This paper reports experimental results of an airfoil-based flap actuator that is actuated using high temperature Nickel-Titanium (NiTi) polycrystal and Copper-Aluminium-Nickel (CuAlNi) single crystal wires with a nominal diameter of 1.5 mm. The stress-free transformation temperatures of the commercially available NiTi wires are Mf = 53°C, Ms = 70°C , As = 95°C , Af = 110°C whereas those for the CuAlNi wires are Mf = 80°C ,Ms = 100.5°C, As = 104.5°C , Af = 117°C. Due to a significantly low electrical resistivity of the CuAlNi, the commonly used joule heating approach for thermal actuation is shelved for a heating coil approach. Uniaxial stress measurements, trailing edge flap deflections and temperature measurements are recorded during a typical heating and cooling cycle using a load cell in line with the SMA wire, a LVDT at the trailing edge tip and a thermocouple on the wire (outside the heating coil). It is seen that actuation by the CuAlNi (with a prestrain = 5.5%) leads to about a 50% higher tip deflection and about a 67% lower cooling time after actuation as compared to the corresponding values for NiTi (with a prestrain = 5.6%). The larger tip deflection is attributed to a higher strain recovery for the CuAlNi as compared to the NiTi during phase transformation whereas the lower actuation time is attributed, in part, to the narrow hysteresis in the stress-free transformation temperatures of the CuAlNi (~ 37°C) as compared to the NiTi (~ 57°C).
Novel autophagous (self-consuming) systems combining structure and power functionalities are under development for improved material utilization and performance enhancement in electric unmanned air vehicles (UAV's). Much of the mass of typical aircraft is devoted separately to the functions of structure and fuel-energy. Several methods are proposed to extract structure function from materials that can also serve as fuel for combustion or as a source of hydrogen. Combustion heat is converted to electrical energy by thermoelectric generation, and hydrogen gas is used in fuel cells to provide electrical energy. The development and implementation of these structure-fuels are discussed in the context of three specific designs of autophagous wing spars. The designs are analyzed with respect to mechanical performance and energy storage. Results indicate a high potential for these systems to provide enhanced performance in electric UAV's.
In this study, we identify and survey energy harvesting technologies for small electrically powered unmanned systems designed for long-term (>1 day) time-on-station missions. An environmental energy harvesting scheme will provide long-term, energy additions to the on-board energy source. We have identified four technologies that cover a broad array of available energy sources: solar, kinetic (wind) flow, autophagous structure-power (both combustible and metal air-battery systems) and electromagnetic (EM) energy scavenging. We present existing conceptual designs, critical system components, performance, constraints and state-of-readiness for each technology. We have concluded that the solar and autophagous technologies are relatively matured for small-scale applications and are capable of moderate power output levels (>1 W). We have identified key components and possible multifunctionalities in each technology. The kinetic flow and EM energy scavenging technologies will require more in-depth study before they can be considered for implementation. We have also realized that all of the harvesting systems require design and integration of various electrical, mechanical and chemical components, which will require modeling and optimization using hybrid mechatronics-circuit simulation tools. This study provides a starting point for detailed investigation into the proposed technologies for unmanned system applications under current development.
The need for shape memory alloys (SMA) with high transformation temperatures is urgent. SMA actuators may then be designed that will demonstrate two important attributes: (i) actuation in a high temperature environment and (ii) high actuation frequency in a moderate temperature environment. Copper-Aluminum-Nickel (CuAlNi) single crystal alloy is a promising candidate due to favorable characteristics such as wide range of transformation temperatures, large actuation stress and strain, and lack of plastic deformation under cyclic loading. These characteristics point to the possibility that CuAlNi single crystal high temperature SMA (HTSMA) actuators can be developed that will demonstrate the attributes mentioned above. An initial investigation of these possibilities is carried out computationally by analyzing a HTSMA-actuated airfoil. An existing rate-independent thermomechanical model is calibrated to describe the CuAlNi material behavior. Time-dependent thermomechanically-coupled parametric studies are carried out to yield information about the airfoil stroke (trailing edge deflection), the actuation energy required and the actuation frequency possible for cyclic actuation, all as a function of transformation temperatures. For comparison purposes, a similar parametric study is also carried out for NiTi polycrystalline SMA. The analysis indicates that a CuAlNi HTSMA actuated airfoil will demonstrate a cooling time two to six times lower than its NiTi SMA counterpart.
The main interest of this study is to investigate phase transformation behavior in hybrid SMA under moderate dynamic loads. A hybrid SMA is described as a two-component composite made of dense SMA matrix with pores, and a passive or active material that fills the pores. The dynamic behavior of hybrid SMA under strain rates of 1000 /s is investigated by incorporating the porous mesostructure in the finite element analysis. X-ray computed micro tomography (XCMT) images are employed to synthesize statistical information and probabilistic algorithms are utilized in obtaining suitable representative finite element meshes. The effective constitutive response of hybrid SMA is obtained by simulating the split Hopkinson bar test of the finite element specimen. A parametric study based on volume fraction of the filler material is carried out. A comparison of hybrid SMA and porous SMA is also performed.
