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Joining prepreg composite interfaces with aligned carbon nanotubes

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Abstract

An interlaminar reinforcement using aligned carbon nanotubes (CNTs) is demonstrated for prepreg unidirectional carbon tape composites. Aligned CNTs are grown at high temperature and then transfer-printed to prepreg at room temperature, maintaining CNT alignment in the through-thickness direction. In initial testing, the CNT-modified interface is observed to increase fracture toughness 1.5–2.5X in Mode I, and 3X in Mode II. Both compliant interlayer and bridging are considered as mechanisms of toughening, with evidence of CNT bridging observed in fracture micrographs. Fabrication methods are compatible with existing manufacturing processes and have the potential to enhance the structural and multifunctional properties of advanced composite laminates.

Introduction

New structural concepts harnessing the attractive properties of carbon nanotubes (CNTs) have been pursued with great vigor in recent years [1], [2], [3], [4], [5], [6], [7]. In this work, a processing route for a hybrid advanced composite architecture is introduced utilizing standard prepreg-based manufacturing combined with aligned CNTs. The hybrid system (see Fig. 1) has three parts: advanced (carbon) fibers, an aerospace-grade polymer resin, and aligned CNTs oriented perpendicular to the ply surface (i.e., in the z-direction) to reinforce the laminate’s interlaminar properties. The hybrid composite would not be considered a nanocomposite (comprised solely of CNTs and a matrix) [1], [8], [9], but rather a nano-engineered hybrid composite. The processes developed (see Fig. 2) utilize a rolling transfer scheme to transfer the aligned CNTs to the prepreg, potentially allowing integration into existing processing routes. Mode I and II fracture testing, and fracture-surface inspection are performed to investigate operative toughening mechanisms.

The development of composites utilizing CNTs has been hindered by difficulties in dispersing CNTs in polymers at high weight fractions while achieving uniform and strong interactions with the polymer matrix [1], [3]. Studies of hybrid composites using unoriented CNTs dispersed in polymers report only marginal mechanical property improvements [10], [11] at low CNT loadings. Thus, to realize mechanical improvements that take advantage of CNT properties, the CNTs should be aligned and organized with long-range order [12]. One approach to establishing such order is by spinning yarns or ropes of discontinuous CNTs [13], [14] as a new type of advanced carbon ‘fiber’. Another approach is to modify existing advanced composite systems to create hybrids. Dispersion and alignment challenges for nanocomposites are even more pronounced when the CNTs must be processed into a matrix with a high volume fraction (around 60%) of advanced fibers. The interface between plies in advanced composites is far more accessible from a processing standpoint than the laminate interior, and several studies have realized marginal to significant nano-modified interfacial properties using carbon nanofibers (CNFs) and CNTs at laminate interfaces [11], [15], [16], [17], [18]. Our approach is the first to integrate aligned CNTs with existing carbon fiber prepreg materials and processing. This is accomplished by growing a vertically-aligned CNT (VACNT) forest at high temperature, and then ‘transfer-printing’ the CNTs to prepreg at room temperature, taking advantage of the tack of the prepreg to separate the CNTs from the growth substrate (see Fig. 2). Our prior studies using off-the-shelf commercial complex thermosets indicate that aligned CNT forests readily draw up such polymers through capillary action [21], and bond to the resin [9].

Section snippets

Experimental

Fabrication of the VACNT-reinforced laminates is first presented, followed by the procedures used to test the laminates in Mode I and II. Two aerospace-grade unidirectional prepregs (Cytec IM7/977-3 and Hexcel AS4/8552) are used in Mode I, and AS4/8552 is utilized for Mode II. All SEM images were taken with a FEI/Philips XL30 FEG.

Results and discussion

The fabrication of aligned CNTs oriented perpendicular to the interface is a key aspect of the observed fracture behavior to be discussed subsequently. After the VACNTs are transfer-printed to the prepreg, laminate assembly and curing proceeds per the manufacturer’s recommended processing with no modifications. CNT alignment on the prepreg has been confirmed using SEM and discussed previously (see Fig. 2). It was also shown that the aligned CNTs are wet by epoxy from just two plies of prepreg

Conclusions and recommendations

The interlaminar nanostitched architecture introduced here is realized by introducing VACNTs at the interface between plies of common prepreg material using a simple transfer-printing scheme. Preliminary Mode I and II tests indicate enhanced toughness, mechanistically explained by either interleaving or bridging toughening. CNTs observed on either surface of a Mode I crackface suggest bridging by pullout. While preliminary due to small sample size, the stable (toughened) critical energy release

Acknowledgements

This work was supported by Airbus S.A.S., Boeing, Embraer, Lockheed Martin, Saab AB, Spirit AeroSystems, and Textron Inc. through MIT’s Nano-Engineered Composite aerospace Structures (NECST) Consortium and by MIT’s Karl Chang (1965) Innovation Fund. The authors gratefully thank John Kane, Namiko Yamamoto, and the entire Technology Laboratory for Advanced Materials and Structures (TELAMS) at MIT for valuable discussions and technical support, Dr. Alexander H. Slocum for valuable input, and Dr.

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