1,721,011 research outputs found
Thermally actuated shape-memory polymers: Experiments, theory, and numerical simulations
No full textWith the aim of developing a thermo-mechanically-coupled large-deformation constitutive theory and a
numerical-simulation capability for modeling the response of thermally-actuated shape-memory polymers,
we have (i) conducted large strain compression experiments on a representative shape-memory polymer
to strains of approximately unity at strain rates of 10[superscript −3] s[superscript −1] and 10[superscript −1] s[superscript −1], and at temperatures ranging
from room temperature to approximately 30C above the glass transition temperature of the polymer; (ii)
formulated a thermo-mechanically-coupled large-deformation constitutive theory; (iii) calibrated the material
parameters appearing in the theory using the stress-strain data from the compression experiments; (iv)
numerically implemented the theory by writing a user-material subroutine for a widely-used finite element
program; and (v) conducted representative experiments to validate the predictive capability of our theory
and its numerical implementation in complex three-dimensional geometries. By comparing the numericallypredicted
response in these validation simulations against measurements from corresponding experiments,
we show that our theory is capable of reasonably accurately reproducing the experimental results. As a
demonstration of the robustness of the three-dimensional numerical capability, we also show results from a
simulation of the shape-recovery response of a stent made from the polymer when it is inserted in an artery
modeled as a compliant elastomeric tube.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc
A Thermo-Mechanically Coupled Large-Deformation Theory for Amorphous Polymers Across the Glass Transition Temperature
No full textAmorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent me- chanical properties of polymeric products. In this manuscript we briefly summarize a few results from our own recent research [1–4] which is intended to fill this need. We have conducted large strain compression experiments on three representative amorphous polymeric materials a cyclo-olefin polymer (Zeonex-690R), polycarbonate (PC), and poly(methyl methacrylate) (PMMA) in a temperature range from room temperature to approximately 50C above the glass transi- tion temperature, θ g, of each material, in a strain-rate range of roughly 0.0001 s⁻¹ to 0.1 s⁻¹, and compressive true strains exceeding 100%. We have specialized our constitutive theory to capture the major features of the thermo-mechanical response of the three materials studied experimentally. We have numerically implemented our thermo- mechanically-coupled constitutive theory by writing a user material subroutine for a widely used finite element program Abaqus/Standard. In order to validate the predictive capabilities of our theory and its numerical implementation, we present the following validation experiments: (i) a plane-strain forging of PC at a temperature below θg, and another at a temperature above Tg; (ii) blow-forming of thin-walled semi-spherical shapes of PC above θg; and (iii) microscale hot-embossing of channels in PMMA above θ g. By comparing the results from this suite of validation experiments of some key features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments
Finite element implementation of a gradient-damage theory for fracture in elastomeric materials
No full textWe present a finite element implementation procedure for a phase-field framework for fracture in elastomeric materials based on the gradient-damage theory. Governing equations of macroscopic and microscopic force balances, and constitutive theories for large elastic deformation and damage are summarized, and the computational implementation is described in significant detail. To facilitate the computational implementation of the gradient-damage theory for elastomeric materials in a widely available finite element program, the source codes are provided as online Supplemental Materials to this paper. Furthermore, we provide a comparative study of the gradient-damage models with two distinct driving forces for damage: (1) entropy-driven and (2) internal energy-driven. We then show that the internal energy-driven damage model presents more realistic descriptions of the failure that accompanies extreme stretching and scission in elastomeric networks.
A coupled theory of fluid permeation and large deformations for elastomeric materials
No full textAn elastomeric gel is a cross-linked polymer network swollen with a solvent (fluid). A continuum-mechanical
theory to describe the various coupled aspects of fluid permeation and large deformations (e.g., swelling and
squeezing) of elastomeric gels is formulated. The basic mechanical force balance laws and the balance law
for the fluid content are reviewed, and the constitutive theory that we develop is consistent with modern
treatments of continuum thermodynamics, and material frame-indifference. In discussing special constitutive
equations we limit our attention to isotropic materials, and consider a model for the free energy based on a
Flory-Huggins model for the free energy change due to mixing of the fluid with the polymer network, coupled
with a non-Gaussian statistical-mechanical model for the change in configurational entropy — a model
which accounts for the limited extensibility of polymer chains. As representative examples of application of
the theory, we study (a) three-dimensional swelling-equilibrium of an elastomeric gel in an unconstrained,
stress-free state; and (b) the following one-dimensional transient problems: (i) free-swelling of a gel; (ii)
consolidation of an already swollen gel; and (iii) pressure-difference-driven diffusion of organic solvents across
elastomeric membranes.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc
A thermo-mechanically-coupled large-deformation theory for amorphous polymers in a temperature range which spans their glass transition
No full textAmorphous thermoplastic polymers are important engineering materials; however, their non-linear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. In this paper we extend our recently published theory [Anand, L., Ames, N. M., Srivastava, V., Chester, S. A., 2009. A thermo-mechanically-coupled theory for large deformations of amorphous polymers. Part I: formulation. International Journal Plasticity 25, 1474–1494; Ames, N. M., Srivastava, V., Chester, S. A., Anand, L., 2009. A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part II: applications. International Journal of Plasticity 25, 1495–1539] to fill this need.
