Graded conventional-auxetic Kirigami sandwich structures: Flatwise compression and edgewise loading

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Abstract

The work describes the manufacturing and testing of graded conventional/auxetic honeycomb cores. The graded honeycombs are manufactured using Kevlar woven fabric/914 epoxy prepreg using Kirigami techniques, which consist in a combination of Origami and ply-cut processes. The cores are used to manufacture sandwich panels for flatwise compression and edgewise loading. The compressive modulus and compressive strength of stabilized (sandwich) honeycombs are found to be higher than those of bare honeycombs, and with density-averaged properties enhanced compared to other sandwich panels offered in the market place. The modulus and strength of graded sandwich panel under quasi-static edgewise loading vary with different failure mode mechanisms, and offer also improvements towards available panels from open literature. Edgewise impact loading shows a strong directionality of the mechanical response. When the indenter impacts the auxetic portion of the graded core, the strong localization of the damage due to the negative Poisson’s ratio effect contains significantly the maximum dynamic displacement of the sandwich panel.

Introduction

Sandwich structures with composite skin and honeycomb core have been widely used for a variety of engineering applications, from advanced aircraft design [1], [2], to modern wind turbines [3]. Between the main features behind the use of sandwich structures is the possibility to design lightweight components and structures with high specific bending stiffness, high strength and high buckling resistance [4], [5], [6]. Conventional hexagonal shaped honeycomb constitutes the most fundamental and widely used cellular configuration, which is produced using the expansion method or corrugated process [1], [7]. Hexagonal honeycombs possess a positive Poisson’s ratio, leading to anticlastic behavior (i.e., producing saddle-shaped curvatures when bent out-of-plane), making difficult to manufacture sandwich structures with complex geometry without using postprocessing techniques with high discard rates. An alternative to the hexagonal core is represented by auxetics (negative Poisson’s ratio) honeycombs, which are usually characterized by stiffening geometric effects [8], [9], [10], [11], [12], enhanced in-plane indentation resistance [9], [11], [13], [14], transverse shear modulus [12], [15] and impact energy absorption [15], [16], [17]. The auxetic topologies reported so far mainly involve re-entrant, chiral, double arrow-head and rotating rigid units [18], [19], [20], [21], [22]. Honeycombs with negative Poisson’s ratio illustrated in open literature are made primarily made by different RP (Rapid Prototyping) techniques, like FDM and SLS [23], [24], or general 3D printing process [25] usually confined to the use of thermoplastic materials. However, for practical applications in aerospace or wind turbine blades, more composite production-oriented techniques should be explored to bring non-classical honeycomb topologies into the market place, and efforts in that sense have been recently done about using composite manufacturing techniques [25], [26]. Kirigami (Origami plus ply-cut patterns) is found to be a promising way to produce honeycombs with complex geometries, starting from a thin flat composite prepeg sheet (i.e. Kevlar woven fabric/914 epoxy prepreg, carbon fiber reinforced plastic prepreg) [27]. The Kirigami structures consisting of combinations of fold/valleys and cuts to create a cellular tessellation, and modular molds for the curing of the cellular composite More specifically, the flat composite prepreg material is scored and cut with periodically-distributed slits by ply-cutting, and then folded in a zigzag fashion into a 3D honeycomb structure. Kuribayashi et al. [28] presented a self-foldable stent graft using a single piece of SMA foil made by the Origami concept. Heimbs et al. [29] experimentally and numerically characterized the mechanical behavior of folded core structures made from aramid paper and CFRP under flatwise compression loading. Saito et al. [30] produced cellular morphing wingbox using the Kirigami concept and numerically demonstrated that the cellular wingbox was a major contributor in the torsion stiffness and static divergence of the wing structure. In terms of graded honeycomb core and corresponding sandwich structures, efforts have been produced to design and develop gradient-type cellular and porous structures [29], [30], [31], [32]. Lim [33] theoretically designed a functionally graded cellular structure exhibiting Poisson’s curving by combining the re-entrant and hexagonal honeycomb together, which is called positional semi-auxetics. Also the same author has shown that the combination of conventional laminas (possessing positive Poisson’s ratio) and auxetic laminas (possessing negative Poisson’s ratio) can give rise to effective in-plane composite laminates with a stiffness that surpasses the one predicted by rule-of-mixture [34]. Gradient honeycomb configurations have been produced and tested showing high specific shear stiffness capabilities, together with control of the anisotropy of the cellular panel [35]. Gradient core configurations have been also proposed to design aeroengine fan blades with low dynamic displacement characteristics [36]. Gradient cellular structures with auxetic behavior for sandwich panels have also shown strong localization charcateristics in terms of damage and failure under 3-point bending loading [37]. Gradient cellular structures can be considered a form piezomorphing porous material, in which the structure responds with shape changes due to an external mechanical loading [38]. Ruzzene et al. [39] have investigated the wave propagation characteristics of sandwich plates with periodic honeycomb core. Negative Poisson’s ratio (auxetic) core materials of different geometry placed periodically in the plate have demonstrated to introduce the impedance mismatch necessary to obstruct the propagation of waves over specified frequency bands (stop bands) and along particular directions [39].

