Wednesday, November 27, 2019

Smart Materials in Aerospace Industry free essay sample

Within each subsection, we will draw a relationship between the properties of the smart material and its molecular mechanism. This is followed by presenting an outline of their recent and future applications, and the experimental procedures and results done in recent researches to show the feasibility of these applications. In the Discussion section, we will be delving into the cost-effectiveness and feasibility of using smart materials in aerospace components. Finally, the conclusion will give an insight into the relationship between the use of smart materials and the design of future aircrafts. 1. Introduction Smart materials are defined as materials that can significantly change their intrinsic properties (mechanical, thermal, optical or electromagnetic), in a predictable and controlled manner in response to their environmental stimulus[1]. In general, these materials can be categorized into 3 categories, namely thermal-to-mechanical (shape memory alloys), electrical-to-mechanical (piezoelectric), and magnetic-to-mechanical (magnetostrictive). Materials engineering has undergone a major transformation in the recent decade, as atoms and molecules are no longer viewed and worked upon on the microscopic level, but now on the nanometer level. We will write a custom essay sample on Smart Materials in Aerospace Industry or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page Materials requirements are becoming more complex, especially in the aerospace industry in which safety and cost-effectiveness often conflict against each other. It is no longer acceptable for materials to have a single function; they need to be multifunctional to save costs and weight. Smart materials are now replacing monolithic ones to achieve multiple functions at all scale levels. Hence, smart materials are essentially integrated into the use of aerospace components. What differentiates smart materials from normal monolithic ones? Smart materials exhibit characteristics which most scientists would term as ‘intelligence’. This includes immediacy (the ability to respond in real time); transiency (the ability to respond to more than one environmental states); self-actuation (inherent intelligence within material); selectivity (having a discrete and predictable response); and lastly, directness (response is local to the ‘activating’ event)[2]. The performance characteristics of aircrafts are often limited by properties of materials used in both the airframe and propulsion systems. With the recent advancement of materials technology, high performance materials are created, resulting in a breakthrough in the performance and efficiency of modern aircrafts. The discovery of smart materials provides cost-effective and innovative solutions to the limitations currently faced in the design of aircrafts. These smart materials perform specialized functions when exposed to external stimuli, and they are increasingly being used to replace conventional aircraft parts for better performance. In this report, we shall look at the current and future use of these smart materials in the aerospace industry. 1. Purpose The purpose of this report is to introduce the different types of smart materials and their applications in the aerospace industry. Recent and emerging uses of these smart materials will also be presented, with brief experimental procedures and results obtained from recent researches and experiments to show the feasibility in aerospace applications. 1. 2 Background Smart material transformation was first observed on gold-cadmium, and recorded in 1932. Five year later, in 1938, the same phase transformation was observed in brass. In 1962, Beehler and coworkers discovered the shape memory effects of Nickel-Titanium alloy, and they named this family of alloy as Nitinol. The discovery of Nitinol ignites the discovery of other alloy systems with shape memory effect, and also accelerates the use of smart materials in product development. [3] Since then, aerospace companies are also exploring the use of smart materials in aircraft components. Conventional automatic control systems which use servo-valve or hydraulic actuators face a lot of limitations. These limitations include multiple energy conversions, complexity due to large number of parts resulting in large number of potential failure sites and large weight penalty, high vulnerability of hydraulic network, and frequency limitation. In contrast, the advantages from the use of smart materials actuators include the direct conversion of electrical energy to high frequency linear motion, easier transmission of electrical energy throughout aircraft, and light and compact smart materials induced-strain actuation in place of bulky hydraulic power systems. With this huge potential offered by smart materials, researchers are eager to tap on this potential, by exploring on ways to implement these smart materials into aircraft components. 1. 