Translaminar fracture toughness is pivotal for notch sensitivity and damage tolerance of fibre-reinforced composites. Hybridisation offers a promising pathway for enhancing this parameter in thin-ply composites. Three novel mini-compact tension specimen geometries were investigated for their competence in microscale characterisation of translaminar fracture using in-situ synchrotron radiation computed tomography (SRCT). Only “mini-protruded” design resulted in stable crack propagation with adequate crack increments. Based on this design, five baseline and hybrid cross-ply configurations incorporating low- and high-strain carbon fibres were studied. Crack propagation in low- and high-strain baseline configurations was stable. For interlayer and intrayarn fibre-hybrid configurations, a correlation between load–displacement curves and delamination is observed. The SRCT data confirmed that 90° ply-blocks cushion the interaction between 0° plies, enabling independent fracture. Additionally, crack fronts in 90° plies advance further than those in 0° plies. Moreover, mechanical interlocking and bundle bending within 0° plies serve as supplementary mechanisms for energy dissipation.

Microstructural analysis of resin flow in liquid composite molding is impeded by the absence of a characterization method that possesses both the required spatial and time resolution to capture the ongoing phenomena. An optimized UV-flow freezing methodology is presented to rapidly capture dynamic flow behavior, followed by high-resolution micro-computed tomography (μCT) imaging to extract the flow front morphology. Optimisation of the resin strongly enhances the photopolymerisation kinetics, reducing the gelation time by up to 56%, while an adequate postcuring procedure at moderate temperature is proposed by introducing radical induced cationic polymerization. Additives are identified to facilitate facile variation of the capillary number while distortions of the flow front morphology are minimized by finetuning the experimental procedure. μCT imaging allows for a micron-scale through-thickness assessment of unsaturated flow at range of flow regimes corresponding to both capillary- and viscous-dominated flow regimes while the corresponding saturation curves were derived by segmentation of the resulting images. Highlights: An optimized method for evaluating microstructural flow in fibrous preforms. Optimisation of the resin composition allows for fast UV-photopolymerisation. Additives identified for facile variation of the capillary number. Visualization of “frozen” microstructural flow by micro-CT analysis. Applicable to broad range of flow conditions that normally cannot be captured.

Modeling the consolidation of fiber-reinforced thermoplastic composites at the part level presents a formidable computational challenge due to the multi-scale nature of the process. In this article, a method to bypass the multi-scale problem by homogenizing the micro scale and describing the medium with characteristic parameters is described. The model is intended for press molding of hybrid textiles and considers a free-form plate with non-uniform thickness and can describe consolidation in three dimensions with some restrictions. 2D implementation in FEM shows how in-plane matrix pressure gradients can arise in parts and cause fiber disorientation. Experimental verification demonstrates that fiber disorientation arises at the predicted location, and that defect size is proportional to matrix pressure gradient. This novel consolidation model provides new insights, enables part and process optimization, and paves the way for high-quality composite part production. Highlights: A consolidation model for press molding of hybrid textiles is presented. A method to extend consolidation models for complex geometry is presented. The origin of defect formation in complex geometries is explained.

The transverse permeability of roving/tow-based fiber reinforcement is of great importance for accurate flow modeling in the pultrusion process. This study proposes an experimental approach to characterize the roving-based fiber beds' permeability under different compaction conditions. The experimental permeability results of thick roving-based preforms were reported and compared with the permeability values of roving-based preforms in the literature. A representative preform was infused under vacuum conditions. Its thickness was varied to replicate the different compaction values observed in permeability tests. Micrographs were then collected from it and analyzed to highlight the microscale transformations caused by processing/compaction on the fiber arrangement. The analysis revealed that compaction resulted in the reorganization of filaments along the direction of the applied compaction. Overall, the uniformity of the spatial filament distribution, i.e., the homogeneity within the fibrous domain, increased with increasing compaction. Furthermore, the microstructural analysis demonstrated transverse anisotropy within the tested domains, indicating that the obtained permeability results represented an upper boundary. In addition to the experimental analyses, various transverse permeability models, which were developed based on recently introduced statistical descriptors of fiber distribution, were evaluated by using the statistical descriptors extracted from the analyzed cross-sections. Among these models, the one correlating the second neighbor fiber distance with apparent permeability exhibited good agreement with the experimental results. Highlights: Transverse permeability measurement of a roving-based reinforcement was presented. The influence of compaction on the microstructure was investigated at the filament level. Filament distribution in a pultruded profile was analyzed by using statistical descriptors. The results of the experiments and the models in the literature were compared. The correlation between microstructural features and apparent permeability was discussed.

