Siavouche Nemat-Nasser

Bio: Siavouche Nemat-Nasser is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Ionic polymer–metal composites & Nafion. The author has an hindex of 14, co-authored 20 publications receiving 1579 citations.

Electromechanical response of ionic polymer-metal composites

Abstract: An ionic polymer-metal composite (IPMC) consisting of a thin Nafion sheet, platinum plated on both faces, undergoes large bending motion when an electric field is applied across its thickness. Conversely, a voltage is produced across its faces when it is suddenly bent. A micromechanical model is developed which accounts for the coupled ion transport, electric field, and elastic deformation to predict the response of the IPMC, qualitatively and quantitatively. First, the basic three-dimensional coupled field equations are presented, and then the results are applied to predict the response of a thin sheet of an IPMC. Central to the theory is the recognition that the interaction between an imbalanced charge density and the backbone polymer can be presented by an eigenstress field (Nemat-Nasser and Hori, Micromechanics, Overall Properties of Heterogeneous Materials, 2nd Ed., Elsevier, Amsterdam, 1999). The constitutive parameter connecting the eigenstress to the charge density is calculated directly using a s...

Interacting micro-cracks near the tip in the process zone of a macro-crack

Abstract: F or a linearly elastic and isotropic solid containing two or more cracks, cavities and other interacting defects of complex geometries, a method called “the method of pseudo-tractions” has been recently proposed by H ori and N emat -N asser (1983, 1985a), which can effectively solve two-dimensional problems of this kind, when cracks or cavities with sharp corners are suitably far apart. The method, however, breaks down when a crack or cavity is situated very close to the tip of another crack, which is the case when the process zone at the tip of a crack contains many micro-cracks. In this work, modifications of the method of pseudo-tractions are introduced, which will permit effective calculation of the stress intensity factors when a large crack interacts with small cracks which are situated very close to its tip in its process zone. Explicit asymptotic expressions are obtained for the stress intensity factors of the macro-crack, as well as those of the micro-cracks. It is shown that the presence of the microcracks in the process zone of a macro-crack may induce out-of-plane crack growth even under far-field hydrostatic tension. Several illustrative examples are worked out, including two collinear cracks for which an exact solution exists, arriving at an excellent correlation.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

Abstract: In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers’ complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradat...

Arbitrary branched and intersecting cracks with the eXtended Finite Element Method

Abstract: SUMMARY Extensions of a new technique for the nite element modelling of cracks with multiple branches, multiple holes and cracks emanating from holes are presented. This extended nite element method (X-FEM) allows the representation of crack discontinuities and voids independently of the mesh. A standard displacementbased approximation is enriched by incorporating discontinuous elds through a partition of unity method. A methodology that constructs the enriched approximation based on the interaction of the discontinuous geometric features with the mesh is developed. Computation of the stress intensity factors (SIF) in dierent examples involving branched and intersecting cracks as well as cracks emanating from holes are presented to demonstrate the accuracy and the robustness of the proposed technique. Copyright ? 2000 John Wiley & Sons, Ltd.

Elastic Solids with Many Cracks and Related Problems

Abstract: Publisher Summary This chapter discusses some basic problems in mechanics of elastic solids containing multiple cracks. A number of mathematical aspects that frequently constitute fields of their own (like various numerical techniques) are discussed very briefly in the chapter. The focus is on physically important effects produced by crack interactions and to present results in the simplest form possible. The problems considered in this chapter can be divided into two groups: (1) The impact of interactions on individual cracks, particularly on the stress intensity factors (SIFs), and (2) the effective elastic properties of solids with many cracks. Problems of the first group are, generally, relevant for the fracture-related considerations; solutions are sensitive to the positions of individual cracks. Problems of the second group deal with the volume average quantities; they are relatively insensitive to the information on individual cracks. The chapter discusses, in this connection, whether correlations exist between these two groups of quantities; in particular, whether microcracking can be reliably monitored by measuring changes in the effective elastic moduli.

Micromechanics of actuation of ionic polymer-metal composites

Abstract: Ionic polymer-metal composites (IPMCs) consist of a polyelectrolyte membrane (usually, Nafion or Flemion) plated on both faces by a noble metal, and is neutralized with certain counter ions that balance the electrical charge of the anions covalently fixed to the backbone membrane. In the hydrated state (or in the presence of other suitable solvents), the composite is a soft actuator and sensor. Its coupled electrical-chemical-mechanical response depends on: (1) the chemical composition and structure of the backbone ionic polymer; (2) the morphology of the metal electrodes; (3) the nature of the cations; and (4) the level of hydration (solvent saturation). A systematic experimental evaluation of the mechanical response of both metal-plated and bare Nafion and Flemion in various cation forms and various water saturation levels has been performed in the author’s laboratories at the University of California, San Diego. By examining the measured stiffness of the Nafion-based composites and the corresponding bare Nafion, under a variety of conditions, I have sought to develop relations between internal forces and the resulting stiffness and deformation of this class of IPMCs. Based on these and through a comparative study of the effects of various cations on the material’s stiffness and response, I have attempted to identify potential micromechanisms responsible for the observed electromechanical behavior of these materials, model them, and compare the model results with experimental data. A summary of these developments is given in the present work. First, a micromechanical model for the calculation of the Young modulus of the bare Nafion or Flemion in various ion forms and water saturation levels is given. Second, the bare-polymer model is modified to include the effect of the metal plating, and the results are applied to calculate the stiffness of the corresponding IPMCs, as a function of the solvent uptake. Finally, guided by the stiffness modeling and data, the actuation of the Nafion-based IPMCs is micromechanically modeled. Examples of the model results are presented and compared with the measured data.