3 Scope This report will present the 4 common types of smart materials that are popular in the market. A brief description will be made with regards to the mechanism of how the smart materials function. The properties of the smart materials will then be related to their current and future aerospace application. This is followed by the detailed outline of the experimental procedures undertaken by past researches, as well as results obtained which prove the feasibility of using these smart materials for the aerospace applications. Finally, discussions will be made on the viability of the use of smart materials in the aerospace industry, in terms of safety, cost feasibility and future trends. . Types and Applications of Smart Materials 1. Piezoelectric Material Piezoelectricity is the generation of electrical potential in a material in response to a mechanical stress. This is known as the direct effect. It can also mean mechanical deformation upon the application of electrical charge or signal (Harrison JS and Ounaies Z, 2001). In this case, the material can serve as a sensor to detect m echanical stress. In addition, the materials can serve as an actuator when there’s a large increase of size due to electrical stimuli. The two types of piezoelectric materials that are used as smart materials are piezoelectric ceramic and polymer. Properties Piezoelectric materials are widely used as they possess favorable properties such as fast electromechanical response, wide bandwidth, high generative force and relatively low power requirements (Harrison JS and Ounaies Z, 2001). In addition, piezoelectric polymers are flexible, lightweight, and have low acoustic and mechanical impedance, while piezoelectric ceramics are brittle, heavy and toxic. Mechanism Piezoelectric effect is formed in crystals that have no centre of symmetry. One end of the molecule is more negatively charged while the other end is more positively charged, hence a dipole moment exists within the molecule. This is due to both the atomic configuration of the molecule, and also the molecular shape. Polar axis is the imaginary line that runs through the centre of both charges on the molecule. The orientation of the polar axis determines the type of crystal. For monocrystal, all the molecules’ polar axes are oriented in the same direction (Figure 2. 1. 1), while for polycrystal, the polar axes of molecules are oriented in different direction (Figure 2. . 2) [pic][pic] Figure 2. 1. 1 Figure 2. 1. 2 To create the piezoelectric effect, polycrystal is heated under the application of a strong electric field. The high temperature increases the rate of self-diffusion among the molecules, while the strong electric field forces almost all of the dipoles to orient in nearly the same direction (Figure 2. 1. 3) [pic] Figure 2. 1. 3 Piezoelectric ef fect can now be observed in the crystal (Figure 2. 1. 4). Figure (a) shows the piezoelectric material in its neutral state. When the material is compressed, a voltage of the same polarity as the resultant dipole moment will appear between the electrodes (Figure (b)). Conversely, the voltage will be of opposite polarity when the material is expanded (Figure (c)). Similarly, a voltage applied that is opposite to the poling voltage will cause the material to expand(Figure (d)), while an applied voltage of the same polarity will cause the material to be compressed (Figure (e)). If an alternating voltage is applied across the material, the material will vibrate with the same frequency as the signal. [pic] Figure 2. 1. 4 Advantages and Disadvantages[4] Advantages |Disadvantages | |Compact and lightweight |Brittle due to crystalline structure | |Displacement proportional to applied voltage |Produce small strains compared to SMA and magnetostrictives | |Operate over large temperature range |Cannot withstand high shear and tension | |Fast response to applied voltage(msec) |Aging of material | |Repeatable sub-nanom eter steps at high frequency |Uses active control, which can result in instability | |No moving parts |Can become depolarized (at high temperature, high voltages and large | |Function at high frequencies |stresses) | |Excellent stability | | |Easily embedded into laminated composites. Aerospace Applications Piezoelectric materials are mainly used in the aerospace industry for shape control and vibration control. †¢ Tail-Buffet Suppression High performance aircrafts with twin vertical tails often face the aeroelastic phenomenon of tail buffeting, in which the unsteady vortices that emanates from the wing leading edge extensions burst and immerse the vertical tails in their wake. This results in severe vertical tail response and buffet loads, which lowers airframe life and increases maintenance costs. pic][pic] Before the development of piezoelectric actuators, various method of alleviating buffeting was used. One method was the use of hydraulic actuators to superimpose the oscillations of affected control surfaces about their hinges, so as to effect damping. However, this method has two disadvantages. Firstly, the flight control system and buffeting-minimization system must use the same degree of freedom for the same control surface, thus reducing the availabi lity of the control surface for each role. Secondly, operations are limited to low frequencies due to the difficulty of oscillation a large control surface about its hinges. Experiment The Technical Cooperation Program (TTCP) collaborated with National Research Council Canada (NRC) and Department of Defense of Canada (DND) in researching about the feasibility of using piezoelectric actuators for tail buffet suppression on a full-scale F/A-18. The full-scale aircraft was tested in the International Follow-On Structural Test (IFOST) Program rig in Australia (Yousefi-Koma A Zimcik DG 2003). The procedures for the experiment are as follow: †¢ The starboard fin of the aircraft was instrumented with piezoelectric actuators over a wide area on both sides of the fin, as shown in Figure 2. 