Brian L. Wardle

Aligned Carbon Nanotube Reinforcement of Advanced Composite Ply Interfaces

*† ‡ § ** , This work presents the fabrication and characterization of three hybrid multiscale advanced composite materials. Long (>20 micron), aligned carbon nanotubes (CNTs) are placed at the interface of existing advanced composite plies and used as a reinforcement and to enhance electrical properties of the laminate. Three fabrication routes utilizing aligned CNTs at ply interfaces are presented: transplantation of CNT forests between prepreg carbon/epoxy plies, transfer of aligned CNTs and layup between woven carbon fiber plies that are subsequently infused to form a laminate, and in situ growth of aligned CNTs on the interior (and surface) of alumina fiber woven cloth prior to hand layup. Aerospace-grade thermoset epoxies, without modification, are noted to wet and penetrate the unfunctionalized aligned CNT forests, which is consistent with initial studies on solely CNT-polymer interactions. In all the fabrication routes, aligned CNTs are observed at the interface after laminate fabrication. Both mechanical (interlaminar) and multifunctional (electrical) property modifications are noted for the laminates containing CNTs. Significant interlaminar property enhancement has been observed and the mechanisms of this reinforcement are investigated via optical and scanning electron microscopy. Further improvements in the fabrication routes are discussed, and further testing of additional laminate-level property enhancements are suggested. Nomenclature

Electrical and Thermal Properties of Hybrid Woven Composites Reinforced with Aligned Carbon Nanotubes

Electrical and thermal properties of hybrid advanced composites containing aligned carbon nanotubes (CNTs) are studied experimentally. In situ growth of aligned CNTs on the surface of woven alumina fiber cloth, followed by wetting with a thermoset epoxy, allows CNTs to be distributed throughout the laminate and forming transport pathways. Laminate cross sections are characterized optically with scanning electron microscopy, revealing low (<1%) void fractions as seen in baseline laminates without CNTs. Fabricated samples with ~0.05-5 wt% fractions of CNTs were characterized for alternating current (AC) impedance in the in-plane and the through-thickness directions. Measurements indicate that the impedance is dominated by simple resistance for the hybrid specimens, resulting in significant resistivity decrease (by a factor of x 10 -4 ) compared to the baseline laminate. In addition, a through-thickness functionally-graded laminate having aligned CNTs only in the middle plies, displayed an RC-response similar to the non-conducting baseline laminate in the through-thickness direction, while retaining a purely resistive behavior (low resistivity) in the in-plane direction. This demonstrates nonisotropic impedance laminate design using CNT inclusion in specific plies to achieve macroscopically-tailored impedance. Through-thickness thermal conductivity of the hybrid laminates is experimentally observed with moderate compared with baseline composites, but additional testing is required with techniques better suited to the small specimen sizes, such as the laser-flash method. Aerospace applications for such hybrid laminates that demonstrate multifunctional enhancement include electrostatic and electromagnetic interference shielding, and potentially lightningstrike protection. Future work includes more expansive mechanical and multifunctional characterization and modeling of the existing system, and extension of the architecture to carbon fiber reinforced polymer composites.

Fabrication and Multifunctional Properties of High Volume Fraction Aligned Carbon Nanotube Polymeric Composites

High volume fraction aligned carbon nanotube (CNT) nanocomposite specimens are fabricated using mechanical densification of CNT forests and capillarity-induced wetting of the forests with several thermoset polymers. Such nanocomposites approach the ideal morphology of collimated aligned fiber systems used in aerospace composites, and have long been expected to exhibit substantial engineering property improvements over existing systems. Polymers used are unmodified and include two aerospace-grade complex thermosets and a UV-curing thermoset used in microfabrication. Mechanical densification of the CNT forests prior to polymer introduction results in uniform nanocomposites. High volume fraction (to ~20%) CNT forests are effectively wet by the thermosets studied. Modulus, hardness, and electrical resistivity are characterized as a function of volume fraction for one of the epoxy systems. Multifunctional properties (modulus, hardness, and electrical conductivity) of the nanocomposites are strongly influenced by CNT volume fraction. Such specimens can be used to explore nano-scale interaction effects such as CNT- CNT contact effects on thermal and electrical conductivities.

Electrothermal Icing Protection of Aerosurfaces Using Conductive Polymer Nanocomposites

United States. Dept. of the Navy. Small Business Innovation Research (Contract N68335-11-C-0424)

Aligned Carbon Nanotube Reinforcement of Aerospace Carbon Fiber Composites: Substructural Strength Evaluation for Aerostructure Applications

Vertically aligned carbon nanotubes (VACNTs) are placed between all plies in an aerospace carbon fiber reinforced plastic laminate (unidirectional plies, [(0/90/±45)2]s) to reinforce the interlaminar region in the z-direction. Significant improvement in Mode I and II interlaminar toughness have been observed previously. In this work, several substructural in-plane strength tests relevant to aerostructures were undertaken: bolt/tension-bearing, open hole compression, and L-shape laminate bending. Improvements are observed for the nanostitched samples: critical bearing strength by 30%, open-hole compression ultimate strength by 10%, and L-shape laminate energy (via increased deflection) of 40%. The mechanism of reinforcement is not compliant interlayer creation, but rather is a fiberstitching mechanism, as no increase in interlayer thickness occurs with the nanostitches. Unlike traditional (large-fiber/tow/pin) stitching or z-pinning techniques that damage inplane fibers and reduce laminate in-plane strengths, the nano-scale CNT-based ‘stitches’ improve in-plane strength, demonstrating the potential of such an architecture for aerospace structural applications. The quality of VACNT transfer to the prepreg laminates has not been optimized and therefore the noted enhancement to strength may be considered conservative. Ongoing work has been undertaken to both improve VACNT transfer and expand the data set.

