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Poly(urethane-urea) (PUU) has been infused into ultrahigh volume fraction carbon nanotube (CNT) forests using a heat-curable polymer formula. Polymer nanocomposites with carbon nanotube volume-fractions of 1%, 5%, 10%, 20%, and 30% were fabricated by overcoming densification and infusion obstacles. These polymer nanocomposites were nanoindented quasi-statically and dynamically to discern process-structure-(mechanical) property relations of polymerizing PUU in such densely-packed CNT forests. A 100× increase in indentation modulus has been observed, which is attributed not only to CNT reinforcement of the matrix, but also to molecular interactions in the matrix itself. Quasi-static elastic moduli ranging from 10 MPa–4.5 GPa have been recorded. Storage modulus for all materials is found to track well at loadings of 200 Hz, with little effect observed from increasing CNT volume fraction. Keywords: carbon nanotubes; polymer nanocomposites; polyurethane urea; self-assembly, United States. Army Research Office (Contract W911NF-13-D-0001)

Glass-epoxy composite structures (common aerospace applications) exhibit damage precursors such as micro-cracks, which significantly impact structural performance and life span. This study focused on identifying the stress levels responsible for crazing, micro-crack formation, and coalescence into macro-cracks in S2 glass and Cycom-381 epoxy matrices (8 plies woven). Fatigue tests were performed at 1, 100, 1,000, 10,000, 100,000, and 1,000,000 cycles under tensile loading conditions (R=0.1). Damage monitoring strategies prior to, during, and after cyclic loading conditions include ultrasonic nondestructive evaluation (NDE), acoustic emission (AE), and scanning electron microscopy (SEM). Macro-scale inhomogeneities in the as-fabricated composite structures, such as porosity, debonding, macro-cracking, and ply distortions, were evaluated using an ultrasonic NDE technique. AE was used during the fatigue loading tests to identify and localize the presence of rupture, delaminations, and micro-crack formations in real time. Post-damage microscale examination of the fracture surfaces was conducted using SEM. By this systematic evaluation, the phenomena of crazing, the propagation of micro-cracks, delaminations, and the factors that lead to catastrophic failure were investigated. Based on this study, an empirical framework was developed to map the relationship between the sizing and location of damage precursors and the corresponding matrix/fiber degradation mechanisms., PDF updated 13 Nov 2013.

Army operations have placed a high premium on reconnaissance missions for unmanned aerial vehicles (UAVs) and micro air vehicles (MAVs) (15 cm or less in dimension and less than 20 g in mass). One approach for accomplishing this mission is to develop a biologically inspired flapping wing insect-sized device that can maneuver into confined areas. Analysis of insect flight indicates that in addition to the bending excitation (flapping), simultaneous excitation of the twisting degree of freedom (pitching) is required to adequately control the vehicle. Traditionally, bimorph piezoelectric lead zirconate titanate (PZT) (Pb(Zr0.55Ti0.45)O3) actuators have been used to excite the bending degree of freedom. In laminated or layered structures, bend-twist coupling is governed by having at least one anisotropic layer not aligned with the primary plate axes. By adding a layer of off-axis PZT segments to a PZT bimorph actuator, a functionally modified bimorph now offers a bend-twist coupling, flexural response in the traditional bimorph actuator. This study presents an experimental investigation of both traditional bimorph and functionally modified PZT bimorph designs intended for active bend-twist actuation of cm-scale flapping wing devices. Results are shown comparing aerodynamic parameters for three wing shapes and two PZT designs., The original document contains color images.