Frontal polymerisation has the potential to bring unprecedented reductions in energy demand and process time to produce fibre reinforced polymer composites. Production of epoxy-based fibre reinforced polymer parts with high fibre volume content, commonly encountered in industry, is however hindered by the heat sink created by the fibres and the mould, overcoming the heat output of the chemical reaction, thus preventing front propagation. We propose a novel self-catalysed frontal polymerisation manufacturing method based on the integration of thin resin channels in thermal contact with the composite stack as a strategy for low-energy production of high fibre volume fraction polymer composites without the need for a continuous energy input. Frontal polymerisation inside the resin channel proceeds faster and preheats the fabric stack, thus catalysing the process. Parts with up to 60% fibre content are successfully produced independently of the sample thickness. Fillers added within the resin channels provide means to tailor the frontal polymerisation process kinetics. The parts have a significantly higher glass transition temperature than those produced in a conventional oven, and comparable mechanical properties while energy consumption is reduced by over 99.5%.

We propose a methodology to monitor the progressive saturation of a non-translucent unidirectional carbon fabric stack through its thickness by means of X-ray radiography and extract the dynamic saturation curves using image analysis. Four constant flow rate injections with increasing flow speed were carried out. These were simulated by a numerical two-phase flow model for both capillary and viscous leading flow conditions. The hydraulic functions describing pressure and relative permeability versus saturation were determined by fitting the saturation curves using a heuristic optimization routine. As the fluid velocity increases and the flow regime at the flow front shifts from capillary to hydrodynamically driven, the resulting capillary pressure curves for a given saturation level are shifted to higher values, from negative to positive. These as well as the capillary pressure calculated from the pressure drop within the unsaturated region of the fabric correlate well with a corresponding change in the averaged dynamic contact angle.

Three different J-integral formulations to derive the experimental translaminar toughness of composites from compact tension tests with a large-scale fracture process zone are implemented and discussed. They improve the existing approaches by taking advantage of stereo-digital image correlation to acquire full-field displacement fields. A field fitting procedure based on robust and efficient piecewise cubic smooth splines addresses noise-related issues reported in previous studies. Additionally, the paper proposes a novel crack tip extraction procedure to report the energy release rate as a function of the crack increment, even if knowledge of the crack tip is not required for the proposed J-integral method. The three methods are discussed in light of a parametric study conducted on synthetic and experimental data, including artificially noisy data. The study reveals that the proposed J-integral methods are suitable for translaminar toughness evaluation of a wide range of materials without the need for restrictive assumptions. However, variations in propagation values were observed when applied to experimental data. Finally, guidelines are drawn to chose the most suitable parameters for the algorithms that are proposed as a Python package.

A new model is proposed for the consolidation of hybrid textiles, in which air entrapment and dissolution are considered. One of the key parameters is tow permeability, which is described by the analytical model of Gebart and validated at very high fibre volume fractions by direct tow permeability measurement. The model also takes into account the presence of fibres limiting gas diffusion in the molten polymer. Experimental validation of the proposed model is then conducted with quasi-unidirectional glass textile and either polypropylene or polyethylene by measuring the impregnation degree as a function of the consolidation time. Good agreement is found between predictions and measurements for the two matrix systems at different pressures. It is shown that entrapped air significantly influences impregnation. The model offers new and comprehensive insights about the phenomena taking place during consolidation and enables future process optimization.

