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Understanding the fatigue behaviour of hybrid fibre-reinforced plastics is desirable for exploiting their features in safe, durable and reliable industrial components. The fatigue performance of hybrid composites has not been extensively investigated yet. The paper presents an overview of the available knowledge on the fatigue of hybrid fibre-reinforced plastics, and, more specifically, reports the fatigue behaviour of a quasi-isotropic pseudo-ductile all-carbon fibre interlayer hybrid composite by experimental measurements and observations, with emphasis on the damage development. The fatigue conditions are tension-tension stress- and strain-controlled cyclic loading. The results include fatigue life for different maximum stress and strain levels, stiffness evolution and damage observations by X-ray micro-computed tomography. The studied hybrid all-carbon fibre quasi-isotropic composite exhibits pseudo-ductility in quasi-static testing. For stress-controlled fatigue, the fatigue load over the limit of elastic response is not sustained. Contrary to that, the composite retains its load-carrying ability in the pseudo-ductile regime for a strain-controlled regime, albeit with lowered stiffness. This article is part of the theme issue 'Ageing and durability of composite materials'.

This paper presents the development of novel Carbon-Fibre Reinforced Polymer (CFRP) laminates, tailored for the application of Digital Volume Correlation (DVC) and Computed Tomography (CT) to experimental mechanics analyses of these materials. Analogous to surface-based Digital Image Correlation (DIC), DVC is a relatively novel volumetric method that utilizes CT data to quantify internal three-dimensional (3D) displacements and implicit strain fields. The highly anisotropic and somewhat regular/self-similar microstructures found in well-aligned unidirectional (UD) materials at high fibre volume fractions are intrinsically challenging for DVC, especially along the fibre direction at microstructural length-scales on the order of a few fibre diameters. To permit the application of DVC to displacement and/or strain measurements parallel to the fibre orientation, the matrix was doped with a sparse population of sub-micrometre particles to act as displacement trackers (i.e. fiducial markers). Barium titanate particles (400 nm, ∼1.44 vol. %) were found to offer the most favourable compromise between contrast in CT images and the ability to obtain a homogeneous distribution in 3D space with sufficient particle compactness for local DVC analyses. This property combination was selected following an extensive Micro-focus Computed Tomography (µCT)-based qualitative assessment on a wide test matrix, that included 38 materials manufactured with a range of possible particle compositions, mean sizes and concentrations. By comparing the tensile behaviour of the particle-adapted material alongside its particle-free counterpart, we demonstrate through the application of in situ Synchrotron Radiation Computed Tomography (SRCT) that the macro- and micromechanical responses of the newly developed CFRP are consistent with standard production materials indicating its suitability as a model system for mechanistic investigations. ispartof: Journal Of Composite Materials vol:55 issue:14 status: Published online

Longitudinal tensile failure of unidirectional fibre-reinforced composites remains difficult to predict accurately. The key underlying mechanism is the tensile failure of individual fibres. This paper objectively measured the relevant input data and performed a detailed experimental validation of blind predictions of six state-of-the-art models using high-resolution in-situ synchrotron radiation computed tomography (SRCT) measurements on two carbon fibre/epoxy composites. Models without major conservative assumptions regarding stress redistributions around fibre breaks significantly overpredicted failure strains and strengths, but predictions of models with at least one such assumption were in better agreement for those properties. Moreover, all models failed to predict fibre break (and cluster) development accurately, suggesting that it is vital to improve experimental methods to characterise accurately the in-situ strength distribution of fibres within the composites. As a result of detailed measurements of all required input parameters and advanced SRCT experiments, this paper establishes a benchmark for future research on longitudinal tensile failure The research leading to these results has been conducted in the framework of the FiBreMoD project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 722626. Compressive tests on the neat epoxy resin were performed at the mechanical lab of the Institute of Mechanics, Materials and Civil Engineering (IMMC), Université Catholique de Louvain (UCLouvain) with the kind support of J. Chevalier and T. Pardoen. M.J. Emerson from QIM, Technical University of Denmark (DTU) is thankfully acknowledged for the handson training sessions and support of the Insegt Fibre segmentation toolbox. Support of C. Schlepütz from TOMCAT beamline at Swiss Light Source for optimising the acquisition settings during the beamtimes under proposal IDs 20161157 and 20171494 is thankfully acknowledged. J.M. Guerrero would like to acknowledge the grant BES-2016-078270 from the ‘Subprograma Estatal de Formación del MICINN’ cofinanced by the European Social Fund, and all authors from the University of Girona acknowledge the funding from the Spanish project RTI2018-097880-B-I00. The work of M. Hajikazemi forms part of the research programme of DPI, project 812T17. S. Pimenta acknowledges the funding from the Royal Academy of Engineering in the scope of her Research Fellowship on “Multiscale discontinuous composites for high-volume and sustainable applications” (2015-2019). R.P. Tavares and P.P. Camanho would like to thank the financial support provided by FCT - Fundação para a Ciência e a Tecnologia through National Funds in the scope of project MITP-TB/PFM/0005/2013

