Modeling the Behavior of Crystallizable Shape Memory Polymers Subject to Inhomogeneous Deformations

The constitutive model for the mechanics of crystallizable shape memory polymers (CSMP) has been developed in the past [1, 2]. The model was developed using the theory of multiple natural configurations and has been successful in addressing a diverse class of problems. In this research work, the efficacy of the developed CSMP model is tested by applying it to the torsion of a cylinder, which is an inhomogeneous deformation. The crystallization of the cylinder is studied under two different conditions i.e. crystallization under constant shear and crystallization under constant moment.

The Modelling of Hysteresis in Magnetorheological Dampers Using a Generalised Prandtl-Ishlinskii Approach

The major drawback of magnetorheological dampers (MR) lies in their non-linear and hysteretic force-velocity response. To take full advantage of the operating characteristics of these devices a high fidelity model is required for control analysis and design. In this contribution the ability of a generalised PI operator-based model to represent the characteristics of a commercially available MR damper is examined. This approach allows the user to define the PI operator to best match the hysteresis characteristics. For the MR damper the force-velcoity hysteresis characteristic is ‘S’ shaped and constrained. Two possibilities will be examined here for the generalised play operator; an hyperbolic tan function and a symmetric sigmoid function.

Cyclic Behavior of Concrete Confined by Active and Passive Jackets

Shape memory alloy (SMA) wire jackets for concrete are distinct from the conventional jackets of steel or FRP since they provide active confinement that can be easily archived due to the shape memory effect of SMAs. This study uses NiTiNb SMA wires of 1.0 mm diameter to confine concrete cylinder with the dimension of 300 mm × 150 mm (L × D). The NiTiNb SMAs have a relative wider temperature hysteresis than NiTi SMAs and, thus, are more applicable for severe temperature-variation environment which civil structures are exposed to. Steel jackets of passive confinement are also prepared to compare the cyclic behavior of active and passive confined concrete cylinders. For this purpose, monotonic and cyclic compressive loading tests are conducted to obtain axial and circumferential strain. The both of strains are used to estimate volumetric strains of concrete cylinders. Also, plastic strains from cyclic behavior are also estimated. For the NiTiNb SMA jacketed cylinders, the monotonic axial behavior differs from the envelope of cyclic behavior; this should be studied in future. The plastic strains of the active confined concrete show a similar trend to those of the passive confinement. The trend of plastic strain of this study does not match with that of CFRP (Carbon Fiber Reinforce Polymer) jackets. For the volumetric strain, the active jackets of the NiTiNb SMA wires provide more energy dissipation than the passive jacket of steel.

Analysis of Wrinkled Membranes Bounded With Macro-Fiber Composite (MFC) Actuators

In this paper we focus on a study which involves quantifying the effects of Macro Fiber Composite (MFC) actuators on the pattern and magnitude of wrinkles in a membrane when exposed to various loadings. An ABAQUS finite element code is employed for this research. The membrane in this study has a rectangular shape which is clamped at one edge and is free to move in the horizontal direction at the other edge. MFC actuators are bounded to the membrane to make a bimorph configuration.

Design of Switching Damping Control for Smart Structure Self-Powered Adaptive Damping Control

This paper presents a new damping control circuit which named adaptive VSPD [1] (adaptive velocity-controlled switching piezoelectric damping) that can be used to vary control voltage of VSPD damping circuit with the amplitude variation and be self-powered itself as well. Since the voltage source in the VSPD damping control circuit may cause a stability problem at small vibrations, an adaptive voltage source can be designed to purposely solve this problem. The design concept of an adaptive VSPD unit is not only to dampen the residual vibration but also to maintain the system stability by incorporating an adaptive control voltage. In fact, the energy needed for the extra voltage source within the control circuit can be provided by the storage capacitor and the energy stored can be harvested from the structure vibration energy. With this design, the damping performance can be maximized while maintaining system stability at the same time and also does not add complexity to the circuit. All the theoretical modeling, simulation and experimental results will all be detailed in this paper.

Experimental Study of a Self-Excited Piezoelectric Energy Harvester

In the present experimental work, we explore the possibility of using piezoelectric based fluid flow energy harvesters. These harvesters are self-excited and self-sustained in the sense that they can be used in steady uniform flows. The configuration consists of a piezoelectric cantilever beam with a cylindrical tip body which promotes sustainable, aero-elastic structural vibrations induced by vortex shedding and galloping. The structural and aerodynamic properties of the harvester alter the vibration amplitude and frequency of the piezoelectric beam and thus its electrical output. This paper presents results of energy-harvesting tests with one configuration of such a self-excited piezoelectric harvester using a PZT bimorph. In addition to the electrical voltage output, the strain on the surface of beam close to its clamped tip was also measured The measured strain and voltage output were perfectly correlated in the frequency range containing the first natural mode of vibration of the system. It was observed that about 0.24 mW of electrical power can be attained with this harvester in a uniform flow of 28 m/s.