We seek to address how air entrapment mechanisms during infiltration are influenced by the wetting characteristics of the fluid and the pore network formed by the reinforcement. To this end, we evaluated the behavior of two model fluids with different surface tensions, infiltrating three carbon fiber reinforcements, by means of X-ray radiography. We also assessed initial (dry) and final (wetted) states for each experiment by performing X-ray CT scans. We found that the fluid characteristics strongly affect the flow front patterns and pore filling events for a given fabric architecture. Two main promoters of snap-off events are involved in capillary dominated flows: a very wetting system leading to corner flows and the fabric bundles oriented perpendicular to the flow acting as obstacles, specifically in fabric architectures prone to variations in nesting. Finally, we evaluated the applicability of a pore network model to further link preform architecture and void formation.

Radical induced cationic frontal polymerisation (RICFP) is considered a promising low energy method for processing of fibre reinforced polymers (FRPs). Optimisation of the local heat balance between reinforcement, epoxy resin and the surrounding mould is required to pave the way for its adaptation to an industrial processing method for high volume fraction structural fibre reinforced composites. In this work, we investigate several methods to control the governing heat balance in RICFP-processing of FRPs. Heat generation was controlled by tuning the initiator concentration while limitation of heat losses using highly insulating moulds was found beneficial to the front characteristics and resulting curing degrees. An optimised mould configuration allowed for self-sustaining RICFP in FRPs with fibre volume fractions (V_fs) up to 45.8%, exceeding previously reported maxima of similar systems. A process window was moreover established relating the V_f and required heat generation to the potential formation of a self-sustaining or supported front.

Thin-ply composites are recognized as a key solution for the manufacturing of high-performance composite structures due to the unique mechanical properties and the increased design versatility that they offer. They are obtained with state-of-the-art fiber spreading methods where high-count (6-24K filaments) tows of technical fibers (carbon, glass) are thinned by spreading into flat unidirectional tapes which are then combined with a polymer matrix to create pre-impregnated (prepregs) tapes of reduced thickness. In recent years, the industrialization of fiber spreading and impregnation processes enabled the large-scale production of homogenous thin-ply prepregs with thicknesses down to about 15μm per ply, which attracted the interest of the research community. However, the high production cost due to the complexity of the manufacturing methods and the inherent brittleness of thin-ply composites limit their wider adoption by the composites industry[1]. Fiber hybridization (i.e combining at least two types of fibers in a common matrix) is emerging as a promising approach for alleviating these drawbacks towards laminates with balanced characteristics in terms of mechanical properties and cost-efficiency. Currently, most studies on thin-ply hybrids employ simple interlayer (ply-by-ply) configurations mainly due to difficulties in manufacturing of more complex hybrid architectures[2]. However, simulation tools predict that notable improvements can be obtained from more complex intralayer (tow-by-tow) and intrayarn (fiber-by-fiber) hybrid architectures[3]. This work focuses on the study of existing fiber spreading methodologies, the development of equipment, and the optimization of composite processing at North Thin Ply Technology (NTPT) Renens, Switzerland, that allowed the manufacturing of hybrid composites with a high degree of fiber dispersion and controlled microstructure. Hybrid prepregs were produced by combining various ratios of dissimilar fibers following different processing routes. Composite laminates were manufactured and a versatile microstructural analysis tool was developed that enabled correlations between the manufacturing route, the resulting microstructural features describing the degree of co-dispersion, and the mechanical performance of the final part. Acknowledgments The research leading to these results has been performed within the framework of the HyFiSyn project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 765881. Delamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation).