The work described in this paper is concerned about the physical realization of the concept proposed in Ref. [39], the graded core sandwich panel. The complex geometry of the core is produced using the Kirigami technique using woven Kevlar prepreg and modulus molds for autoclave curing. A portion of the graded honeycomb is made with hexagonal conventional cells, while the other section is represented by a re-entrant (negative Poisson’s ratio) butterfly-type core. The graded honeycombs are then embedded into CFRP sandwich panels, and subjected to ASTM standards tests related to flatwise compression and edgewise loading. A drop-tower impact edgewise test is also carried out, with the samples facing up the indenter alternatively along the conventional and auxetic face. The experimental results are compared against available cores and sandwich panels from open literature and the market place. It will be shown that the graded core concept provides some significant enhancements in terms of specific flatwise and edgewise compressive strength against commercial sandwich panels and core materials. The graded cellular concept shows also a strong dependence over the direction of dynamic loading, with interesting potential implications for energy absorption during impact (see Fig. 1).

Section snippets

Kirigami sandwich panels manufacturing process

Both hexagonal and auxetic honeycombs are manufactured using one layer woven Kevlar/914 epoxy prepreg from Hexcel Composites Ltd., Duxford, UK (Es = 29 GPa, ρ = 1380 kg/m3, thickness 0.250 mm).

The Kirigami manufacturing process consists in the following five steps: cutting, molding, curing, folding and bonding. Periodic distributions of slits are introduced within the plain weave woven using an Auto Prepreg Cutting machine following the patterns shown in Fig. 2a. The ply-cut pattern is programmed

Flatwise compression tests of graded sandwich panels

The flatwise compression tests of the hexagonal-auxetic graded sandwich panels have been performed using a Zwick machine with a calibrated loading cell of 100 kN. Compressive buckling tests were performed until the load–displacement curve indicated a collapsed structure, i.e. with significantly reduced stiffness. The test rig has been is setup according to the ASTM Standard ASTM C365/C365M-11a [42] (Fig. 5). The specimens used have dimensions 51 mm × 51 mm × 28 mm (Fig. 5a and b). The flatwise

Flatwise compression

The stress and strain curves of the graded sandwich panels under flatwise compression are shown in Fig. 8. Table 1 shows also the compressive modulus and strength for different conventional, auxetic and graded configurations. Samples #1 and #2 are the referred to the Kevlar woven epoxy (KWEP) fabric specimens, with hexagonal and auxetic topology respectively [44]. Sample #3 is referred to KWEP graded sandwich core developed in this work. The table contains other comparative data available from

Conclusions

Kirigami techniques applied to cellular composite structures show significant promise in manufacturing composite honeycomb with diverse and arbitrary geometry. The graded core configuration proposed in this work makes use of a dual conventional-negative Poisson’s ratio cellular structure that shows interesting capabilities in terms of flatwise compression and edgewise loading against existing sandwich panels offered in the market place. The edgewise impact loading shows a different mechanical

Acknowledgements

The UK Royal Society Grant RG2625 has supported this work. RN acknowledges the UK Engineering and Physical Sciences Research Council (EPSRC) for his PhD bursary through the ACCIS Doctoral Training Centre in Composites. FS acknowledges also the contribution of the European Commission to the logistics of the manufacturing and testing rigs through the project NMP4-LA-2010-246067-, Acronym: M-RECT. YH would also like to thank the Chinese Scholarship Council for the provision of a bursary to help

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