Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes

Abstract

Three-dimensional reinforcement of woven advanced polymer–matrix composites using aligned carbon nanotubes (CNTs) is explored experimentally and theoretically. Radially-aligned CNTs grown in situ on the surface of fibers in a woven cloth provide significant three-dimensional reinforcement, as measured by Mode I interlaminar fracture testing and tension-bearing experiments. Aligned CNTs bridge the ply interfaces giving enhancement in both initiation and steady-state toughness, improving the already tough system by 76% in steady state (more than 1.5 kJ/m² increase). CNT pull-out on the crack faces is the observed toughening mechanism, and an analytical model is correlated to the experimental fracture data. In the plane of the laminate, aligned CNTs enhance the tension-bearing response with increases of: 19% in bearing stiffness, 9% in critical strength, and 5% in ultimate strength accompanied by a clear change in failure mode from shear-out failure (matrix dominated) without CNTs to tensile fracture (fiber dominated) with CNTs.

Introduction

Carbon nanotubes continue to interest researchers for their exceptional mechanical, thermal, and electrical properties. While amply demonstrated on the nanoscale, taking advantage of intrinsic (and even scale-dependent) properties in macroscale structural systems presents significant challenges. A promising structural application for CNTs is reinforcement of traditional fiber-reinforced plastic (FRP) advanced composites [1], [2], [3], [4], [5], [6]. In traditional composites, properties are tailored by controlling the direction of the reinforcing fibers as in a unidirectional composite. Aligned CNTs are envisioned as a second, albeit nanoscale, fiber that can be combined within the microscopic framework of existing FRPs to create hybrid, or nano-engineered, composites [7], [8], [9], [10], [11]. A particular focus for reinforcement with aligned CNTs is the interface between FRP plies, a region well known as a ‘weak link’ in laminated composites. The interlaminar region is devoid of fiber reinforcement and fails via various modes, primarily delamination and matrix cracking, which may develop during the service life of the structure. Relatively poor interlaminar properties severely limit the overall performance of advanced laminated composites systems. Several solutions have emerged to strengthen the interface: 3D-textiles, stitching, and z-pinning, all of which reinforce the composite in the through-thickness (composite z-axis) direction with typically micron-diameter advanced fibers in bundles or pins with lengths on the order of millimeters [12], [13], [14], [15], [16], [17], [18], [19], [20]. The primary and unaddressed drawback of these existing solutions is that there is an unavoidable, and typically significant, reduction of in-plane mechanical properties due to lamina and laminate damage and in-plane fiber volume loss, e.g., lamina fiber and matrix damage due to z-direction pin insertion [12], [18], [21], [22].

Due to their size, nanoscale fibers such as CNTs can be introduced into the polymer matrix region (order of 1 μm spacing) between existing micron-diameter fibers to reinforce the laminate. Work by others with CNTs and other nanomaterials has either focused solely on the interlaminar area, or has dispersed small quantities (by volume) of unaligned CNTs within the matrix via mixing or other methods for intralaminar reinforcement. Due to significant issues such as agglomeration, lack of alignment, poor dispersion, and damage to CNTs during mixing, only marginal mechanical property improvements are observed for both nanocomposites [23] and hybrid composites [24], [25], [26] when CNTs are mixed into the bulk matrix. Somewhat more success has been achieved with nanoscale modification of the interlaminar region (between plies), either by growing CNTs on the surface of cloth [27] or placing unaligned CNTs at low volume fraction at the interface [28], [29], [30], [31]. An area of particular activity has been in growing CNTs on the surface of carbon fibers to reinforce carbon FRP (CFRP), with the additional unfortunate difficulty that typical CNT growth processes significantly damage tensile properties of the carbon fibers [32], [33], [34], [35].

Here, aligned CNTs are introduced in both the interlaminar and intralaminar regions of the laminate, providing three-dimensional reinforcement (see Fig. 1). This architecture has been introduced previously into the literature as a fuzzy fiber-reinforced plastic (FFRP) laminate [10]. Prior work with this architecture has shown property enhancement due to the aligned CNTs (interlaminar shear strength as measured by short-beam shear increased ∼70%, and electrical conductivity increased by more than 10⁶). Due to advances in larger-scale synthesis [36], the work here investigates the magnitude and mechanisms of Mode I interlaminar toughness and tension-bearing strength enhancement. Significant toughness enhancement observed in steady-state is attributed to CNT-pull-out toughening, or bridging, consistent with observations of CNT pull-out from crack surface inspection. Bridging is modeled using a recently developed analytical approach [37] and shown to predict the magnitude of observed toughening. Aligned CNTs in the FFRP architecture also reinforce the intralaminar region of the laminate. In-plane strength and stiffness enhancement due to the aligned CNTs is assessed via a standard through-hole tension-bearing strength testing. Tension-bearing strength is a critical property for composites given the pervasive use of mechanical fasteners, and the poor performance of composites relative to metals in this area [38]. Thus, the effectiveness of aligned CNTs in providing intralaminar and interlaminar reinforcement between fibers and between plies is assessed in this work for the FFRP architecture, building on the prior observed positive interlaminar shear strength improvement assessed via short-beam-shear (SBS) testing [10].