Multi-scale interlaminar fracture mechanisms in woven composite laminates reinforced with aligned carbon nanotubes Sunny S. Wicks, Wennie Wang 1 , Marcel R. Williams, Brian L. Wardle ⇑ Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, United States article info Article history: Received 16 March 2014 Received in revised form 19 May 2014 Accepted 5 June 2014 Available online 13 June 2014 Keywords: A. Carbon nanotubes A. Laminate A. Hybrid composites B. Fracture toughness D. Fractography abstract Aligned nanoscale ﬁbers have been shown to provide inter and intralaminar reinforcement of ﬁber-rein- forced polymer composites. In one hierarchical architecture, aligned carbon nanotubes (CNTs) grown on advanced microﬁbers in a woven ply creates a ‘fuzzy ﬁber’ reinforced plastic (FFRP) laminate. Here, the mechanisms of Mode I fracture toughness enhancement for such laminates are elucidated experimentally by varying the type of epoxy and aligned CNT length. Reinforcement effectiveness is found to vary from reduced initiation toughness to over 100% increase in steady-state fracture toughness, depending upon the interlaminar fracture mechanisms. Fracture-surface morphology investigations reveal that interlam- inar toughness enhancement is signiﬁcantly less for an aerospace infusion resin than that for a much tougher hand lay-up marine epoxy. Long (20 micron) aligned CNTs add signiﬁcant toughness in steady state (>1 kJ/m 2 increase for marine epoxy) via CNT pullout and by driving the crack through tortuous paths around and through microﬁber bundles/tows, whereas shorter CNTs produce less toughening (or even reduced toughness in aerospace epoxy), which is attributed to increased microﬁber–matrix cohesive failure. These ﬁndings reveal the multi-scale nature of the aligned-CNT reinforced composite ply inter- face, and the mechanisms at work at the micro and nanoscales that inﬂuence the macroscopic behavior. These new insights provide impetus for using aligned CNTs to tailor microﬁber polymer composites by adjusting microﬁber orientation, steering cracks through tortuous paths, and maximizing fracture surface area through both microﬁber and nanoﬁber pullout. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Aerospace structural components are increasingly being made of (micro)ﬁber reinforced plastics (FRPs) due to their excellent mass-speciﬁc mechanical properties. While properties in the plane of the microﬁbers are strong, stiff, and tailorable, the interlaminar region is relatively weak and compliant, leading to failure due to delamination and other damage modes. To address these deﬁcien- cies at the interlaminar interface, several architectures have been developed to enhance strength and/or toughness in the through- thickness direction of the laminate, including Z-pinning, stitching, and 3-D weaving [1–3]. These processes result in a signiﬁcant reduction in in-plane properties of the laminate due to damage from insertion of through-thickness direction reinforcements that are hundreds of microns in diameter, and loss of microﬁber volume fraction in the in-plane direction [1,4]. Alternative routes to increasing interlaminar toughness include modiﬁcation of matrix properties through added toughening agents, compliant or rein- forced interlayers [5,6] or particles [7–9] and nanoﬁbers [10,11], including in particular carbon nanotubes (CNTs) that possess advantageous speciﬁc stiffness and strength [12]. Their excellent electrical and thermal conductivity make them attractive as a nanoﬁller for aerospace composites due to their ability to add multifunctional properties and capabilities [13–16]. While CNTs possess signiﬁcant potential for composite reinforcement, only small improvements in composite properties have been realized due to difﬁculties in processing and controlling CNT orientation during manufacturing. Many approaches to CNT incorporation have been explored in industry and in the literature, dominated by mixing CNTs in poly- mer matrices before inﬁltration [17–23] in low loading to mini- mize inﬂuence on matrix viscosity. CNTs tend to agglomerate into bundles, and techniques to separate the CNTs by sonication [24], surface modiﬁcation [25,26] and/or shear mixing [27] can easily damage the CNTs. Alignment of the CNTs in the matrix is also difﬁcult to obtain even with severe processing techniques like extrusion [28] that are not compatible with laminated composites, though some orientation has been controlled through directional http://dx.doi.org/10.1016/j.compscitech.2014.06.003 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 617 252 1539. E-mail address: wardle@mit.edu (B.L. Wardle). 1 Current address: University of California, Santa Barbara, United States. Composites Science and Technology 100 (2014) 128–135 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech