Mechanical Performances Of A Novel Honeycomb Design With Zero Poisson's Ratio And Its Application In Camber Morphing Wings

Morphing aircraft concept has been proposed against the conventional aircraft concept,aiming at solving the design deficiency of the conventional aircraft.The conventional aircraft has the optimal aerodynamic performance for only one design point,while in the whole flying processes the flying parameters are always changing for varying flying environments and missions.In other words,the conventional can't keep the optimal aerodynamic shape for most conditions.Morphing aircraft has the ability to change its aerodynamic shape according to varyin g flying environments and missions,and thus can keep the optimal states during the whole flying processes.Light-weight and well-arranged wing morphing structures have been the key technology of morphing aircraft,because of the requirement of bearing the aerodynamic loading as well as the requirement of large deformations.To bear the aerodynamic loading requires the wing structures to be stiff enough,while to achieve large deformations need the wing structures to have certain flexibility.That's why the wing morphing structures have been the bottleneck for the development of morphing aircraft.This work presents a novel honeycomb design with zero Poisson 's ratio(ZPR),its mechanical performances and its application in camber morphing wings.This honeycomb configuration features a new mechanism to achieve ZPR,which consists in inserting a hexagonal part to bear the out-of-plane compression and to produce in-plane flexibility,and connecting a thin plate for the large out-of-plane flexibility.Thus different parts bring about different mechanical properties leading to a separately design for the in-plane and out-of-plane performances.For the existing honeycomb structures,there are only two ways to increase their out-of-plane bending compliance: increase the size of their unit cell or decrease the thickness of cell walls.However,both the two ways lead to a side effect of the reduction of in-plane tensile stiffness and the flatwise compressive stiffness.The novel ZPR honeycomb design could solve this problem by changing the thickness of the thin plate to achieve an increase of the out-of-plane bending compliance without affect on the in-plane tensile stiffness and the flatwise compressive stiffness.Honeycomb structures can be classified into three categories by their Poisson' ratio property: positive,negative and zero.The out-of-plane deformation of positive Poisson's ratio(PPR)honeycombs exhibits anticlastic or saddle-shape curvature that does not facilitate their use in sandwich structures with complex out-of-plane geometry.Structures with negative Poisson's ratio(NPR)behavior feature synclastic curvature when subjected to out-of-plane bending.On the opposite,no anticlastic or synclastic curvature could be found for structures exhibiting ZPR under out-of-plane bending,which makes zero-? cellular configurations more suitable for cylindrical sandwich panels and morphing applications in which the structure needs either to undergo pure cylindrical bending or one-dimensional(span)morphing.In conclusion,the novel honeycomb design has two advantages: firstly,the separate design of in-plane and out-of-plane mechanical performances,and secondly,the ZPR property.In Chapter Two,we illustrate the honeycomb design and the in-plane mechanical properties of the novel ZPR structures through a combination of theoretical analysis,FE homogenization and experimental tests.The novel honeycomb topology is composed by two parts that provide separate in-plane and out-of-plane deformations contributions.The hexagonal component provides the out-of-plane load-bearing compression and in-plane compliance,while a thin plate part that connects the hexagonal section delivers the out-of-plane flexibility.Parametric analyses have also been carried out to determine the dependence of the in-plane stiffness versus the geometric parameters that define the zero-? honeycomb.Chapter Three describes the out-of-plane bending performances of the ZPR structures also through a combination of theoretical analysis,FE homogenization and experimental tests.A comparison of the out-of-plane bending behavior of six different types of cellular topologies with the same relative density including the ZPR honeycomb has also been carried out by using three-point bending tests.The novel ZPR lattices show the highest bending compliance at large strains,and highly tailorable mechanical properties for the design of composite structures for airframe morphing applications.And also in this chapter,a special honeycomb structures based on the novel ZPR structures and shape memory polymer has been proposed to achieved the variable stiffness of bending property.The out-of-plane bending performance,the shape memory effect and the variable stiffness of the novel zero Poisson's ratio honeycomb structures have been studied by experimental tests.Three-point bending tests have been performed to determine the out-of-plane bending performance of the ZPR structures at varying temperatures while results of the tests have also been compared with those from the analytical model of the equivalent bending modulus.Chapter Four features a multi-stiffness topology optimization of the ZPR cellular structure for morphing skin applications.The optimization is performed with stiffness constraints to minimize the weight by using a state-of-the-art solid isotropic microstructure with penalty(SIMP)method.The topology optimization has been performed to minimize flatwise compressive and transverse shear moduli for aerodynamic pressures and shear forces.The multi-stiffness topology optimization is performed using a norm method with weighting coefficients.B oth the single-stiffness and the multi-stiffness topology optimization have generated new honeycomb design by imposing symmetry conditions and geometric post-processing to avoid the presence of stress concentrations.The mechanical performances of the new honeycomb designs have been validated using two approaches: one based on force boundary conditions and another with displacement BCs.In the last chapter,we present the application of the ZPR honeycomb design in a camber morphing wing concept.The light-weight honeycomb design generated in Chapter Four have been used.To solve the problem of the wing skin,a novel ZPR elastomeric skin design using the spandex woven and carbon fiber rods reinforced silicone rubber matrix has been proposed.And the elastomeric skin samples have been manufactured through a four step method.Planar tensile tests and tear strength tests show a hyperelastic tensile ability and a reinforced tear strength.To solve the actuation problem and to achieve bidirectional bending deformation of the camber morphing wing,a flexible Pneumatic Artificial Fiber(PAF)has been chosen as the actuation.Free contraction tests and the tensile tests have been done to understand its actuation performance.An active honeycomb design has also been pr oposed combining the PAF actuation and the light-weight ZPR honeycomb.Unidirectional and bidirectional bending tests show its deforming performances.The camber morphing wing sample has been established by combining the active honeycomb,the elastomeric skin,the leading and trailing edge sections with fixed shape.Its bending performances and aerodynamic properties have been tested using bidirectional bending tests and the wind tunnel tests.