Polycation Coated Polymeric Particles as Vehicles of RNA Delivery Into Immune Cells

The purpose of this work is to develop a carrier system for delivering RNA molecules aimed to downregulate specific functions in T cells. In many forms of cancer, T cells that express the protein Forkhead Box P3 (Foxp3) are associated with cancer progression. These cells can be identified by CD4 and CD25, molecules express on the cell surface. Studies have shown that downregulation of Foxp3 can increase the ability of other immune cells to destroy tumors. A class of RNA molecules, commonly referred to as “siRNA”, bind to and degrade specific messenger RNA (mRNA) in a sequence-dependent manner such that expression of the encoded protein is terminated. Because mRNA molecules are located inside cells, a carrier system is required to facilitate the uptake of siRNA, which does not passively diffuse through the plasma membrane. To this end, nanosized polymeric particles coated with the polycation, ornithinex10-histidinex6 (or O10H6) were used to adsorb siRNA that bind to the mRNA encoding Foxp3. The RNA-loaded particles are spherical and uniform in size (normally distributed, polydispersity index = 0.072). Loading of RNA to the particles was confirmed using gel electrophoresis. RNA complexed with the particles are protected from serum destabilization: 83.1% of RNA were recovered compared to 36.1% in RNA that were not associated with the particles. Association with the particles increased the uptake of the RNA in mouse T cells from 3.2±0.2% (free RNA) to 20.1±3.9%. Specifically, uptake of the RNA in T cells that express CD4 increased from 2.7±0.2% to 27.1±1.3% when particles were employed. These differences are statistically significant in three experiments conducted (p < 0.01). Internalization of the RNA into T cells was confirmed using confocal imaging. Flow cytometric analysis showed that the particle-complexed RNA reduced the percentage of T cells that express both CD4 and CD25 in mice carrying tumors from 24.0% when free RNA molecules were used to 13.5%. In these cells, the level of Foxp3 mRNA was reduced by 30%. In conclusion, the particles facilitate the uptake of siRNA molecules into a population of T cells that is known to promote cancer growth.

Non-Contact Technique for Characterizing Full-Field Surface Deformation of Shape Memory Polymers

Thermally-actuated shape memory polymers (SMPs) typically display two phases separated by the glass transition temperature (Tg). At temperatures well below the Tg, the polymer exhibits a relatively high elastic modulus. Well above the Tg the elastic modulus drops by several orders of magnitude. In this high temperature region, SMP materials can achieve strain levels well above 100 %. The complex behavior of SMPs (stiffnesses dropping to the order of 1 GPa and extremely high strain levels) precludes the use of traditional strain gages and low-contact force extensometers. The present study presents a detailed expansion of state-of-the-art thermomechanical testing techniques used to characterize the material behavior of SMPs. An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A laser extensometer is used for measurement and active control of axial strain. The upper limit on the strain rate capability of the laser extensometer is established. In addition, the photographic strain measurement method known as digital image correlation (DIC) is incorporated, allowing for full field measurement of axial and transverse strains of SMPs over a range of temperatures and strain rates. The strain measurements of the DIC and laser extensometer are compared to each other as well as to clip-on extensometers and strain gages. The comparisons provide insight into the limitations of the traditional strain measurement systems. A series of tensile tests are performed on a commercial SMP from 25 °C up to temperatures of 130 °C and strain levels above 100 %. The laser extensometer provides a robust method for controlling the strain in the gage section of the samples. In addition, results show that the full field measurements of both the axial and the transverse strain are essential for characterizing the constitutive response of SMPs at room and elevated temperatures.

Low Frequency Piezoelectric Synthetic Jets

Active cooling is often required for circuit boards with high heat generation densities. Synthetic jets driven with piezoelectric actuators offer interesting capabilities for localized active cooling of electronics due to their compact size, low cost and substantial cooling effectiveness. The design of synthetic jets for specific applications requires practical design tools that capture the strong fluid structure interaction without long run times. There is particular interest in synthetic jets that have a low operating frequency to reduce noise levels. This paper describes how common finite element (FE) and computational fluid dynamics (CFD) codes can be used to calculate parameters for a synthetic jet fluid structure interaction (FSI) model that only requires a limited number of degrees of freedom and is solved using a direct approach for low frequency synthetic jets. Tests are performed based on impinging on a heated surface to measure heat transfer enhancement. The test results are compared to the FSI model results for validation and agreement is found to be good in the frequency range of interest from 200 to 500 Hz.

Computational Study of Inclusion Burst via the Proton Sponge Hypothesis

Biological proteins embedded in either a biological or an engineered membrane will actively maintain electrochemical balance across that membrane through transport of fluid and charge. While membrane studies are often planar, in nature they typically take the form of inclusions (∼spherical). Study and ultimately manipulation of the protein transporter types and density, and interior/exterior states of these inclusions lend insight into burst mechanisms appropriate to a broad array of engineering and biological applications, such as intracellular burst release of a vaccine. To explore these phenomena the governing equations of each transporter, as well as the membrane state are established. The result is a model requiring the simultaneous solution of a stiff system of differential equations. Presented is the computational solution of this system of equations for a specific burst scenario — the hypothesis that a proton sponge may be employed to expedite intracellular burst release of a DNA vaccine is explored.