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This work studies the hybrid effect in a high performance Quasi-Isotropic (QI) carbon/glass hybrid composite under tensile loading. The hybrid effect is the enhancement in the strain at failure of the carbon fibres in a hybrid composite compared with a pure carbon composite. Different types of QI layups were tested and analysed to understand the interaction between the layers and the effect of the stiffness of the adjacent sub-laminates. A numerical model using the Finite Element method was proposed to evaluate the strain distribution considering an initial failure of one of the 0° carbon plies. The results show that the stiffness of the layers and the stacking sequence play an important role in the failure strain of the carbon plies; the layups with stiffer adjacent layers showed a higher hybrid effect.

The aim of this work was to investigate the notched behaviour of thin ply pseudo-ductile carbon/glass hybrid laminates with in-plane Over-height Compact Tension (OCT) specimens. Therefore, a series of tests were performed on two different thin-ply carbon/glass hybrid configurations fabricated from Quasi Isotropic (QI) hybrid constituents and architectures. The first consisted of thin ply carbon M46JB/thin ply glass (X-strand) and the second thin-ply carbon TC35/standard thickness S-glass prepregs. For comparison, all glass and all carbon laminates were investigated as well. A detailed assessment of the damage evolution was carried out using X-ray Computed Tomography (CT). Results show that the hybrid configurations have a higher stiffness compared to the all glass samples. A progressive and gradual failure of the hybrids resulted in gradual load-pin opening displacement (POD) curves, where there was no significant sudden load-drop that happened for the all carbon laminates due to the catastrophic failure mechanisms. The damage in the hybrid configurations was visually detectable and agreed well with the CT-scan results, whereas it was not feasible to see the damage easily for the all glass and all carbon configurations.

In this paper, a viscoelastic-damage cohesive zone model is formulated and discussed. The interface element constitutive law has two elastic and damage regimes. Viscoelastic behaviour has been assumed for the shear stress in the elastic regime. Three element Voigt model has been used for the formulation of relaxation modulus of the material. Shear Stress has been evaluated in the elastic regime of the interface with integration over the history of the applied strain at the interface. Damage evolution proceeds according to the bilinear cohesive constitutive law up to the complete decohesion. Numerical examples for one element model has been presented to see the effect of parameters on cohesive constitutive law.

Fragmenting hybrid laminates create a pseudo-ductile response and a striped pattern of fractures and delaminations which can be used as a simple visual strain overload sensor. The mechanism is explained and it is shown how it may be used to detect if a specified strain threshold or a range of different strains have been exceeded. The novel hybrid composite sensor may be used as an indicator patch cocured with composite components or retrofitted/bonded onto the surface of composite or metal components, or be incorporated as an integral load-carrying surface layer with sensing functionality.

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The effect of different parameters on the gradual failure and pseudo-ductility of thin UD hybrids is studied using an analytical method developed recently. Damage mode maps are proposed to show the effect of different geometric parameters for a specific material combination. This type of map is a novel and efficient method to find the optimum configuration of UD hybrids and also indicates the importance of thin layers to achieve the optimum geometric parameters in practice. The material parametric study reveals that there is always a trade-off between the “yield stress” and the amount of pseudo-ductility; higher yield stresses leads to lower pseudo-ductility and vice versa. However, application of high-stiffness fibres with high strengths as the low strain material can provide both better pseudo-ductility and yield stress.

A new analytical modelling method is developed and subsequently used to predict the pseudo-ductile, metal-like stress-strain responses of thin ply CFRP [±θ/0]s laminates. Three combinations of thin ply materials containing standard (235 GPa) and high (540 GPa, 720 GPa, 780 GPa) modulus fibres have been utilised. The modelling method is used to determine the range of ±θ values for each material combination that will exhibit pseudo-ductility. It is shown that a high modulus 0◦ ply and standard modulus ±θ layers can produce considerable pseudo-ductility, via fragmentation of the 0◦ ply and local delaminations at the 0/-θ interfaces. With the use of the various thin ply materials and a range of ±θ angles between 20◦-30◦, the stress-strain response of these laminates can be finely controlled to achieve a desired 'yield' stress plateau, pseudo-ductile strain or laminate strength.

Hybridisation is one of the approaches to introduce pseudo-ductility to brittle composite materials. In this approach, two or more different types of fibre are combined and if the configuration and material constituents are well selected, the tensile response shows a gradual failure and pseudo-ductile strain. Different types of hybrid composites with continuous layers have been studied to produce pseudoductile tensile behaviour. However, most hybrid material studies to date have been focused on Uni- Directional (UD) laminates which are not usually applied in industry due to poor transverse mechanical properties.In this study, the behaviour of multi-directional hybrid laminates made with UD hybrid sub-laminates is studied. The final goal is to introduce pseudo-ductility to layups with wider industrial applications. The effect of layup as well as the UD building-block stress-strain curve on the final stress-strain curve of the laminate is also studied.A new analytical approach based on Classical Laminate Theory is introduced in which the effect of different damage modes in UD hybrid laminates (fragmentation and dispersed delamination) is taken into account. The output of this method is the non-linear stress-strain curve of a multi-directional laminate with UD hybrid sub-laminates. This method is then used to study the effect of different parameters such as the mechanical properties of the constituents (low and high strain materials) and layup on the pseudo-ductility.

The aim of this research is to manufacture intermingled hybrid composites using aligned discontinuous fibres to achieve pseudo-ductility. Hybrid composites, made with different types of fibres that provide a balanced suite of modulus, strength and ductility, allow avoiding catastrophic failure that is a key limitation of composites. Two different material combinations of high strength carbon/E-glass and high modulus carbon/E-glass were selected. Several highly aligned and well dispersed short fibre hybrid composites with different carbon/glass ratios were manufactured and tested in tension in order to investigate the carbon ratio effect on the stress-strain curve. Good pseudo-ductile responses were obtained from the high modulus carbon/E-glass composites due to the fragmentation of the carbon fibres. The experimental results were also compared with an analytical solution. The intermingled hybrid composite with 0.25 relative carbon ratio gave the maximum pseudo-ductile strain, 1.1%, with a 110 GPa tensile modulus. Moreover, the initial modulus of the intermingled hybrids with 0.4 relative carbon ratio is 134 GPa, 3.5 times higher than that of E-glass/epoxy composites. The stress-strain curve shows a clear "yield point" at 441 MPa and a well dispersed and gradual damage process.

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.

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