The main goal of this work is to obtain a scientific understanding of the possibilities provided by, and the behavioral features of, a novel type high performance carbon nanotube (CNT) reinforced composite material incorporated in the interfaces of composite laminates and bonded joins with the following two purposes: (a) providing enhancement of the interlaminar fracture toughness and strength and (b) serving as a continuous strain monitoring sensor.

Illustration of the interlayer concept and classification of the known interleaves comprised within interlayer.

The material under investigation is a relatively thin (in the range of several hundred microns), high CNT volume fraction, shear-pressed sheet (SPS) fabricated from the vertically grown aligned CNT array (a.k.a. “CNT forest”). The intrinsic high fracture toughness of composites reinforced with such CNT SPS should provide significant increase in the interlaminar fracture toughness of composite laminates and bonded joints. Simultaneously, the piezoresistive nature of the aligned CNTs and their interconnected networks opens new opportunities for monitoring both the strain and progressive failure in the composite joints. The stated principal outcome of this research was to establish a starting point for the future development of field deployable multifunctional CNT SPS interface-enhancing materials enabling, at the same time, for strain sensing of composite laminates and bonded joints of aerospace structures.

The interleaf material is fabricated from CVD vertically grown CNT arrays by the shear pressing method. The shear pressing is realized here on a specialty automated device which allows one to delicately control the CNT array alignment, CNT volume fraction and the shear pressed sheet (SPS) thickness. The produced thin dry SPS can be strategically placed within some region between composite plies and, after conventional laminate fabrication it becomes its integral part. Under this scenario, the SPS can be partly or fully impregnated during the cure process with the resin which is already contained in the prepreg plies. An alternative manufacturing approach could be to first impregnate the SPS preform with some other, desirably lower viscosity, resin and partially cure it, then embed the “SPS prepreg” between the plies of composite and complete the manufacturing cycle.

Extensive experimental results show how the Double Cantilever Beam (DCB) testing methodology can be used to evaluate the effect of different CNT SPS interleaves on Mode I interlaminar fracture toughness of conventional carbon/epoxy laminated composites. The experimental studies also included traditional optical microscopy and SEM imaging of the fractured DCB samples. They showed that the delamination propagation path is dramatically altered in the presence of CNT SPS interleaves. It changes from a nearly straight one to highly tortuous; the latter one typically consists of a sequence of jagged or saw-tooth type microcracks often combined with even more subtle sub-micro scale cracks developing within the interleaf, and with microcracks propagating between the interleaf and adjacent composite ply. The overall delamination fracture mechanism shows highly variable from sample to sample and very sensitive to such factors as CNT functionalization, epoxy resin viscosity, cure cycle, the resin infusion technique into the SPS, to the presence of residual voids, and other fine manufacturing peculiarities.

Overall, results of this research in the part of enhancing fracture toughness of traditional laminates by embedding CNT SPS interleaves are very encouraging. They showed that both dry and pre-infused CNT SPS interleaves significantly, up to two times, increase the critical strain energy release rate of the baseline non-interleaved laminate. The non-functionalized, plasma treated and acid treated SPSs were used. Both functionalization methods maintained the high alignment and aspect ratio of the CNTs.

Although adding the CNT functionalization step does not result in further significant toughening versus the non-functionalized interleave case, the characteristics of the fracture surfaces appear to be dramatically different. As evidenced by the much “smoother” load vs. displacement curves, the pre-infused SPS interleaves show better ability than the non-interleaved and dry SPS interleaved laminates to resist catastrophic failure. Apart from the Mode I fracture toughness performance (which was in the focus of this study), the CNT SPS reinforcements provide high dimensional stability to the interleaves and structural joining elements. This feature may be especially beneficial for interfaces and joints in high-precision devices and structures (particularly, miniature ones) which require very thin preforms with well-defined dimensions - a requirement that may be difficult to satisfy with traditional adhesives and bonding methods.

This work was done by Dr. Alexander Bogdanovich and Dr. Philip Bradford of NC State University College of Textiles for the Air Force Research Laboratory. AFRL-0242

This Brief includes a Technical Support Package (TSP).
Multifunctional Shear Pressed CNT Sheets for Strain Sensing and Composite Joint Toughening

(reference AFRL-0242) is currently available for download from the TSP library.

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This article first appeared in the June, 2016 issue of Aerospace & Defense Technology Magazine.

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