Thermoplastic compression resin transfer moulding coupled with injection moulding is an appealing process for the production of thermoplastic composites. However, its implementation at an industrial scale remains challenging as variotherm injection moulding could prevent solid skin formation in the parting line, making cavity sealing difficult. In this study, a tool for thermoplastic compression resin transfer moulding and the related methods and process parameters for an implementation at an industrial scale are presented. The validity of the concept is proved by producing and characterizing composite plates with elevated fibre volume fractions and advantageous mechanical properties at a range of production temperatures within a cycle time not exceeding 20 min. The best mechanical properties were obtained at a production temperature of 270°C with a bending strength of 477 MPa, a flexural modulus measured at 25.7 GPa and a fibre volume content of 67%.

Knowledge of permeability of fibrous microstructures is crucial for predicting the mold fill times and resin flow path in composite manufacturing. Herein we report a method to rapidly predict the permeability of 3D fibrous microstructures. Our method relies on predicting the permeability of 2D cross-sections via deep neural networks and extending this capability to 3D microstructures via circuit analogy as a means of reduced order modeling. Approximately 50% of the permeability predictions of 2D cross-sections have 10% or less deviation from the permeability results obtained via flow simulations in Geodict. Computational time required for predicting the permeability of 3D microstructures is reduced from hours to less than 10 seconds. This framework enables fast and accurate prediction of micro-permeability and serves as the first building block towards prediction of fabric mesostructures’ permeability via deep learning based methods.

In this study, three novel thermoplastic impregnation processes were analyzed towards automotive applications. The first process is thermoplastic compression resin transfer molding in which a glass fiber mat is impregnated in through thickness by a thermoplastic polymer. The second process is a melt-thermoplastic Resin Transfer Molding (RTM) process in which the glass fibers are impregnated in plane with the help of a spacer. The third process, stamp forming of hybrid bicomponent fibers, coats the fibers individually during the glass fiber production. The coated fibers are used to produce a fabric, which is then further processed by stamp forming. These three processes were compared in a life cycle analysis (LCA) against conventional resin compression resin transfer molding with either glass or carbon fibers and metal processes with either steel or aluminum that can be new, partly or fully recycled using the case study of the production, life and disposal of a car bonnet. The presented LCA includes the main phases of the process: extraction and preparation of the raw materials, production and preparation of the mold, process, and energy losses. To include the life of the analyzed bonnet, the amount of diesel that is used to drive the weight of the bonnet for 300′000 km is calculated. In this LCA, the disposal of the bonnet is integrated by analyzing the used energy for the recycling and the incineration. The results show the potential of the developed thermoplastic impregnation processes producing automobile parts, as the used energy producing a thermoplastic bonnet is in the same range as the steel production.

The COVID-19 pandemic resulted in shortages of personal protective equipment and medical devices in the initial phase. Agile small and medium-sized enterprises from regional textile industries reacted quickly. They delivered alternative products such as textile-based community masks in collaboration with industrial partners and research institutes from various sectors. The current mask materials and designs were further improved by integrating textiles with antiviral and antimicrobial properties and enhanced protection and comfort by novel textile/membrane combinations, key factors to increase the acceptance and compliance of mask wearing. The safety and sustainability of masks, as well as taking into account particular needs of vulnerable persons in our society, are new fields for textile-based innovations. These innovations developed for the next generation of facemasks have a high adaptability to other product segments, which make textiles an attractive material for hygienic applications and beyond.

Side-by-side hybrid textiles consist of layers of woven reinforcing fibres and thermoplastic fibres fabrics alternatively stacked on each other, as represented in Figure 1. Press moulding of side-by-side hybrid textiles is a manufacturing method where a near-net shape final part is produced in a single step, unlike pre-impregnated materials which require a separate impregnation step. This intermediate material offers a great drapeability and vast design freedom as the hybrid system can be easily and locally modified by changing the textile architecture of a single layer or locally adding a polymer layer for instance. These advantages could be used to locally tailor the composite properties, such as using the full potential of composites to create high-performance parts. However, the prerequisite for such an optimization approach is a consolidation model that can be implemented into a finite element analysis, which is not available yet. We demonstrate the concept's relevance for the aerospace industry by producing and consolidating such a hybrid textile with carbon fibres and polyetherimide into semi-complex shapes. A quasi unidirectional fibre architecture is used for the carbon fibres to keep the properties close to a unidirectional layer, as proved by mechanical characterization. As a first step towards a consolidation model, we develop an impregnation model that considers entrapped air, including dissolution into the polymer melt. The model is validated by consolidating flat plates, which confirm that air entrapment can play a major role in impregnation and needs to be accounted for the prediction of porosity.

