Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy-glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates

Acoustic Emission Monitoring of Thin Ply Hybrid Composites Under Repeated Quasi-static Tensile Loading

This paper investigates the applicability of the acoustic emission (AE) technique for identification of the damage onset and accumulation in S-Glass/TR30-Carbon hybrid laminates under repeated quasi-static tensile loading. The samples were made of 2 layers of unidirectional thin carbon prepreg plies which were sandwiched between 2 standard thickness S-glass prepreg plies. Analysis of the AE results shows that there are two types of events regarding the AE parameters; those with high values of energy and amplitude, and low values which are assumed to be related to the fragmentation of the carbon layer and delamination of the carbon/glass interface, respectively. There are more friction related AE signals during the unloading stage than the loading stage due to collision and rubbing between existing crack faces. Increasing the strain level increases the number of fragmentations and the AE technique is able to quantify this. It is concluded that the AE technique can be used to evaluate the number of fragmentations and can identify the damage evolution of the hybrid laminate under repeated quasi-static tensile loading

High Velocity Impact and Blast Loading of Composite Sandwich Panels with Novel Carbon and Glass Construction

This research investigates whether the layup order of the carbon-fibre/glass-fibre skins in hybrid composite sandwich panels has an effect on impact response. Composite sandwich panels with carbon-fibre/glass-fibre hybrid skins were subjected to impact at velocities of 75 ± 3 and 90 ± 3 m s−1. Measurements of the sandwich panels were made using high-speed 3D digital image correlation (DIC), and post-impact damage was assessed by sectioning the sandwich panels. It was concluded that the introduction of glass-fibre layers into carbon-fibre laminate skins reduces brittle failure compared to a sandwich panel with carbon-fibre reinforced polymer skins alone. Furthermore, if the impact surface is known, it would be beneficial to select an asymmetrical panel such as Hybrid-(GCFGC) utilising glass-fibre layers in compression and carbon-fibre layers in tension. This hybrid sandwich panel achieves a specific deflection of 0.322 mm kg−1 m2 and specific strain of 0.077% kg−1 m2 under an impact velocity of 75 ± 3 m s−1. However, if the impact surface is not known, selection of a panel with a symmetric yet more dispersed hybridisation would be effective. By distributing the different fibre layers more evenly within the skin, less surface and core damage is achieved. The distributed hybrid investigated in this research, Hybrid-(GCGFGCG), achieved a specific deflection of 0.394 mm kg−1 m2 and specific strain of 0.085% kg−1 m2 under an impact velocity of 75 ± 3 m s−1. Blast loading was performed on a large scale version of Hybrid-(GCFGC) and it exhibited a maximum deflection of 75 mm following a similar deflection profile to those observed for the impact experiments

Blast resilience of composite sandwich panels with hybrid glass-fibre and carbon-fibre skins

The development of composite materials through hybridisation is receiving a lot of interest; due to the multiple benefits, this may bring to many industries. These benefits include decreased brittle behaviour, which is an inherent weakness for composite materials, and the enhancement of mechanical properties due to the hybrid effect, such as tensile and flexural strength. The effect of implementing hybrid composites as skins on composite sandwich panels is not well understood under high strain rate loading, including blast loading. This paper investigates the blast resilience of two types of hybrid composite sandwich panel against a full-scale explosive charge. Two hybrid composite sandwich panels were mounted at a 15 m stand-off distance from a 100 kg nitromethane charge. The samples were designed to reveal whether the fabric layup order of the skins influences blast response. Deflection of the sandwich panels was recorded using high-speed 3D digital image correlation (DIC) during the blast. It was concluded that the combination of glass-fibre reinforced polymer (GFRP) and carbon-fibre reinforced polymer (CFRP) layers in hybrid laminate skins of sandwich panels decreases the normalised deflection compared to both GFRP and CFRP panels by up to 41 and 23%, respectively. The position of the glass-fibre and carbon-fibre layers does not appear to affect the sandwich panel deflection and strain. A finite element model has successfully been developed to predict the elastic response of a hybrid panel under air blast loading. The difference between the maximum central displacement of the experimental data and numerical simulation was ca. 5% for the hybrid panel evaluated