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Many models for prediction of longitudinal tensile failure of unidirectional (UD) composites have been developed in the last decades. These models require significant assumptions and simplifications, but their consequences for the predictions are often not clearly understood. This paper therefore presents a blind benchmark of seven different models applied to two virtual materials. Reliably capturing the localisation of stress concentrations was vital in predicting the effect of matrix stiffness and strength on composite failure strain and strength as well as fibre break and cluster development. Although the models have different assumptions regarding stress redistributions around fibre breaks, the 2-plet (clusters of two fibre breaks) development was similar. Distance-based criteria were shown to be inadequate for monitoring cluster development. The discussions provide detailed insight into how the model assumptions are linked to the differences in the predictions. European Union’s Horizon 2020 research and innovation programme published

We performed in-situ tensile tests on two carbon fibre/epoxy composites with continuous scanning using synchrotron computed tomography (CT). Both composites were cross-ply laminates, and two specimens were tested for each compos- ite. The voxel size was sufficiently small to recognize individ- ual fibres and fibre breaks. For each test, 16-19 volumes were reconstructed, cropped down to the 0° plies and analysed to track fibre break and cluster development. This dataset provides the last CT volume before failure for each of the four specimens as well as the individual fibre break locations in all reconstructed volumes. These data are then plotted against predictions from six state-of-the-art strength models. The target is that these data become a benchmark for the development of new models, inspiring researchers to set up refined experiments and develop improved models. © 2021 Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Void growth and failure in an epoxy resin is investigated. Tensile tests are carried out on doublenotched round bars. Stress triaxiality in the net section is controlled by the use of two different notch root radii. SEM and computed tomography observations are conducted on regions of interest to identify voids and evaluate their growth. Failure initiation sites are analysed, revealing critical defects in the form of voids or particles. The macroscopic and microscopic experimental results are used to optimize the Gurson–Tvergaard– Needleman model. A finite element analysis is performed to study the mechanical response of the notched specimens. The peaks of the maximum principal stress are found to coincide with the locations of the failure initiation sites observed experimentally, and the model is shown to be able to handle non-uniform initial void distributions. ispartof: Continuum Mechanics And Thermodynamics vol:32 issue:2 pages:1-18 status: Published online

Wind turbine blades are a key growth market for fibre-hybrid composites, as they offer the potential for higher turbine efficiency at a reduced cost. This paper therefore overviews the developments of KU Leuven models for mechanical properties relevant to wind turbine blades, including fibre-hybridisation, size scaling effects, flexure, transverse cracking, stress relaxation and fatigue. The strengths and limitations of the models will be highlighted with a particular focus on relevance for wind turbine blades. While significant steps forward have been made, the various developments still need to be incorporated in one all-encompassing model, and further work is needed on gathering the required input data and detailed experimental validation studies. ispartof: IOP Conference Series: Materials Science and Engineering vol:942 ispartof: 41st Risø International Symposium on Materials Science: "Materials and Design for Next Generation Wind Turbine Blades" location:Roskilde, Denmark date:7 Sep - 10 Sep 2020 status: Published online

