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# Multifunctional auxetic and honeycomb composites made of 3D woven carbon fibre preforms

Multifunctional 3D woven composites have the ability to absorb energy through progressive failure, whilst maintaining gradual load profile decay beyond the failure onset1,2. Consequently, they are of great interest for situations where the ability to withstand crash or impact loading is a design requirement. 3D woven composites are starting to find applications in various sectors, particularly aerospace and automotive applications. Several OEMs and Tier 1 manufacturers are actively investigating these structures. In aerospace, 3D woven structures are already used in fan blades and fan casings. Development is at an early stage and there are many opportunities for improving impact performance and optimising the weight of the structure. It is important that crash structures used in vehicles like cars, buses and trains are accurately predictable and the manufacturing is repeatable. There is also an opportunity using 3D weaving to add an additional functionality into composites.

3D weaving is a specialist activity and there are very few centres capable of conducting the research needed. Textile manufacturers such as DORNIER and STAUBLI manufacture 3D weaving machines but 3D woven fabrics for composites applications are currently in their infancy. In the UK, companies such as Sigmatex UK Ltd, M Wrights & Sons and Antich & Sons have developed internal capabilities to utilise 3D weaving, but more R&D is required to deploy such technology throughout the supply chain. Recently the University of Sheffield AMRC established 3D weaving capabilities which will be used to bridge the gap and support industry.

3D woven preforms have the capability to demonstrate multifunctionality in the manufacture of advanced composites. One of the 3D multifunctional structures is the auxetic functionality which needs to be investigated and demonstrated to industry. This could be in the form of expandable honeycomb type structures3, which could be woven and tested to show capability and potentially improved mechanical performances with high damage tolerance such as crash, compression and impact. Figure 1 explains what the auxetic structure is compared to the conventional honeycomb structure in terms of its geometry, i.e., an auxetic material exposed to tension would increase in dimensions in the direction that is lateral to an applied tensile force. An auxetic structure has several advantages in a crash situation for example good energy absorption, however, the repeatable manufacture of an auxetic structure with a predictable behavior needs further work4.

Poisson’s ratio, which is the ratio of the strain normal to the applied load to the extension strain (or axial strain) in the direction of the applied load. Poisson’s ratio ($$\nu$$) of standard material can be expressed as:

$${\upnu } = – \frac{{{\upvarepsilon }_{{\text{t}}} }}{{{\upvarepsilon }_{{\text{l}}} }}\;where\;{\upvarepsilon }_{{\text{t}}} = \frac{{\Delta {\text{T}}}}{{{\text{T}}_{{\text{o}}} }}\;and\;{\upvarepsilon }_{{\text{l}}} = \frac{{\Delta {\text{L}}}}{{{\text{L}}_{{\text{o}}} }}$$

where, εt = transverse strain, εl = longitudinal or axial strain, ∆L = change in length, Lo = initial length, ∆T = change in width and To = initial width.

Most conventional materials show positive Poisson’s ratio (PPR) under tensile loads because they exhibit positive longitudinal and negative transverse strains, but smart materials like auxetics behave oppositely and show negative Poisson’s ratio (NPR).

It is known that conventional materials such as rubber and metals laterally contract when stretched and laterally expand when compressed in the longitudinal direction; such materials have a PPR. In contrast, there are some special materials which possess a NPR which laterally expand when stretched or laterally shrink when compressed in the longitudinal direction. The materials with NPR are also called ‘auxetics’, which originated from the Greek word ‘auxetos’ meaning ‘that which may be increased’5. Auxetics could be materials and/or structures, they have been investigated in the literature from different perspectives such as developing materials and structures, comparing behaviours and testing performances.

In comparison with conventional materials, auxetic structures have many improved properties. They have higher shear modulus, hence better shear resistance. Auxetic materials have enhanced indentation/impact resistance and energy absorbance properties. When conventional material is subjected to an impact force, the material moves away from the impact point, but exhibiting the opposite behaviour, the auxetic material flows towards to impact point, which makes the auxetic materials harder to be indented. They also have other advantages, such as enhanced fracture toughness, improved crack growth resistance and higher damping resistance. Due to these advantages, auxetic composite structures could find suitable applications in high value manufacturing, such as aerospace and automotive sectors. The disadvantage of auxetic composites is that they may be difficult to manufacture on a large scale 5, but such difficulty has been challenged in this work.

Many studies have been conducted to develop and investigate new auxetic structures and materials based on different material scales. The examples include auxetic fibers6,7, auxetic fabrics8,9, auxetic foams10,11, and auxetic composites12,13. Auxetic woven-composite structures are investigated in this project. Zhou et al.14 developed auxetic composites made of 3D orthogonal woven textile and polyurethane foam. They prove that the auxetic composites exhibited NPR and behave more like damping material with lower compression stress, while the non-auxetic composites behaves more like stiffer material with higher compression stress. In another study15 3D-woven structures were produced and the effect of float length of ground weave and binding yarn on auxeticity of the fabric was investigated. A set of different 3D orthogonal woven structures were produced on a rapier dobby loom by changing the float length in the ground weave and binding yarns. The results showed that the 3D-woven materials with equal and maximum float length of ground weave and binding yarn showed greater auxetic behavior. Also, the impact energy absorption of the developed composites was found to increase with the increase in float length, justifying that the structures are auxetic and possess NPR. Zulifqar and Hu16 reported that the woven fabric could be auxetic through a combination of loose weave and tight weave in the same structure. They showed that the developed fabrics exhibit NPR effect in both weft and warp directions in a large range of tensile strain.

In this work, a Staubli 3D Weaving System was utilised, including Unival jacquard to weave 3D honeycomb fabrics using Toray T300-6 k carbon fibres fed from warp and weft directions. With the aid of polyester foam, the developed 3D-woven fabrics were converted to two different preforms: conventional honeycomb and new auxetic structures. The preforms were infused using epoxy resin to manufacture large composite structures investigated in this study. Tensile and compression tests were carried out to assess the functionality of honeycomb and auxetic composite structures through their Poisson’s ratio measurements.