HyFiSyn: Hybrid Fibre-Reinforced Composites: Achieving Synergetic Effects through Microstructural Design and Advanced Simulation Tools

Although many analytical and some numerical models have been developed to predict the interface stress/strain state, experimental evidence to verify them is scarce. In the current study, we combine synchrotron computed tomography with Digital Volume Correlation (DVC) to acquire, for the first time, 3D strain maps around a glass fiber embedded in epoxy and loaded in-situ in tension. DVC requires a volumetric speckle pattern inside the material, which in this study is achieved by a barium titanate nano-powder. The resulting axial DVC strain map shows a localization at the fiber break, due to the local opening of the break, and higher strain values in the matrix in the vicinity of the fiber break, which diminishes further away from the break. The resulting shear strain has a symmetric distribution at the sides of the fiber with regard to the fiber break. This analysis allows for the definition of real values of strain recovery length.

A novel design of a compact tension specimen, allowing for 4D synchrotron radiation computed tomography, is developed and used, for the first time, to characterise the translaminar fracture of thin-ply composites at the microscale. The mini compact tension specimens are manufactured with thin-ply carbon-epoxy prepregs in [90₂/0/90₂/0/90₂/0/90₂] lay-up configuration. Afterwards, software tools for the training of deep neural networks are employed to segment the acquired images and build a 3D visualisation of the progressive crack. The results reveal advancing crack fronts in the 90° plies and lagging fronts in the 0° plies.

Translaminar fracture toughness is a key property that determines the notch sensitivity and the damage tolerance of fibre-reinforced composites. By establishing a novel downscaled compact tension specimen configuration designed for 4D synchrotron radiation computed tomography, a comprehensive perception of developed failure mechanisms can be attained. In this study, first, an initial finite element model based on limited literature data for a compact tension of a [90/0]₈ₛ T300/913 carbon-epoxy laminate is developed. Next, the cohesive law parameters between 0°/0° plies in a [90₂/0/90₂/0/90₂/0/90₂] thin-ply HS40/736LT laminate are directly extracted from in-situ synchrotron radiation computed tomograms of miniaturised compact tension specimens. The interlaminar (90°/90°) parameters are obtained from double cantilever beam test results on a similar material system. To accurately predict the R-curve, the crack extension and crack opening displacements, in both 90° and 0° plies, were captured in a multi-linear cohesive law. Consequently, in the proposed finite element approach, two distinct cohesive surface definitions are assigned to the fracture plane. The numerical results confirm the lagging crack fronts in 0° plies.

One of the challenges in longitudinal strength models of unidirectional composites is to comprehensively simulate the stress redistribution around a broken fibre. This redistribution is affected by the interfacial debonding of the broken fibre from the matrix. In the present work, a novel approach using finite element modelling is developed that considers all primary contributors to the longitudinal debonding in randomly packed fibre configurations. The parametric study revealed that a larger friction coefficient, interfacial fracture toughness and interfacial shear strength shorten the debond length and increase the stress concentration factor (SCF) on the neighbouring intact fibres. The magnitude of the SCFs strongly depends on the local fibre volume fraction, the radial distance between the intact fibres and the broken fibre, and the debond length. Incorporating interfacial debonding considerably reduces the SCF overpredictions by the well-bonded models. The presence of a matrix crack, encircling the broken fibre, locally increases the SCFs in the adjacent intact fibres and marginally shortens the debond length of the broken fibre. By incorporating these results into a strength model, the influence of the longitudinal debonding and co-existing matrix cracks on failure strain of unidirectional composites can be further tuned.

In the course of longitudinal loading of fibre-reinforced composites, the stress redistribution around a fibre break is affected by the interfacial debonding of the broken fibre from the matrix. In the present work, a 3D finite element modelling approach is developed that incorporates major interrelated phenomena during interfacial failure, including thermal residual stresses, frictional sliding and matrix plastic deformation, while steers clear of a pre-imposed debond length. The improved accuracy of the stress concentration factor (SCF) prediction, on the neighbouring fibres, enables generating more robust fibre break models and optimising composite microstructures at the fibre level. The random fibre packings, around a central fibre as the broken fibre, are generated based on fibre radius and fibre volume fraction (FVF), which for the current carbon-epoxy system, are set to 3.5 μm, 30%-50%-70%, respectively. The size of the RVE is 84 μm× 84 μm × 2 mm. The broken fibre-matrix interface is modelled by adopting a surface-based cohesive contact with a bilinear traction-separation law coupled with the Coulomb friction model. The conducted study reveals that the magnitude of the SCFs strongly depends on FVF, the radial distance between the intact fibres and the broken fibre, and the debonded length. By lowering FVF, the number of nearby fibres reduces and therefore the load that is exerted by the broken fibre is shed onto a sparser fibre population, yielding higher SCFs on them. For each FVF of 30% and 50%, six different random fibre distributions were generated, while for FVF= 70%, since many fibres exist in one configuration, only one distribution was sufficient to capture the trend. The maximum SCF of the nearest fibre when the interfacial failure is not allowed in the 30%, 50% and 70% FVF cases were calculated to be above 16%, 14% and 12%, which is a clear overprediction compared to debonding model values of 5.3%, 2.9%, 1.7%, respectively. With interfacial failure inclusion, while the ranking remains the same, the maximum SCF values decline and their dependency on the distance to the broken fibre alters.

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume.