We have conducted large strain compression experiments on three representative amorphous polymeric materials – a cyclo-olefin polymer (Zeonex-690R), polycarbonate (PC), and poly(methyl methacrylate) (PMMA) – in a temperature range from room temperature to approximately 50 °C above the glass transition temperature, ϑg [theta subscript g], of each material, in a strain-rate range of ≈10-4 [10 superscript -4]to 10-1 s-1 [10 superscript -1 s superscript -1], and compressive true strains exceeding 100%. We have specialized our constitutive theory to capture the major features of the thermo-mechanical response of the three materials studied experimentally.
We have numerically implemented our thermo-mechanically-coupled constitutive theory by writing a user material subroutine for a widely used finite element program. In order to validate the predictive capabilities of our theory and its numerical implementation, we have performed the following validation experiments: (i) a plane-strain forging of PC at a temperature below ϑg [theta subscript g], and another at a temperature above ϑg [theta subscript g]; (ii) blow-forming of thin-walled semi-spherical shapes of PC above ϑg [theta subscript g]; and (iii) microscale hot-embossing of channels in Zeonex and PMMA above ϑg [theta subscript g]. By comparing the results from this suite of validation experiments of some key features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments.National Science Foundation (U. S.) (Grant no. DMI-0517966)Singapore MIT Alliance Programme in Manufacturing Systems and Technolog
A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part I: Formulation
No full textIn this Part I, of a two-part paper, we present a detailed continuum-mechanical development of a thermomechanically
coupled elasto-viscoplasticity theory to model the strain rate and temperature dependent largedeformation
response of amorphous polymeric materials. Such a theory, when further specialized (Part II)
should be useful for modeling and simulation of the thermo-mechanical response of components and structures
made from such materials, as well as for modeling a variety of polymer processing operations.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc
A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part II: Applications
No full textWe have conducted large-strain compression experiments on three representative amorphous polymeric materials: poly(methyl methacrylate) (PMMA), polycarbonate (PC), and a cyclo-olefin polymer (Zeonex-690R), in a temperature range spanning room temperature to slightly below the glass transition temperature of each material, in a strain rate range of View the MathML source to View the MathML source, and compressive true strains exceeding 100%.
The constitutive theory developed in Part I [Anand, L., Ames, N.M., Srivastava, V., Chester, S., 2009. A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part 1: Formulation. International Journal of Plasticity] is specialized to capture the salient features of the thermo-mechanically coupled strain rate and temperature dependent large deformation mechanical response of amorphous polymers. For the three amorphous polymers studied experimentally, the specialized constitutive model is shown to perform well in reproducing the following major intrinsic features of the macroscopic stress–strain response of these materials: (a) the strain rate and temperature dependent yield strength; (b) the transient yield-peak and strain-softening which occurs due to deformation-induced disordering; (c) the subsequent rapid strain-hardening due to alignment of the polymer chains at large strains; (d) the unloading response at large strains; and (e) the temperature rise due to plastic-dissipation and the limited time for heat-conduction for the compression experiments performed at strain rates [View the MathML source]. We have implemented our thermo-mechanically coupled constitutive model by writing a user material subroutine for the finite element program [Abaqus/Explicit, 2007. SIMULIA, Providence, RI].
In order to validate the predictive capabilities of our constitutive theory and its numerical implementation, we have performed the following validation experiments: (i) isothermal fixed-end large-strain reversed-torsion tests on PC; (ii) macro-scale isothermal plane-strain cold- and hot-forming operations on PC; (iii) macro-scale isothermal, axi-symmetric hot-forming operations on Zeonex; (iv) micro-scale hot-embossing of Zeonex; and (v) high-speed normal-impact of a circular plate of PC with a spherical-tipped cylindrical projectile. By comparing the results from this suite of validation experiments of some key macroscopic features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments.National Science Foundation (U.S.) (grant number DMI-0517966)Singapore-MIT Allianc
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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