Forming of composite spars that include carbon nanotube interlayers

13 May 2016
Per Hallander, Jens Sjölander, and Malin Åkermo
An experimental study illustrates how the manufacture of composite parts is affected by the introduction of multiwall carbon nanotubes between pre-impregnated layers.

The use of carbon nanotubes within novel, multifunctional composite materials enables improved matrix properties. Indeed, by using highly aligned multiwall carbon nanotubes (MWCNTs) with thermoset resins, good dispersion and distribution of the nanotubes can be achieved. There has already been a considerable amount of research conducted with the aim of improving the MWCNT growth process.1–4 To date, however, there has been little work conducted on the processing of composites that include MWCNTs as interlayers in the stack.

The previous studies have shown that because of the fragile nature of an MWCNT mat, the mat itself is sensitive to manual handling. The mats are also sensitive to the large-scale deformations (e.g., shearing) that are necessary to form them into a 3D geometry. To avoid damaging MWCNT mats, we have previously shown that they must therefore be carefully transferred5 onto the surface of a flat composite layer (which is used to carry the mat). It is thus necessary to conduct composite part forming after the composite lay-up process.

The aim of our work6 was to perform an experimental study to see how aligned MWCNTs are affected by the forming process during a composite part manufacturing process. We used hot drape forming (HDF)—a commonly used process in the aircraft industry—as the forming process in this study. To strengthen our description of local forming behavior and support our experimental study, we also performed a brief numerical study. To conduct this numerical study we required material models. Consequently, limited experimental investigations of the interply friction7–9 and the intraply shear resistance7, 10,11 of materials containing MWCNTs were also included in this work.

In our experiments we used an aerospace-graded 180°C-cure epoxy pre-impregnated (‘prepreg’) layer, with unidirectional, high-tenacity carbon fibers (at approximately 57% fiber volume content). The aligned MWCNTs were grown (with a height of about 20μm1) on silicon wafers. We then transferred these onto the prepreg surface and performed the HDF. To ensure that the MWCNT areas were subject to several—but realistic—forming mechanisms, we chose a C-shaped spar with a recess area. The geometry of this spar, which we have also used in a previous study,12 is described in Figure 1 and Table 1. In addition, we used a [–45°, 45°, 90°, 0°, 90°] lay-up because a [45°, –45°, 90°] lay-up would have been more prone to intraply deformation than a [45°, 90°, –45°] version.13 We placed MWCNTs between all the plies in the recess area, as shown in Figure 2.

Geometry of the C-shaped spar with a recess area. This spar was used in hot drape forming (HDF) experiments to ensure that the multiwall carbon nanotubes (MWCNTs) were subject to several forming mechanisms. The specific details of the geometry are also given in Table 1.

Details of the C-shaped spar (see Figure 1) geometry.

Spar length (mm)480
Web width (mm)70
Flange length (mm)55
Transition zone length (mm)125
Recess depth (mm)6.25
Nominal thickness (mm)0.655
Radius recess flange (mm)2
Radius straight flange (mm)6

Orientation of the MWCNTs in the flat lay-up before HDF was conducted. Dimensions are given in millimeters. J: Joggle area.

The results of our work can potentially shed light on some of the detailed forming mechanisms in composite lay-ups and on the possibility of using MWCNT layers as ‘markers’ for some deformation modes (see Figure 3). In particular, we found (see Figure 4) that cross-plied prepreg material with MWCNT mats at the ply interfaces promoted seemingly continuous shear, according to pin-jointed net theory.14 It has previously been observed10 that intraply shear normally starts in a continuous shear mode (dominated by shear within the tows). After a specific point, however, the inter-tow shear starts to dominate. This inter-tow shear also causes a measurable degree of interply shear at the crossing and rotating fiber tows (in accordance with a previously developed theory15). When the MWCNT interlayers are introduced, the interply shear is resisted by the z-pinning of the MWCNTS and by the increased interply friction. Shear within the tows therefore continues to be less-energy-consuming and the dominating deformation mode, even at larger levels of deformation.

(a) Scanning electron microscope (SEM) image of a MWCNT mat in a C-shaped spar that shows shear in the [–45°] and [45°] layers (i.e., the bottom two layers in the image). Image not to scale, original magnification 350×. (b) A zoomed-in part of the image in (a), showing the part of the sample that has undergone shear. Image not to scale, original magnification 5500×. (Images are provided courtesy of Camilla Söderström, Exova.)

(a) Comparison of the difference in pin-jointed net behavior between a reference sample (left) and a sample containing MWCNTs (right). (b) SEM image of the MWCNT mat in an intraply shear test sample after 5mm of deformation. Image is not to scale, original magnification 5500×. (Image courtesy of Camilla Söderström, Exova.)

Some other important findings from our work include that the MWCNT mat was more prone to shear than to buckling during the forming process. In addition, the intraply shear stiffness and the interply friction of the prepreg increased significantly when the MWCNT mats were introduced at the prepreg ply interfaces. Lastly, the results of both our numerical study and our experimental study show that adding MWCNTs affects the forming properties of the prepreg material.

In summary, we have performed an experimental study to investigate how aligned multiwall carbon nanotubes are affected during the forming process of composite part manufacturing. In addition to our hot draping forming experiments, we conducted a numerical study to support and verify our results. The results of our work indicate that the MWCNTs are affected by shearing, which occurs during the part forming, but that their integrity is maintained. In addition, the MWCNT shear pattern that is observed can provide some indication of the deformation modes. Our study is relevant, for instance, to the aerospace industry because modern aircraft structures are built from thin layers of epoxy that are pre-impregnated with carbon fiber laminates. In our future work we will focus on steering the forming behavior by using tailored prepreg interlayer properties.


Per Hallander
Saab AB

Per Hallander has been a senior material and process engineer since 2007 and PhD candidate in the Department of Aeronautical and Vehicle Engineering of the Royal Institute of Technology, Sweden, since 2011.

Jens Sjölander
Department of Aeronautical and Vehicle Engineering, Royal Institute of Technology

Jens Sjölander has been a PhD candidate studying lightweight structures, with a special focus on modeling composite forming processes, since 2013.

Malin Åkermo
Department of Aeronautical and Vehicle Engineering, Royal Institute of Technology

Malin Åkermo is an associate professor in lightweight structures, with a special interest in process modeling for composites and sandwich manufacturing. She has been active in this area for more than 20 years. Her research covers material modeling for forming simulations, process simulation, as well as the development of novel manufacturing processes and cost modeling.


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DOI:  10.2417/spepro.006447

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