Creating more gradual failure in high performance composites via hybridisation: 28th Annual Technical Conference of the American Society for Composites 2013, ASC 2013

Hybrid glass laminates with thin carbon plies were tested in tension. Results are presented showing the effects of the proportion and absolute thickness of the carbon on the response and failure mechanisms. It is demonstrated that with the right choice of parameters catastrophic failure can be avoided, and a more gradual, pseudo-ductile failure achieved

Experimental determination of the Weibull parameters in tensile failure using hybrid laminates

Composites trend to failure in their defects, which are randomly distribuited. This causes that the tensile strength decreases when the volumen of material tested increases. This fenomenon is known as size effect or volume effect. Knowing the value of the Weibull parameters is key for modeling and the correct design of large components. In this work, one tensile test are proposed, named the fragmentation test to obtain the shape and scale Weibull parameters that are validated with the scaled test. Carbon/glass hybrid composites can exhibit pseudo-ductile response in the stress-strain curves by having a part with small slope or plateau. The specimen design promotes fragmentation or gradual fracture of carbon layer and suppresses unstable delamination at the plateau. The facture events have been identified by video and accoustic emission monitoring in 11 specimens. The data of fracture strain has been adjusted to the Weibull distribution following the proposed iterative process. The process has been validated using previous results of Finite Elements. The volumen effect has been validated with the results of series of tensile tests, with dimensions scaled by factors of 2,4 and 8 in each direction. Another important advantage of hybridization is the suppression of the stress concentration in the carbon layer, which makes simple end-tab free specimens feasible. The results of two tests have been compared, obtaining very close values

Reducing the notch sensitivity of quasi-isotropic layups using thin-ply hybrid laminates: 30th Annual Technical Conference of the American Society for Composites, ASC 2015

The concept of stress concentration suppression using thin-ply pseudo-ductile hybrid composites is presented in this paper. In previous works, it has been shown that it is possible to achieve high amounts of pseudo-ductility in Uni-Directional (UD) and multi-directional laminates. In this work, the requirements for thin-ply hybrid sublaminates to suppress/reduce notch sensitivity in quasi-isotropic laminates with an open hole is studied. A User Material subroutine (UMAT) has been developed in ABAQUS which can replicate the typical response of pseudo-ductile thin-ply hybrids. This UMAT was then used to study several idealised responses and find the important parameters controlling the fibre failure in notched specimens. The stress concentration factor in the 0° layer has been selected as a measure of notch sensitivity. The ratio of pseudo-ductile strain to initiation strain has also been found to be a useful parameter in comparing different fibre direction tensile responses. The stress concentration factors of the 0° layer found from two different tensile response shapes of UD sub-laminates were found to be very similar when they were plotted versus the ratio of pseudo-ductile strain over initiation strain. These results show that the stress concentration factor varies between 3 for linear elastic material response to 1 for sub-laminates with pseudo-ductile strain to initiation strain ratio of 3. This suggests that optimal structural response can be achieved with thin ply hybrids if the failure strain of the low strain material to high strain material is about 25%

Unidirectional hybrid composite overload sensors: 21st International Conference on Composite Materials, ICCM 2017

A purpose-designed, thin-ply interlayer glass/carbon hybrid composite overload sensor concept is presented, which can be used for structural health monitoring (SHM) of composite structures, with potential for safer operation in service. It has been demonstrated that the sensors work satisfactorily and the striped pattern in the composite structure gives a visual indication of overload of the substrate. An analytical model developed here allows for these sensors to be tailored to suit different substrate materials and design strains. The sensors - comprising a single layer of Ultra-High Modulus (UHM) carbon/epoxy and S-glass/epoxy material - were characterised by experimental strain measurements, and finite element analysis (FEA) regarding their accuracy and the effect of their stiffness on the utilized substrate