Unidirectional composite tapes are extensively used in automated manufacturing to produce high-performance composite parts with significant design flexibility. Their superior mechanical properties stem from the alignment of continuous fibres in a single direction, making them ideal for advanced engineering applications. However, the performance of these components is closely linked to their microstructure, and accurately predicting their behaviour requires a detailed understanding of their microstructural characteristics....

Understanding the microstructural variability in unidirectional composite prepreg tapes is relevant to investigating mechanisms of tape microstructure formation, their impact on its processability and the mechanical performance of the final composite part. It has been shown that three-dimensional microstructural variability at the single-fibre level can be resolved by X-ray microcomputed tomography (XCT). However, to define a representative microstructural fingerprint of a given tape, investigations at the required small voxel size lead to limited volumes of observation, which might not be representative. This research aims to extend these findings via a multiscale approach, considering scales of observations, from microscopic (single fibre) up to mesoscopic (dimension of tape) length scale, to generate further insight into the microstructural organisation of thermoplastic prepreg tapes. By exploring the ability of XCT imaging for carbon fibre-reinforced thermoplastic composites at different voxel sizes, the work aims to identify the limitations of the use of different scales of observations to capture features of microstructures and their propagation from micro- to mesoscale level. While structure tensor analysis appeared to correctly capture misaligned regions in XCT images with small voxel size (1/10 of the fibre diameter), the method proved ineffective for larger voxel size images (1/2 of the fibre diameter).

This work proposes a methodology for the characterisation of complex pore features in unidirectional composite prepregs, and provides insights into the interaction between fibre architecture and pores. The method showcased allows to compare spatial distributions at a three-dimensional level, highlighting in the tape analysed a significant correspondence between regions of elevated tortuosity and increased pore fractions. Regions associated with highly tortuous meandering fibres exhibit a pronounced association with porosity located both in the bulk and at the tape surface, suggesting a strong interaction between non-collective fibre displacement and the probability of pore location. Furthermore, our study quantifies the length scale of feature propagation, shedding light on the spatial extent of microstructural pore occurrence within the composite. These findings have significant implications from a characterisation perspective to aid modelling approaches and manufacturing processes for high-performance composite prepregs tapes.

This study presents the first steps of a benchmarking exercise on image processing of composite materials. Employing three distinct imaging protocols and five different image processing algorithms, the research explains the variability in capturing microstructural features of common micrograph dataset. Results highlight the sensitivity of different methods to factors like illumination inhomogeneities and pixel density, influencing the accuracy and consistency of obtained results. By comparing methodologies from different researchers in a blind format, the study identifies strengths and limitations, laying the groundwork for future benchmarking activities. Moving forward, this research sets the stage for standardized protocols and guidelines, aiming to enhance the reproducibility and reliability of microstructural analysis in composite materials. Such efforts are crucial for advancing material design and development, ultimately creating tailored composite materials with enhanced performance and functionality.

Finding new ways to evaluate the variability of microstructures, and its effect on macroscopic properties such as permeability and mechanical performance [1,2] is of increasing interest in the composite field. The variability of microstructural features at a three-dimensional level is not fully understood and its effect on macroscale properties is not well established, and mostly analyzed at a phenomenological level [3]. We introduced in recent work a method based on X-ray Computed Tomography for the threedimensional reconstruction of the fibrous microstructure of unidirectional tapes at a single fibre resolution [4]. A schematic of the workflow is represented in Figure 1. Three descriptors are introduced in the work to describe increasing level of complexity in the microstructural organization, from a single fiber path level with differential tortuosity, to group behavior with collective motion, to fibre network connectivity with length of contact. These descriptors and their interdependence highlight local effects like edge-core segregation in microstructural characteristics. However, in order to achieve a more complete definition of the unidirectional tape domain, understanding of matrix-based features and its interrelation with fiber architecture descriptors is needed. In this work, we expand the methodology of Gomarasca et al. [4], to account for matrix-based phenomena such as tape boundary variability, and void formation and morphology. This will be showcased on a unidirectional composite tape including both fiber-based and matrix-based analysis. These methods enable advanced characterization and modelling of microstructural formation and evolution during composite manufacturing.

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).

Understanding the three-dimensional variability of unidirectional composites is relevant to the material performance and the development of advanced material modelling strategies. This work proposes a new methodology for the characterization of unidirectional composites, showcased on carbon fibre/poly(ether-ether-ketone) tapes. Three microstructural descriptors were here introduced, each representing an increasing level of complexity in the fibre architecture: from a tortuosity-based single fibre trajectory analysis to fibre groups’ behaviour, to fibre network interconnectivity. The methodology was developed and validated on real material datasets acquired via X-ray computed tomography. A facile method for image analysis was used to reconstruct the three-dimensional fibrous architecture at a single fibre path resolution. The approach bridges a gap in the traditional approach and nomenclature typical of the composite field to describe and quantify complex fibre organization in unidirectional composites, highlighting micro- and mesoscopic features, such as edge-core effects in the fibre arrangement, possibly occurring in tow spreading. The study of the parameter interdependence showed relationships, which will provide further insight for future research in the study of microstructure formation of unidirectional composites, its evolution during processing or loading, and input for advanced modelling techniques based on Representative Volume Elements.