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Gibbs, Albert Oswald (2023) Orientation control during 2D-3D composite preforming. PhD thesis, University of Nottingham.

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Abstract

Non-crimp fabrics (NCFs) are employed in composite structures as an alternative to woven fabrics when there is a requirement for improved tensile strength and modulus. To exploit NCF properties, components are designed with optimised fibre directions that reinforce the predicted load paths. 2D to 3D composite forming is a manufacturing method that has been developed for automation, to improve the labour economy of components and reduce per unit costs. Currently it is difficult to maintain accurate fibre orientation control due to the constraints of the 2D pre-form and the lack of interaction with the fabric during its transition from a 2D to a 3D form.

This thesis explores process alterations that improve fibre orientation control in 2D-3D forming through the introduction of multiple forming stages. Thus enabling the implementation of optimised NCF layups in formed components. The approach has been broken down into the following research areas:

Analysis of fibre angle distribution: A robust method for the full field measurement of fibre angle distribution has been created and validated to ±0.5 degrees for in-plane testing and ±3 degrees on double curvature surfaces. It has been shown that the theoretical calculation of shear angle from the bias extension test for an NCF is incorrect above shear angle values of 15 degrees. Whereas, theoretical shear angle calculations for NCFs in the picture frame test are accurate to within ±1 degree. It was found that shearing causes a non-linear variation in the tensile properties of an NCF due to fibre misalignment that has previously been unmeasured. NCFs can experience an undesirable reduction in tensile properties at shear angles that are commonly found in 2D-3D formed components.

Analysis of fibre misalignment due to inter-stitch buckling defects: It was found that a deformation mode occurs in NCFs at very low shear angles (0 degrees to 1 degree) where frictional interactions between the yarns prevents slippage at the stitch points. This causes a non-linear shear region at low shear angles observed for many NCF shear force/angle graphs. An analytical model was created to show the link between the initial non-linear shear region and the inter-stitch buckling defects that has been proved to impact fibre misalignment at higher shear angles. To improve fibre alignment, two buckling defect reduction strategies were developed as a result of the modelling and applied to 2D and 3D samples. During in-plane testing localised stitch removal showed a 51% reduction in fibre misalignment and resin lubrication presented a 57% reduction. These strategies combat the undesirable reduction in properties due to fibre misalignment, enabling multiple forming cycles to be conducted without negatively impacting the fabric structure.

Modelling multiple forming cycles: A novel multi-cycle finite element material model was created, that accurately captures the previously undocumented hysteresis phenomenon found in NCFs when subjected to multiple shear cycles. This was validated to within 8% of experimental values during in-plane testing. A multi-stage double diaphragm forming process has been developed that locally induces regions of high shear with the objective of taking advantage of the multi-cycle hysteresis in the fabric. The process alteration generated a 25% reduction in the maximum defect size found on a double curvature component, highlighting the formability benefits of a multiple stage processes and validating the model.

Fibre continuity control during forming: A process alteration has been developed to show that plies running longitudinally along a formed component can be successfully pre-sheared before the forming operation to locally align fibres in the desired orientation. A structural simulation was created that combined the multi-cyclic material model and the non-linear structural behaviour of sheared fabrics. The results for a simple beam showed that a pre-sheared laminate has a higher peak stress under all the tested load cases and an improvement to mechanical stiffness which was shown to be transferable to a component weight reduction of 17% through ply removal. The pre-shearing process also generated a reduction in the wastage from trimming fabrics. The overall fabric area needed for simple beam like geometries was reduced by 15.5%-34.5%. A complex beamlike demonstrator was modelled and showed a 31% improvement to material utilisation and 11% improvement to mechanical stiffness.

The thesis chapters progress the idea of fibre alignment control in 2D-3D forming from: measurement), to understanding, into modelling, and finally a demonstration of the application. Ideas from each chapter can be applied to current industrial processes and improve the capabilities of components made using 2D-3D forming.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Warrior, Nicholas Harper, Lee
Keywords:	Industrial fabrics; Forming; Double diaphragm; Material modelling; Finite element analysis (FEA); Non-crimp fabric; Carbon fiber-reinforced plastics
Subjects:	T Technology > TS Manufactures
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	74304
Depositing User:	Gibbs, Albert
Date Deposited:	31 Dec 2023 04:40
Last Modified:	31 Dec 2023 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/74304

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