One of the main limitations of advanced composite materials is their poor z-direction mechanical properties due to the unreinforced pure polymer region at ply interfaces, a known Achilles heel of advanced composites. There are several approaches to reinforce such composites in the through-thickness direction including 3D weaving, stitching, and Z-pinning. As these approaches are based on micron-diameter fibers and their assemblies (tows), in-plane fiber movement and/or damage, fiber volume loss, and stress concentrations are produced as unavoidable artifacts during manufacturing. These act to significantly reduce the in-plane mechanical properties of the laminate, such that these technologies are not in significant use. Thus, the problem of weak interfaces in composites, and concomitant issues such as damage resistance and tolerance, and their implications for over-design, are outstanding limitations in composite structural performance.

In order to avoid this reduction of the in-plane properties, carbon nanotubes (CNTs) can be used as a secondary or hybrid reinforcement that can be integrated within advanced composites. … Z-direction nanofibers, specifically aligned carbon nanotubes (A-CNTs), have been introduced into the interlaminar region of unidirectional composite plies in prior work (termed ‘nanostitching’). … Here, we show that in-plane properties can be significantly increased, thereby giving concomitant improvement in both in-plane and out-of-plane laminate properties, in stark contrast to typical out-of-plane reinforcements (stitching, z-pinning, weaving) that degrade in-plane properties.

—Guzman de Villoria et al.

Nanostitching concept in which aligned carbon nanotubes (A-CNTs) bridge ply interfaces in laminated composites: (left) concept illustration of A-CNTs at a laminate interface, and (right) scanning electron micrographs of representative A-CNT forests showing CNT alignment. Guzman de Villoria et al. Click to enlarge.

In their study, the team found that using the A-CNT nano-stitches suppressed delamination damage modes associated with pre-ultimate failure in the in-plane loaded laminates, significantly increasing load-carrying capability:

• tension-bearing (bolt pull out) critical strength increased by 30%
• open-hole compression ultimate strength increased by 14%
• L-section bending energy and deflection increased by more than 25%

There was no increase in interlaminar or laminate thickness due to the A-CNTs. The researchers found that the ∼10 nm diameter carbon nanotubes interdigitated between carbon fibers in the adjacent laminae—i.e., the observed reinforcement is not due to formation of a thicker interlayer.

These increases in substructural in-plane strengths are in stark contrast to degradation that typically occurs with existing 3D reinforcement approaches such as stitching, weaving and z-pinning.

—Guzman de Villoria et al.

This work was supported by Airbus Group, Boeing, Embraer, Lockheed Martin, Saab AB, Spirit AeroSystems Inc., Textron Systems, ANSYS, Hexcel, and TohoTenax through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium and, in part, by the U.S. Army.

Resources

• R. Guzman de Villoria, P. Hallander, L. Ydrefors, P. Nordin, B.L. Wardle (2016) “In-plane strength enhancement of laminated composites via aligned carbon nanotube interlaminar reinforcement,” Composites Science and Technology, Volume 133, Pages 33-39 doi: 10.1016/j.compscitech.2016.07.006