Sunny S. Wicks

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

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)

Processing and Characterization of Infusion-Processed Hybrid Composites with In Situ Grown Aligned Carbon Nanotubes

Hybrid composite materials enhanced with aligned CNTs directly grown on fibers of woven cloths are fabricated by a vacuum-assisted resin infusion process for the first time. A vacuum-assisted resin infusion process can significantly build upon and expand the hybrid composite work already developed using hand-layup to a more scalable and relevant processing technology. Chemical vapor deposition (CVD) is used for growing aligned CNTs on the alumina fiber surfaces of woven fabrics, creating a nanostructured “fuzzy fiber (FF)” hierarchical architecture. The aligned CNTs grown in situ on alumina-fiber woven fabrics serve as interlaminar and intralaminar reinforcement. FF woven fabrics and baseline (wihout CNTs) alumina woven fabrics are formed on a mold plate, and an aerospace-grade resin-transfer molding (RTM) resin is infused into the laminates by vacuum-assisted resin infusion. Optical and scanning electron microscopy (SEM) are used to characterize these composites. In these microphotographs, no difference in terms of void fraction (less than 1%) and overall distribution of CNTs and resin-fiber ratios are observed between the baseline and FF composites. CNTs are noted to remain on the surface of alumina fibers during the infusion process and are not observed in the excess resin pulled through the laminates. The effect of the CNT distribution within the FF composite is further assessed using electrical impedance spectroscopy testing in the in-plane and transverse directions. Employment of resin infusion process for FF woven fabrics should greatly simplify the development of new composite materials with significantly-enhanced mechanical and electrical as well as thermal properties.

Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites

As carbon nanotube (CNT) infused hybrid composites are increasingly identified as next-generation aerospace materials, it is vital to evaluate their long-term structural performance under aging environments. In this work, the durability of hierarchical, aligned CNT grafted aluminoborosilicate microfiber-epoxy composites (CNT composites) are compared against baseline aluminoborosilicate composites (baseline composites), before and after immersion in water at 25 °C (hydro) and 60 °C (hydrothermal), for extended durations (90 d and 180 d). The addition of CNTs is found to reduce water diffusivities by approximately 1.5 times. The mechanical properties (bending strength and modulus) and the damage sensing capabilities (DC conductivity) of CNT composites remain intact regardless of exposure conditions. The baseline composites show significant loss of strength (44 %) after only 15 d of hydrothermal aging. This loss of mechanical strength is attributed to fiber-polymer interfacial debonding caused by accumulation of water at high temperatures. In situ acoustic and DC electrical measurements of hydrothermally aged CNT composites identify extensive stress-relieving micro-cracking and crack deflections that are absent in the aged baseline composites. These observations are supported by SEM images of the failed composite cross-sections that highlight secondary matrix toughening mechanisms in the form of CNT pullouts and fractures which enhance the service life of composites and maintain their properties under accelerated aging environments.

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.

Fracture Toughness of Aligned Carbon Nanotube Polymer Nanocomposites

Aligned carbon nanotubes (CNTs) are being investigated for application to numerous material disciplines including structural composite materials due to their unique scaledependent physical properties. Prior work on nanoengineered composite architectures demonstrated improved interlaminar fracture toughness was dependent on epoxy type. In this work, we seek to understand the reinforcement mechanisms of aligned CNTs in a polymer matrix in the absence of fibers, of aligned polymer nanocomposites in different epoxy types. Preliminary investigations were performed using single edge notch beam specimens to isolate the fracture toughness of small CNT nanocomposites, and the mechanisms of reinforcement can be elucidated through tight control over CNT alignment and loading epoxy. While crack initiation off a sharpened precrack are unchanged with added aligned CNTs, an increase in fracture surface area and added crack arrest capability reveal mechanisms by which CNTs can add absorb energy and increase toughness post crack initiation.

Effect of Manufacturing Route on Mode I Fracture Toughness of Aligned Carbon Nanotube Reinforced Composites

Several hybrid composite architectures with aligned carbon nanotubes have been shown to provide interlaminar reinforcement of fiber reinforced composites. Aligned carbon nanotubes (CNTs) grown on the surface of woven fibers is one such ‘fuzzy fiber’ reinforced plastics (FFRP) composite that provides mechanical reinforcement in the interlaminar and intralaminar regions, with large (> 1 kJ/m 2 ) increases in Mode I toughness observed previously. Here, the effects of cloth weave and CNT length on Mode I toughness is investigated as part of ongoing work to elucidate mechanisms contributing to interlaminar reinforcement in such hybrid systems. The system considered here differs from prior work in that an aerospace-grade resin is utilized and the composites are fabricated via infusion rather than hand lay-up. The Mode I response for two cloth weaves is studied, as well as a preliminary study on the effect of CNT length using one of the weaves. A moderate increase in steady-state Mode I toughness of ~20% is observed in the new laminate system for CNTs of ~20 µm in length, with a decrease in steady-state toughness noted for shorter (6 µm) CNTs in preliminary tests. The potential of these multi-scale interface hybrid composites for aerospace applications has yet to be fully harnessed without further investigation along the directions begun in this work.

Fracture Toughness of Aligned Carbon Nanotube Polymer Nanocomposites for the Reinforcement of Hierarchical Composite Materials

Aligned carbon nanotubes (CNTs) are being investigated for application to numerous material disciplines including structural composite materials due to their unique scaledependent physical properties. Nanoengineered architectures called Fuzzy Fiber Reinforced Plastics (FFRP) have been developed, where CNTs are grown radially on advanced fibers and extend into inter and intralaminar spaces. The aligned CNT forests induce a swelling of the tows and cloth causing fibers to spread apart, and the demonstrated improvements in Mode I toughness could be attributed to changes in matrix properties, interlaminar region morphology due to the swelling of the plies, or a combination of both. The work here aims to decouple the two effects by determining the toughness of the matrix region in the absence of fibers, employing aligned-CNT polymer nanocomposites with relevant CNT loadings (volume fractions). A single edge notch beam specimen has been demonstrated to isolate the fracture toughness of such aligned-CNT reinforced polymer nanocomposites.