal polymer networks, mechanical behavior of materials under radiations, and micromechanics of heterogeneous materials. In §III, we mainly address the multiscale mechanics of biological nanocomposites, biological adhesive surface mechanics, wetting and dewetting phenomena on microstructured solid surfaces. The phononic crystals and manipulation of elastic waves were elaborated in §IV. Finally, we conclude with a series of perspectives on solid mechanics. This review will set a primary goal of future science research and engineering application on solid mechanics with the effort of social and economic development. Supports from NSFC and MOST are acknowledged. · 2 · ACTA MECHANICA SOLIDA SINICA 2011 I. MICRO-AND NANO-MECHANICS 1.1

Frequency-dependent mechanical damping in alloys

Physical Review B, 2017

We perform oscillatory shear simulations to determine the loss modulus for three solids with identical interaction yet distinct structures: ordered, random and glassy alloys. Random and glassy alloys show more pronounced high-frequency loss in the THz regime than the ordered alloy. Ordered and random alloys exhibit a power-law decay in damping strength as frequency decreases over nearly five decades. Glassy alloy, with a limited frequency range of power-law decay, retains significant damping at low frequencies extending down to ~100 MHz due to slow irreversible deformation of local clusters.

A spectral method solution to crystal elasto-viscoplasticity at finite strains

International Journal of Plasticity, 2013

a b s t r a c t A significant improvement over existing models for the prediction of the macromechanical response of structural materials can be achieved by means of a more refined treatment of the underlying mic romechanics. For this, achieving the highest possible spatial resolution is advantageous, in order to capture the intricate details of complex microstructures. Spectral methods, as an efficient alternative to the widely used finite element method (FEM), have been established during the last decade and their applicability to the case of polycrystalline materials has already been demonstrated . However, until now, the existing implementations were limited to infinitesimal strain and phenomenol ogical crystal elastoviscoplasticity. This work presents the extension of the existing spectral formulation for polycrystals to the case of finite strains, not limited to a particular constitutive law, by considering a general material model implementation. By interfacing the exact same material model to both, the new spectral implementation as well as a FEM-based solver, a direct comparison of both numerical strategies is possible. Carrying out this comparison, and using a phenomenological constitutive law as example, we demonstrate that the spectral method solution converges much fa ster with mesh/grid resolution, fulfills stress equilibrium and strain compatibility much better, and is able to solve the micromechanical problem for, e.g., a 256 3 grid in comparab le times as required by a 64 3 mesh of linear finite elements. ) in the context of computational homogenizatio n. This spectral method operates in FOURIER space and is very efficient compare d to FEM due to the repetitive use of a fast FOURIER transforms (FFT) as part of an iterative solution algorithm. Lebensohn (2001) extended this FFT-based method to the context of crystal viscoplastic ity, and applied it to a number of studies for which the use of FEM-bas ed approach es would have been preclusive (Prakash and Lebensohn, 2009 ). Very recently, an extension of this spectral method to the case of crystal elasto-viscopla sticity has been reported , however restricted to the kinematic framework of infinitesimal strains. The feasibilit y of extending the FFT-based methodology to finite strains was first elaborated by for the case of composites with isotropic phases, from which the present work draws upon.

Modeling the Effect of Polymer Chain Stiffness on the Behavior of Polymer Nanocomposites

Due to their central role in industrial formulations spanning from food packaging to smart coatings, polymer nano-composites have been the object of remarkable attention over the last two decades. Incorporating nanoparticles (NPs) into a polymer matrix modifies the conformation and mobility of the polymer chains at the NP−polymer interface and can potentially provide materials with enhanced properties as compared to pristine polymers. To this end, it is crucial to predict and control the ability of NPs to diffuse and achieve a good dispersion in the polymer matrix. Understanding how to control the NPs' dispersion is a challenging task controlled by the delicate balance between enthalpic and entropic contributions, such as NP−polymer interaction, NP size and shape, and polymer chain conformation. By performing molecular dynamics (MD) simulations, we investigate the effect of polymer chains' stiffness on the mobility of spherical NPs that establish weak or strong interactions with the polymer. Our results show a sound dependence of the NPs' diffusivity on the long-range order of the polymer melt, which undergoes an isotropic-to-nematic phase transition upon increasing chain stiffness. This phase transition induces a dynamical anisotropy in the nematic phase, with the NPs preferentially diffusing along the nematic director rather than in the directions perpendicular to it. Not only does this tendency determine the NPs' mobility and degree of dispersion in the polymer matrix, but it also influences the resistance to flow of the polymer nanocomposite when a shear is applied. In particular, to assess the role of the chains' conformation on the macroscopic response of our model PNC, we employ reverse nonequilibrium MD to calculate the zero-shear viscosity in both the isotropic and nematic phases, and unveil a plasticizing effect at increasing chain stiffness when the shear is applied along the nematic axis.

Investigation on Viscoelastic-Creep Behavior of the Phase-Field Crystal Method

IOP Conference Series: Materials Science and Engineering, 2018

The phase-field crystal (PFC) method is a promising computational model with atomistic resolution and diffusive timescale. In this study, we investigated the viscoelastic-creep behavior exhibited by the PFC model. We considered a one-dimensional crystal subjected to step changes in pressure and studied the time-dependent strain response from the system. The parametric study shows that the PFC model predicts the materials with higher density to exhibit higher stiffness and lower damping capacity while an increase in temperature results in lower stiffness and damping capacity. These predictions agree with experimental observations and show promising capability of the PFC method to model viscoelastic phenomena.