1. 5. [pic] Figure 2. 1. 5 †¢ Accelerometers and strain gauges are placed strategically to measure displacements, and hence calculate the vibration amplitude. †¢ Electrodynamic shakers are attached to the fin to induce structural vibration. These shakers are controlled by the test rig control system to model actual flight structural loads. †¢ In the experiment, four custom-made high-power amplifiers of 2kVA rating over 200Hz bandwidth were used. Results Conclusion The experimental results have shown that the active control system using piezoelectric actuators was able to effectively suppress the buffet response of the vertical fin at high angle of attack. Amplitude reductions of up to 60% at the normal flight configuration and close to 10% in the worst case scenario were observed (Yousefi-Koma A Zimcik DG 2003). It was estimated even a small 10% reduction in vibration amplitude would double the durability of the fin. Hence, it can be concluded that with the use of piezoelectric actuators in active-control surface modal (ACSM) device to deform the control surface, the control surface not only can respond to buffeting-minimization signals, but also flight control commands. †¢ Wing Flutter Damping When a structure is placed in a flow of sufficiently high velocity, an aeroelastic self-excited vibration takes place, which has a sustained or divergent amplitude. This results in dynamic instability that can get violent. This is because at high speed, the effect of the airstream can cause the coupling of two or more vibration modes such that the vibrating structure will extract energy from the airstream. The extracted energy equals the amount of energy that the structure can dissipate at the critical speed, and a neutrally stable vibration exists. However, above this critical speed, the vibration amplitude will diverge, causing structural failure. Experiment and Result The Piezoelectric Aeroelastic Response Tailoring Investigation was conducted at MIT with the support of NASA, and it aims to achieve the following objectives: determining the power consumption of the piezoelectric actuators while controlling the response of the structure; investigating optimal piezoelectric actuator placement; and, testing disturbance rejection controllers at zero airspeed (Anna-Maria Rivas McGowan). The major components of the 4-feet test model, as shown in Figure 2. 1. , consist of two primary structures: an exterior fiberglass shell, which is used to obtain aerodynamic lift; an interior composite plate that contains the piezoelectric actuators, and is made up of an aluminium honeycomb core sandwiche d by graphite epoxy plates. The plates are of [20 °2/0 °]s laminate, referenced to the wings quarter-chord which is swept 30 °, and this provides a static coupling of the bending and torsional behaviour. Fifteen groups of piezoceramic actuators patches are placed at the top and bottom of the interior plate, and they are configured to impart moments to the plate. Together with the orientation of the graphite epoxy and the wing sweep angle, the actuators can affect bending and torsional vibration of the model. Forces on the model were monitored using ten strain gauges and four accelerometers. To acquire time history data, each of the 15 piezoelectric actuator groups was activated individually, as well as in five sets of several actuator groups. The experimental results is shown in the graphs in Figure 2. 1. 7. In summary, it shows that the control system can effect successful flutter suppression and gust reduction in the model, with a 12% increase in flutter damping and 75% decrease in root-bending moment caused by gusts. This clearly shows the potential of the use of piezoelectric actuators in suppressing the detrimental effects of wing flutter. Rotor Blade Twist Outboard portion of the blade travel faster, and with the same lift coefficient, higher lift force is concentrated near the blade tip. To distribute the lift force evenly among the blades, the angle of attack is made to be lower near the blade tip, and higher near the blade root, such that the lift coefficient decreases with increasing distance from the blade root. This can be done by induced blade twisting, through the embedment of piezoelectric materials into the blade skin. Active fibre composites (AFC) are actually used, which consists of continuous, aligned PZT fibres in an epoxy layer (Figure 2. 1. ), and copper electrode films that are etched into an inter-digitated pattern to effect the electric field along the fibre direction, as shown in Figure 2. 1. 9 (Rodgers and Hagood, 1998). [pic][pic] Figure 2. 1. 8 Figure 2. 1. 9 Experiment Active Fiber Composite (AFC) was fitted into the construction of a 1/6th scale replicate of CH-47D blade model (60. 619in span and 5. 388in chord). The blade was sent for wind tunnel testing at Boeing Helicopters, PA. Three AFC plies were diagonally placed in the co-cured D-spar blade lay-up. When the fibres are activated, it causes a shear in the spar skin, which creates the blade twist effect.

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