The need for a better aircraft performance is increasingly prompting designers towards the concept of ‘morphing’ or ‘shape-adaptable’ structural systems. To improve the performance of the aircraft, multi-stable composites could provide an interesting alternative to traditional designs. One approach to morphing aircraft is to use bi-stable structures that have two stable equilibrium configurations to define a discrete set of shapes for the morphing structure. Moving between these stable states may be achieved using an actuation system or by aerodynamic loads. Studding the application of bi-stable composite plate in the morphing wing is aim of forthcoming investigation in this project. Figures 1 and 2 show two conceptual novel designs of morphing airplane.
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Fig. 1 Advanced concepts NASA envisions for an aircraft of the future
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Fig. 2 RoboSwifts : tiny micro planes with unique "morphing" wings have been developed by engineering students in the Netherlands
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The interest in bi-stable composite structures comes from the fact that these structures can have two different shapes and they are completely stablein both shapes without need to a continuous power supply and mechanical hinges. It is well known that in the asymmetric composite laminates bi-stability occurs due to residual stresses resulting from differences in coefficients of thermal expansion and elastic properties in each lamina. Consequently these structures when subject by a thermal load can have large out of plane deflection.
In this project mechanical and thermal behavior of bi-stable composite plates was considered using non-linear Finite Element Analysis (FEA). Experimental investigations were also carried out to study the cured shape of plate and critical load that cause snapping between two different stable shapes. Several specimens were manufactured from graphite-epoxy asymmetric laminate with composition [0/90], [-30/60], [-20/70] and cured in an industrial autoclave. The maximum curing temperature was 180 °C. The first and the second stable shapes of a manufactured specimen with composition [0/90] after curing process have been shown in Figure 3.
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(a) the first stable shape
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(b) the second stable shape
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Fig. 3 The first and the second stable shapes for graphite-epoxy asymmetric laminate with composition [0/90]
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In order to create an accurate finite element model, laminate composition was characterized by optical microscopy from different zone of laminate thickness and an average thickness of each lamina and resin layers was determined and modeled. Figure 4 shows typical images from optical microscopy of a [0/90] laminate.
Fig. 4 Typical images from optical microscopy of thickness of a [0/90] laminate with 50X magnification.
Many proposed applications of graphite-epoxy composite require the material to operate at elevated or low temperatures. This is particularly true for applications to orbiting space structures, where the cold of space coupled with radiant solar heating effects can lead to a wide range of operating temperatures. Because of this, it is necessary to understand how temperature affects the basic properties of the material. Obviously, stiffness and thermal expansion are two properties which are of fundamental importance. Therefore, by considering the temperature dependency of mechanical and thermal expansion properties of graphite-epoxy, it is expected that the cured shape of the laminate has better consistency with the experimental results in compare with the case that the temperature dependency was not considered.
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Fig 5 (a),(b) The first and second stable shapes respectively by considering the temperature dependency of mechanical and thermal expansion properties , (c),(d) The first and second stable shapes respectively without considering the temperature dependency of mechanical and thermal expansion properties
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The results shown in Figure 5 indicate that by considering temperature effect, the height of the plane middle point is closer to the experimental value in compare to that when temperature effect is not considered.