This paper presents multifunctional structure-plus-power developments being pursued under DARPA sponsorship with the focus on structure-battery components for unmanned air vehicles (UAV). New design strategies, analysis methods, performance indices, and prototypes for multifunctional structure-battery materials are described along with the development of two UAV prototypes with structure-battery implementation.
In multifunctional material design, two or more functions performed by distinct system components or materials are incorporated into a single component or material system to improve system performance. The aim of this paper is to present a framework for the design of structure-battery (power) multifunctional composite materials for unmanned air vehicle (UAV) applications. The design methodology is based on optimization of composite material performance indices and the use of material design selection charts introduced by Ashby and coworkers in a series of papers for homogeneous and two-phase composite materials. Performance indices are derived for prismatic structure-battery composites under various loading conditions. The development of simple design tools in the form of spreadsheet templates is also discussed. Finally, results based on the above-mentioned framework and actual material properties will be presented for structure-battery circular and square struts.
Shape memory alloys (SMAs) have emerged as a class of materials with unique thermal and mechanical properties that have found numerous applications in various engineering areas. While the shape memory and pseudoelasticity effects have been extensively studied, only a few studies have been done on the high capacity of energy dissipation of SMAs. Because of this property, SMAs hold the promise of making high-efficiency damping devices that are superior to those made of conventional materials. In addition to the energy absorption capability of the dense SMA material, porous SMAs offer the possibility of higher specific damping capacity under dynamic loading conditions, du to scattering of waves. Porous SMAs also offer the possibility of impedance matching by grading the porosity at connecting joints with other structural materials. As a first step, the focus of this work, is on establishing the static properties of porous SMA material. To accomplish this, a micromechanics-based analysis of the overall behavior of porous SMA is carried out. The porous SMA is modeled as a composite with SMA matrix, which is modeled using an incremental formulation, and pores as inhomogeneities of zero stiffness. The macroscopic constitutive behavior of the effective medium is established using the incremental More-Tanaka averaging method for a random distribution of pores, and a FEM analysis of a unit cell for a periodic arrangement of pores. Results form both analyses are compared under various loading conditions.
In the area of underwater vehicle design, the development of highly maneuverable vehicles is presently of interest with their design being based on the swimming techniques and anatomic structure of fish; primarily the undulatory body motions, the highly controllable fins and the large aspect ratio lunatic tail. The tailoring and implementation of the accumulated knowledge into biomimetic vehicles is a task of multidisciplinary nature with two of the dominant fields being actuation and hydrodynamic control. Within this framework, we present here our progress towards the development of a type of biomimetic muscle that utilizes shape memory alloy (SMA) technology. The muscle is presently applied to the control of hydrodynamic forces and moments, including thrust generation, on a 2D hydrofoil. The main actuation elements are two sets of thin SMA wires embedded into an elastomeric element that provides the main structural support. Controlled heating and cooling of the two wire sets generates bi-direction bending of the elastomer, which in turn deflects or oscillates the trailing edge of the hydrofoil. The aquatic environment of the hydrofoil lends itself to cooling schemes that utilize the excellent heat transfer properties of water. The modeling of deflected shapes as a function of input current has been carried out using a thermomechanical constitutive model for SMA coupled with the elastic response of the elastomer. An approximate structural analysis model, as well as detailed FEM analysis has been performed and the model predictions are been compared with preliminary experimental measurements.
KEYWORDS: Shape memory alloys, Photoelasticity, Composites, Polymers, Finite element methods, Epoxies, Temperature metrology, Data modeling, Fringe analysis, Vibration control
Shape memory alloy (SMA) wires can be embedded in a host material to alter the stiffness or modal response and provide vibration control. The interaction between the embedded SMA and the host material is critical to applications requiring transfer of loads or strain from the wire to the host. Although there has been a significant amount of research dedicated to characterizing and modeling the response of SMA alone, little research has focused on the transformation behavior of embedded SMAs. Photoelastic experiments with SMA wires in polymer matrices had previously provided a qualitative understanding of stress transfer in SMA composites. In the current work, 2D photoelasticity is utilized to quantify the internal stresses induced by the actuation of a thin SMA ribbon in a pure polymer matrix. Through the use of a CCD camera and a frame grabber, photoelastic images are digitally recorded at discrete time increments. Shear stresses induced during the actuation are calculated as a function of time. Computational predictions of shear stress are made using finite element analysis and compared with experimental observations.
Shape memory alloy (SMA) wires can be embedded in a host material to alter the stiffness or modal response and provide vibration control. The interaction between the embedded SMA and the host material is critical to applications requiring transfer of loads or strain from the wire to the host. Although there has been a significant amount of research dedicated to characterizing and modeling the response of SMA alone, little research has focused on the transformation behavior of embedded SMA wires. In the current work, photoelasticity is utilized to quantify the development of internal stresses induced by the actuation of a SMA wire embedded in a pure polymer matrix. Through the use of a CCD camera and a frame grabber, photoelastic images are digitally recorded at discrete time increments. Shear stresses induced during the actuation are calculated as a function of time. Computational predictions of the transformation fronts are made using finite elements analysis and compared with experimental observations.
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