Elastomer-matrix composites show promise for high Poisson's ratio and negative Poisson's ratio (auxetic) applications due to high orthotropy. There are approximately five orders of magnitude between the axial stiffness of high modulus graphite fibres and the stiffness of low durometer elastomers. Although the maximum Poisson's ratio for isotropic elastomers is 0.5, it is easily shown that inplane Poisson's ratios twice unity can be obtained with a graphite/epoxy angle-ply laminate at 25°. Chou and others have predicted inplane Poisson's ratios greater than 7 for certain cord-rubber combinations. Peel previously predicted inplane Poisson's ratios higher than 32 and less than -60. Preliminary experimental results have produced inplane Poisson's ratios as high as 14. Certain combinations of un-balanced highly orthotropic laminates may also produce inplane negative Poisson's ratios. Inplane Poisson's ratios, experimentally obtained from two laminate configurations with approximately the same axial stiffness are compared. A symmetric, balanced laminate produced an inplane Poisson's ratio of 3.7; while an unbalanced laminate with an equivalent axial stiffness produced an average inplane Poisson's ratio of -1.5. Certain high and negative Poisson's ratio elastomer-matrix laminates appear to have quasi anti-symmetric relationships about 0°, although laminate designers may consider dual angle-ply laminates to be the more likely counterpart to the unbalanced dual angle auxetic laminates. Unbalanced symmetric laminates experience significantly more inplane shear deformation when axially loaded than their balanced counterparts. The high shear may be desirable for damping applications, but less desirable otherwise.

Recently, carbon fiber-reinforced thermoplastics (CFRTPs) have become popular choices in desktop-based additive manufacturing, but there is limited information on their effective usage. In Fused Deposition Modeling (FDM), a structure is created by layers of extruded beads. The degree of bonding between beads, bead orientation, degree of interlayer bonding, type of infill and the type of material, determines overall mechanical performance. The presence of chopped fibers in thermoplastics increases melt viscosity, changes coefficients of thermal expansion, may have layer adhesion issues, and causes increased wear on nozzles, which makes FDM fabrication of thermoplastic composites somewhat different from neat thermoplastics. In the current work, best practices and the effect of annealing and infill patterns on the mechanical performance of FDM-fabricated composite parts were investigated. Materials included commercially available PLA, CF-PLA, ABS, CF-ABS, PETG, and CF-PETG. Two sets of ASTM D638 tensile and ASTM D790 flexural test specimens with 3 different infill patterns and each material were fabricated, one set annealed, and all tested. Anisotropic behavior was observed as a function of infill pattern. As expected, strength and stiffness were higher when the beads were oriented in the direction of the load, even for neat resins. All fiber-filled tensile results showed an increase in stiffness, but surprisingly, not in strength (likely due to very short fiber lengths). Tests of annealed specimens resulted in clear improvements in tensile strength, tensile stiffness and flexural strength for PLA, CF-PLA, and PETG, CF-PETG but a reduction in flexural stiffness. Also, annealing resulted in mixed improvements for ABS and CF-ABS and is only useful in certain infill patterns. This work also establishes ‘Best Practices’ of FDM-type fabrication of thermoplastic composite structures and documents the minimum critical fiber lengths and fiber fractions of several CF-filled FDM filaments.

Morphing wings are desired for their ability to reduce drag, change flight characteristics, and perhaps reduce weight by eliminating flap / aileron mechanisms. Development of two generations of a student morphing wing project is documented. The second wing was further developed by Peel. The work shows how a relatively low cost but realistic morphing wing test-bed can be fabricated. Wing skin, actuator, and actuator attachment development are discussed, as well as possible auxetic skin behavior. Aerodynamic characterization of the wing will be discussed in another paper. A very simple morphing wing was fabricated in phase one. The nose was able to elastically camber down ∼ 25° and the tail 20°. Actuation was provided by three pneumatic “Rubber Muscle Actuators” (RMA) that produce high contractive forces. Upper and lower wing skins were fabricated from carbon fiber / polyurethane elastomer laminates. Lower skin buckling, actuator air leaks and actuator attachment problems were resolved in the second phase. A finite element model of the generation II wing was developed and is being used to refine/ explore the morphing wing test-bed. The second generation wing fabrication methodology shows smooth elastic cambering and no buckling or waviness in the skins. The nose cambered down 23° and the tail cambered down to 15°. Improved leak-free biomimetic actuators and attach points now include no metal parts, have higher actuation forces due to new braided sheaths and functionally gradient matrix properties.

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Morphing wings are desired for their ability to reduce drag, to change flight characteristics, and perhaps to reduce weight by eliminating flap/aileron mechanisms. Development of two generations of a morphing wing project is documented. The work shows how a relatively low cost but realistic morphing wing test-bed can be fabricated. Wing skin, actuator, and actuator attachment development are discussed, as well as possible auxetic skin behavior. Aerodynamic characterization of the wing will be discussed in another paper. A very simple morphing wing was fabricated in generation one. The nose was able to elastically camber down approximately 25 deg and the tail 20 deg. Actuation was provided by three pneumatic “rubber muscle actuators” that produce high contractive/tensile forces. Upper and lower wing skins were fabricated from carbon fiber/polyurethane elastomer laminates. Lower skin buckling, actuator air leaks, and actuator attachment problems were resolved in the second generation. A finite element model of the second wing was developed and is being used to refine the morphing wing test-bed. The second wing fabrication methodology enabled smooth elastic cambering with no buckling or waviness in the skins. The nose cambered down 14 deg and the tail cambered down to 13 deg, and is capable of larger deformations. Improved leak-free biomimetic actuators and attach points now include no metal parts and have higher actuation forces due to new braided sheaths and functionally gradient matrix properties.

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