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  • The Colossal Piezoresistive Effect in Nickel Nanostrand Polymer Composites and a Quantum Tunneling Model
  • Abstract A novel nickel nanostrand-silicone composite material at an optimized 15 vol% filler concentration demonstrates a dramatic piezoresistive effect with a negative gauge factor (ratio of percent change in resistivity to strain). The composite volume resistivity decreases in excess of three orders of magnitude at a 60% strain. The piezoresistivity does decrease slightly as a function of cycles, but not significantly as a function of time. The material's resistivity is also temperature dependent, once again with a negative dependence.
    The evidence indicates that nickel strands are physically separated by matrix material even at high volume fractions, and points to a charge…
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  • The Scalar Homotopy Method for Solving Non-Linear Obstacle Problem
  • Abstract In this study, the nonlinear obstacle problems, which are also known as the nonlinear free boundary problems, are analyzed by the scalar homotopy method (SHM) and the finite difference method. The one- and two-dimensional nonlinear obstacle problems, formulated as the nonlinear complementarity problems (NCPs), are discretized by the finite difference method and form a system of nonlinear algebraic equations (NAEs) with the aid of Fischer-Burmeister NCP-function. Additionally, the system of NAEs is solved by the SHM, which is globally convergent and can get rid of calculating the inverse of Jacobian matrix. In SHM, by introducing a scalar homotopy function and…
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  • A Quasi-Boundary Semi-Analytical Approach for Two-Dimensional Backward Heat Conduction Problems
  • Abstract In this article, we propose a semi-analytical method to tackle the two-dimensional backward heat conduction problem (BHCP) by using a quasi-boundary idea. First, the Fourier series expansion technique is employed to calculate the temperature field u(x, y, t) at any time t < T. Second, we consider a direct regularization by adding an extra termau(x, y, 0) to reach a second-kind Fredholm integral equation for u(x, y, 0). The termwise separable property of the kernel function permits us to obtain a closed-form regularized solution. Besides, a strategy to choose the regularization parameter is suggested. When several numerical examples were tested,…
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  • On Pseudo-Elastic Models for Stress Softening in Elastomeric Balloons
  • Abstract The phenomenon of stress softening observed in the cyclic inflation of spherical balloons or membranes is quantitatively and qualitatively examined. A new measure of the stress softening extent is proposed which correctly captures the main feature of this phenomenon. This measure of the stress softening is related to the relevant response functions in the constitutive models proposed in the literature to describe this effect. Using these relationships, the predictive capability of the theoretical models is examined. It is shown that only those theoretical models which admit a non-monotone character of the stress softening can properly describe this phenomenon.
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  • Meshless Local Petrov-Galerkin (MLPG) Method for Laminate Plates under Dynamic Loading
  • Abstract A meshless local Petrov-Galerkin (MLPG) method is applied to solve laminate plate problems described by the Reissner-Mindlin theory. Both stationary and transient dynamic loads are analyzed here. The bending moment and the shear force expressions are obtained by integration through the laminated plate for the considered constitutive equations in each lamina. The Reissner-Mindlin theory reduces the original three-dimensional (3-D) thick plate problem to a two-dimensional (2-D) problem. Nodal points are randomly distributed over the mean surface of the considered plate. Each node is the center of a circle surrounding this node. The weak-form on small subdomains with a Heaviside step…
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  • Statistics of High Purity Nickel Microstructure From High Energy X-ray Diffraction Microscopy
  • Abstract We have measured and reconstructed via forward modeling a small volume of microstructure of high purity, well annealed nickel using high energy x-ray diffraction microscopy (HEDM). Statistical distributions characterizing grain orientations, intra-granular misorientations, and nearest neighbor grain misorientations are extracted. Results are consistent with recent electron backscatter diffraction measurements. Peaks in the grain neighbor misorientation angle distribution at 60 degrees (∑3) and 39 degrees (∑9) have resolution limited widths of ≈ 0.14 degree FWHM. The analysis demonstrates that HEDM can recover grain and grain boundary statistics comparable to OIM volume measurements; more extensive data sets will lead to full, five…
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  • Measurements of the Curvature of Protrusions/Retrusions on Migrating Recrystallization Boundaries
  • Abstract Two methods to quantify protrusions/retrusions and to estimate local boundary curvature from sample plane sections are proposed. The methods are used to evaluate the driving force due to curvature of the protrusions/retrusions for partially recrystallized pure nickel cold rolled to 96% reduction in thickness. The results reveal that the values calculated by both these methods are reasonable when compared with the stored energy measured by differential scanning calorimetry. The relationship between protrusions and the average stored energy density in the deformed matrix is also investigated for partially recrystallized pure aluminum cold rolled to 50%. The results show that the local…
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  • EBSD-Based Microscopy: Resolution of Dislocation Density
  • Abstract Consideration is given to the resolution of dislocation density afforded by EBSD-based scanning electron microscopy. Comparison between the conventional Hough- and the emerging high-resolution cross-correlation-based approaches is made. It is illustrated that considerable care must be exercised in selecting a step size (Burger's circuit size) for experimental measurements. Important variables affecting this selection include the dislocation density and the physical size and density of dislocation dipole and multi-pole components of the structure. It is also illustrated that simulations can be useful to the interpretation of experimental recoveries.
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  • Identification of Particle Stimulated Nucleation during Recrystallization of AA 7050
  • Abstract Mechanical properties of polycrystalline metals are dependent upon the arrangement of microstructural features in the metal. Recrystallization is an important phenomenon that often occurs during thermo-mechanical processing of metals. This work focuses upon aluminum alloy 7050 and uses crystallographic texture and pair correlation functions of recrystallized grains to characterize the dominance of particle stimulated nucleation in the recrystallization process. The randomization of the recrystallization texture and similar pair correlation functions for the particle distribution and the recrystallization nuclei distribution indicate that particle stimulated nucleation controls the recrystallization behavior in this alloy.
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  • An Eulerian-Based Formulation for Studying the Evolution of the Microstructure under Plastic Deformations
  • Abstract In this paper, a model is introduced to examine the evolution of the microstructure function under plastic deformations. This model is based upon a double continuity relationship that conserves both material particles in the mass space and orientations in the orientation space. An Eulerian description of the motion of material particles and orientations is considered, and continuity relations are derived for both spaces. To show how the proposed model works, two different case studies are provided. In the mass space, the continuity relation is used to examine the evolution of the microstructure function of a two-phase (isotropic) material; while, in…
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