Tomographic Electrical Resistance-based Damage Sensing in Nano-Engineered Composite Structures

Abstract : Aligned carbon nanotubes (CNTs) are being investigated as a means for enhancing structural performance of composite structures. Inherent in introducing CNTs into existing polymer-matrix composites are new multifunctional attributes such as significantly enhanced electrical conductivity and piezoresistivity that may be used for damage sensing and inspection. Here, fiber-reinforced polymer-matrix laminates with aligned CNTs grown in-situ are coupled with a non-invasive sensing scheme utilizing the enhanced electrical conductivity of the laminates to infer damage based on resistance changes. The laminates contain long (~10 micron) aligned CNTs throughout the woven plies of the laminate, including at the ply interfaces. Electrodes are written onto the laminate surfaces using a direct-write process, and 3D damage inspection (in-plane and through-thickness) is demonstrated for impacted composite plates.

Interlaminar Fracture Toughness of a Woven Advanced Composite Reinforced with Aligned Carbon Nanotubes

We present the fabrication and interlaminar fracture testing of a novel multi-scale composite architecture incorporating aligned carbon nanotubes (CNTs). Forests of long (>20 μm) radially-aligned CNTs are grown directly on fibers in alumina fiber woven cloth forming “fuzzy fiber” plies that contain CNTs on the surface and inside the woven fabric. Epoxy is introduced during hand layup creating a fuzzy fiber reinforced plastic (FFRP) composite with improved interlaminar properties. The interlaminar Mode I fracture toughness of these nano-engineered composites are compared to those of composites made without CNTs. Mode I fracture toughness is considered in detail, including discussion of Rcurves and optical and scanning-electron microscopy of fracture surfaces that help to elucidate the mechanisms of observed significant (up to 100%) increase in toughness. In addition to improved fabrication and characterization, Mode II testing and modeling of fracture are areas of future work.

Interlaminar Fracture Toughness of Laminated Woven Composites Reinforced with Aligned Nanoscale Fibers: Mechanisms at the Macro, Micro, and Nano Scales

United States. National Aeronautics and Space Administration (Space Technology Research Fellowship)

Molecular Dynamics and Finite Element Investigation of Polymer Interphase Effects on Effective Stiffness of Wavy Aligned Carbon Nanotube Composites

Interphase effects have been studied for their effect on composite properties for many decades, and it is well documented that an interphase can exist in polymer composites comprised of nanofibers as well. We present a first study of interphase effects on the basic elastic response of wavy aligned carbon nanotube (A-CNT) polymer nanocomposties (PNCs). Waviness is characterized by ex situ pre-fabrication imaging of A-CNT forests and used as an input to finite element analyses of the PNCs containing an interphase region in the thermoset polymer defined using molecular dynamics (MD) simulations. The interphase thickness of ~1nm is found to be independent of crosslink density and contain regions of both higher and lower mass density than the bulk polymer. Finite element analyses of wavy single and double-wall A-CNT PNCs incorporating this interphase, allow the effective stiffness based on a representative volume element to be calculated. Waviness of the A-CNTs dominates the effective axial stiffness of the PNCs, with the interphase having a negligible effect. The interphase changes the stress and strain distribution local to the CNT ‘fiber’ and this is expected to play an important role in failure, such as CNT pullout, of the CNTpolymer system. These findings, in addition to the relatively high volume fraction (Vf) of the interphase in PNCs with high CNT Vf, suggests that the interphase may play a more important role in PNCs than in micron-scale (typical) collimated fiber composites. Future work in this area includes inelastic polymer response during nanofiber pullout.

Factors Controlling Infusion Processing of Laminated Composites Containing Aligned Carbon Nanotubes

The manufacturing of fiber-reinforced plastic (FRP) laminates containing aligned carbon nanotubes (CNTs) is investigated in this paper. Three-dimensional reinforcement of the laminates is achieved using long (>10   m) CNTs in both the interlaminar and intralaminar regions. Radially-aligned CNTs were grown in situ on the surface of alumina fibers in woven fabrics by chemical vapor deposition (CVD), creating a nanoscale, ‘fuzzy fiber (FF)’ hierarchical architecture. The effect of CVD conditions on CNT growth morphology and length is determined for a new cloth weave, and a CVD process for long, consistent aligned CNT growth is established. Additionally, the effect of CNT length on laminate permeability is explored to study the manufacturability of fuzzy fiber reinforced plastic (FFRP) laminates. The parametric study on CVD conditions revealed the robustness of the CVD process in growing aligned CNTs on alumina fibers under a range of conditions. Permeability testing revealed a 10x decrease in permeability of laminates with the longest CNTs, in contrast to the over 20x increase in surface area. Results indicate that infusion processing of FFRP laminates with an unmodified aerospace-grade resin can be accomplished with standard infusion setups.