Capillarity plays a crucial role in many natural and engineered systems, ranging from nutrient delivery in plants to functional textiles for wear comfort or thermal heat pipes for heat dissipation. Unlike nano- or microfluidic systems with well-defined pore network geometries and well-understood capillary flow, fiber textiles or preforms used in composite structures exhibit highly anisotropic pore networks that span from micron scale pores between fibers to millimeter scale pores between fiber yarns that are woven or stitched into a textile preform. Owing to the nature of the composite manufacturing processes, capillary action taking place in the complex network is usually coupled with hydrodynamics as well as the (chemo) rheology of the polymer matrices; these phenomena are known to play a crucial role in producing high quality composites. Despite its importance, the role of capillary effects in composite processing largely remained overlooked. Their magnitude is indeed rather low as compared to hydrodynamic effects, and it is difficult to characterize them due to a lack of adequate monitoring techniques to capture the time and spatial scale on which the capillary effects take place. There is a renewed interest in this topic, due to a combination of increasing demand for high performance composites and recent advances in experimental techniques as well as numerical modeling methods. The present review covers the developments in the identification, measurement and exploitation of capillary effects in composite manufacturing. A special focus is placed on Liquid Composite Molding processes, where a dry stack is impregnated with a low viscosity thermoset resin mainly via in-plane flow, thus exacerbating the capillary effects within the anisotropic pore network of the reinforcements. Experimental techniques to investigate the capillary effects and their evolution from post-mortem analyses to in-situ/rapid techniques compatible with both translucent and non-translucent reinforcements are reviewed. Approaches to control and enhance the capillary effects for improving composite quality are then introduced. This is complemented by a survey of numerical techniques to incorporate capillary effects in process simulation, material characterization and by the remaining challenges in the study of capillary effects in composite manufacturing.

Radical Induced Cationic Frontal Polymerisation (RICFP) has recently been proposed as a promising strategy for processing of epoxide carbon fibre reinforced polymers. Control of the local heat balance is crucial towards the production of industrial-quality composites, which is typically achieved via controlling the heat generation. In this work we present a comprehensive overview of RICFP processing of cycloaliphatic epoxide composites with enhance heat insulation. The thermal initiating compound was identified as the main component to control heat generation, which correlated well with the front velocity. A processing window was defined as function of the fibre and initiator contents and composites with to 45.8% Vf were successfully produced. Optimisation of resulting mechanical properties was made possible by optimisation of the heat balance, with matrix glass transition temperatures of up to 187°C achieved for the used cycloaliphatic system. Post-curing was found beneficial to overcome suggested inhomogeneous curing due to the dual-scale nature of fabrics.

Permeability of fibrous microstructures is a key material property for predicting the mold fill times and resin flow path during composite manufacturing. In this work, we report an efficient approach to predict the permeability of 3D microstructures from deep learning based permeability predictions of 2D cross-sections combined via a circuit analogy. After validating the network's predictions in 2D and extending it to 3D, we investigate its capabilities for handling images of various sizes obtained from virtual and real microstructures. More than 90% of 2D predictions is within ± 30% of their counterparts obtained via flow simulations, similarly for 3D transverse permeability predictions, while in 3D case computational time is reduced from several thousands of seconds to less than 10 s. This work provides a robust and efficient framework for characterizing the permeability of fibrous microstructures and paves the way for extending this capability to estimate the permeability of fabric mesostructures.