Structural supercapacitors can both carry load and store electrical energy. An approach to build such devices is to modify carbon fibre surfaces to increase their specific surface area and to embed them into a structural electrolyte. We present a way to coat carbon fibres with graphene nanoplatelets by electrophoretic deposition in water. The effect of time and voltage on the mechanical properties of the carbon fibres, the structure of the coating and the specific surface area of the coated carbon fibres are discussed. A specific capacity of 1.44 F/g was reached, which is 130% higher than state-of-the-art structural electrodes. We demonstrate the scalability of the deposition process to continuous production of coated carbon fibres. These carbon fibre electrodes were used to realise large (21 cm long) structural supercapacitor demonstrators without the need for a separator, having a specific capacity of 623 mF/g.

Interleaving the plies of carbon fibre reinforced epoxy composites with thermoplastic interleaves have previously been shown to enable these composites to display controllable stiffness and shape memory properties. However, the incorporation of unreinforced thermoplastic interleaves leads to a decrease in flexural modulus of the interleaved composites. In this study, the flexural modulus of composites with reinforced polystyrene interleaves was investigated. The reinforcements used in this study were: (1) stainless steel mesh (SS), (2) unidirectional carbon fabric (UD), (3) woven carbon fabric, (4) woven carbon fabric with epoxy coating and (5) non-woven short carbon fibre mesh. The flexural moduli of the interleaved composites with reinforced interleaves were predicted theoretically and determined experimentally. Among these composites, significant increases in the flexural modulus were achieved in the interleaves with UD, woven and woven+epoxy reinforcements. Additionally, these interleaved composites were shown to retain their controllable stiffness and shape memory properties.

This paper presents an experimental method for tensile testing of unidirectional carbon fibre composites. It uses a novel combination of a new specimen geometry, protective layer, and a robust data analysis method. The experiments were designed to test and analyze unprotected (with conventional end-tabs) and protected (with continuous end-tabs) carbon fibre composite specimens with three different specimen geometries (straight-sided, butterfly, and X-butterfly). Initial stiffness and strain to failure were determined from second-order polynomial fitted stress–strain curves. A good agreement between back-calculated and measured stress–strain curves is found, on both composite and fibre level. For unprotected carbon composites, the effect of changing specimen geometry from straight-sided to X-butterfly was an increase in strain to failure from 1.31 to 1.44%. The effect of protection on X-butterfly specimens was an increase in strain to failure from 1.44 to 1.53%. For protected X-butterfly specimens, the combined effect of geometry and protection led to a significant improvement in strain to failure of 17% compared to unprotected straight-sided specimens. The observed increasing trend in the measured strain to failure, by changing specimen geometry and protection, suggests that the actual strain to failure of unidirectional carbon composites is getting closer to be realized.

Improvement of the interfacial fracture toughness of the layer interfaces is one way to increase the performance of interlayer hybrid laminates containing standard thickness carbon/epoxy plies and make them fail in a stable, progressive way. The layer interfaces were interleaved with thermoset 913 type epoxy or thermoplastic acrylonitrile-butadiene-styrene (ABS) films to introduce beneficial energy absorption mechanisms and promote the fragmentation of the relatively thick carbon layer under tensile loads. Carbon layer fragmentation and dispersed delamination around the carbon layer fractures characterised the damage modes of the epoxy film interleaved hybrid laminates, which showed pseudo-ductility in some cases. In the ABS film interleaved laminates, a unique phase-separated ABS/epoxy inter-locking structure was discovered at the boundary of the two resin systems, which resulted in a strong adhesion between the fibre-reinforced and the thermoplastic layers. As a result, the delamination cracks were contained within the ABS interleaf films.

Safe, light, and high-performance engineering structures may be generated by adopting composite materials with stable damage process (i.e., without catastrophic delamination). Interlayer hybrid composites may fail stably by suppressing catastrophic interlayer delamination. This paper provides a detailed analysis of delamination occurring in poly(acrylonitrile-butadiene-styrene) (ABS) or polystyrene (PS) film interleaved carbon-glass/epoxy hybrid composites. The ABS films toughened the interfaces of the hybrid laminates, generating materials with higher mode II interlaminar fracture toughness (G IIC), delamination stress (σ del), and eliminating the stress drops observed in the reference baseline material, i.e., without interleaf films, during tensile tests. Furthermore, stable behaviour was achieved by treating the ABS films in oxygen plasma. The mechanical performance (G IIC and σ del) of hybrid composites containing PS films, were initially reduced but increased after oxygen plasma treatment. The plasma treatment introduced O-C=O and O-CO -O functional groups on the PS surfaces, enabling better epoxy/PS interactions. Microscopy analysis provided evidence of the toughening mechanisms, i.e., crack deflection, leading plasma-treated PS to stabilise delamination.

Accurate characterization of fibres is crucial for the understanding the properties and behaviour of fibre-reinforced composite materials. Fibre properties are key parameters for composite design, modelling and analysis. In this study, characterization of mechanical properties of glass and carbon fibres has been performed using a semi-automated single-fibre testing machine. Based on a sample set of 150 glass and carbon fibres, engineering and true stress-strain curves are analyzed. Different modulus determination methods are discussed based on true stress-strain and tangent modulus-strain relationships. For glass fibres, the true stress-strain based tangent modulus is found to be independent of applied strain, whereas for carbon fibres, a tendency of tangent modulus to increase with applied strain is observed. The modulus of glass fibres is found to be independent of fibre diameter, whereas carbon fibres with smaller diameter show higher modulus compared with carbon fibres with larger diameters.