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Title: Pseudo-ductility of unidirectional thin-ply hybrid composites
Author: Suwarta, Putu
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2020
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Fibre composite materials suffer from sudden, brittle failure which limits their usage. It is possible to alleviate this limitation and expand the design space by using unidirectional (UD) thin ply hybrid composites that display non-linear ‘pseudo-ductile’ stress-strain. In real applications, structural parts made of fibre composites are often subjected to different loading conditions. It is important to be able to design a thin-ply hybrid composite that will fail in a pseudo-ductile manner under different loading conditions such as tension, compression, and bending. The durability of thin-ply hybrid composites under cyclic loading is also an important factor when designing structural parts made of thin-ply hybrids. The aim of this thesis is to explore the pseudo-ductility of unidirectional thin ply hybrid composites under tension, compression, bending and fatigue. The incorporation of thin, spread tow ply carbon-epoxy prepreg material (ply thicknesses 0.03-0.06 mm) within a UD interlayer glass/carbon hybrid system yielded a non-linear stress-strain response under tensile loading. Hybrid configurations consisting of thin ply carbon layers with standard thickness glass layers were investigated. Video gauge images showed that fragmentation of carbon layers followed by stable delamination is responsible for the pseudo-ductile response. The loading-unloading-reloading response of glass/carbon hybrids shows a gradual loss of stiffness as fragmentation and localised delamination are present and increase with increasing strain. It was deduced that the low energy release rate of the thin ply carbon layers is responsible for the stable delamination. Favourable pseudo-ductile strain has been achieved for hybrid configurations made of thin high modulus carbon/epoxy layers and standard thickness high strain glass/epoxy layers under longitudinal compression. The pseudo-ductile response is again due to progressive fragmentation and dispersed delamination of the thin ply carbon/epoxy layers. For the hybrid with thicker carbon layers, sudden failure occurs at a lower strain. The different damage behaviours underpin the crucial role of the carbon layer thickness and glass/carbon ratio in the unidirectional glass/carbon hybrid laminates under uniaxial compressive loading. Indirect compression of asymmetric hybrids made with different numbers of layers of high modulus carbon/epoxy sandwiched between high strain glass layers tested in four-point bending also showed a noticeable change of force-strain slope due to fragmentation and dispersed delamination of the carbon/epoxy layers on the compression side. The flexural response of hybrid laminates made of thin high strength carbon epoxy layers and high strain glass layers has been shown to exhibit a noticeable stiffness change, via a combination of gradual failure by fragmentation of the carbon layers on the tensile side and very high fibre strain at failure on the compression side. For the hybrid laminates made of thin high modulus carbon epoxy layers and high strain glass layers, fragmentation has been observed on both the tension and compresson sides of the hybrid beams. Fragmentation and dispersed delamination on the tension side delayed damage accumulation on the compression side until higher final strain to failure. A high fragmentation strain on the tension side was obtained for the hybrid laminates in bending. This value is higher than the fragmentation strain to failure obtained from the static tensile testing. The tensile fatigue behaviour of hybrid laminates made of high strength carbon epoxy and high strain glass epoxy shows a gradual stiffness reduction due to slow delamination growth when the pristine hybrid laminates were fatigued at 90% load level and no significant damage at 80% load level. When the overloaded hybrid laminates were fatigued at 70% and 80% load level, they showed gradual stiffness reduction and slow damage growth compared to the fatigued overloaded hybrids at 90% load level. At 70%, 80% and 90% load level, the overloaded hybrid laminates did not fail immediately but delaminated slowly. The delamination at different numbers of cycles was visually observable and, therefore, this hybrid system can also be used as an early warning for future maintenance and replacements in structural applications subjected to cyclic loading.
Supervisor: Wisnom, Michael Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available