The variability of the statistical physics descriptors of failure in an impregnated fibre bundle model is investigated. Using the same bundle and fibre parameters as in (Lomov et al., 2021), 50 random realisations were built for VF50 and for VF60 models. The same descriptors of the bundle damage, clusters, and avalanches were calculated, and their statistical scatter characteristics were analysed. The conclusions of (Lomov et al., 2021) are confirmed to be statistically representative for an ensemble of random fibre placement realisations.

Recently, in-situ X-ray Computed Tomography (CT) has shown its potential for 3D damage analysis in composite materials. However, the characterization of damage in X-ray tomograms is not always straightforward, and it requires post-processing of the 3D images. In this study, we explore the potential of Digital Volume Correlation (DVC) for the detection and characterization of damage in fiber-reinforced composites, where fibers provide the required 3D speckle pattern. Preliminary analysis via “digital deformation” of 3D images is performed to verify the applicability of DVC for quantification of deformation and damage in a carbon/epoxy laminate and to estimate the measurement errors. Then, real-deformation images, acquired with synchrotron CT during in-situ tensile loading of the laminate, are analyzed with DVC to detect different damage mechanisms. A rough analysis is performed at the mesoscale using subset-based DVC, followed by a more detailed investigation at the microscale via finite-element-based DVC. Damage appears in the DVC strain fields as local strain magnification. Crack opening displacement can be estimated reliably via the jumps in displacement fields. DVC proves to be a promising tool for damage characterization in X-ray tomograms of fiber-reinforced composites, especially when simple methods such as grayscale thresholding are not adequate.

Development of clusters and avalanches of damage events is a key characteristic in statistical physics of structures susceptible to a catastrophic failure. This paper applies the concepts of such descriptions to the process of fibre breakage under tension in an impregnated fibre bundle model (IFBM). The following parameters are analysed: (1) susceptibility (damage caused by change in loading); (2) spatial clustering of breaks and its interdependency with avalanches; (3) the size distribution of fibre break clusters and avalanches; (4) fractal dimensions; and (5) correlation lengths of breaks. A systematic workflow and algorithms for this analysis are presented and applied for two realisations of a random fibre placement of carbon fibre/epoxy bundles with fibre volume fractions of 50% and 60%. For these two realisations: (1) the susceptibility divergence near the failure point is affected by the finite-size effect, being nearly constant for the last ~1/20th of strain life; this behaviour can be considered as a failure predictor; (2) the morphology of the break system is a spatially distributed multi-defect occurrence with the dimensionality of 3 for all breaks together and weak planarity demonstrated only by the largest clusters near failure; (3) the exponents of the cluster-size and avalanche-size power law distributions reached 2.5–3 for both statistics near the bundle failure, which may constitute an imminent failure indicator. Damage evolution of the carbon/epoxy IFBM up to the final failure is found to be short-correlated (with correlation length of the order of one fibre break stress redistribution zone) in the absence of collective long-range defect interactions. During the catastrophic avalanche, the spatially distributed morphology of breaks is gradually reorganized into a translaminar fracture mode.

Digital Volume Correlation (DVC), in concert withinsituSynchrotron Radiation Computed Tomography (SRCT),has been applied to Carbon-Fibre Reinforced Polymers (CFRPs) under quasi-static tensile loading. DVC re-presents a relatively novel tool for quantifying full-field volumetric displacements and implicit strain fields. Thehighly anisotropic and somewhat regular/self-similar microstructures found in well-aligned unidirectional (UD)materials at high volume fractions are shown to be intrinsically challenging for DVC, especially along the fibredirection. To permit the application of DVC to displacement and/or strain measurements parallel to the fibreorientation, the matrix was doped with a sparse population of sub-micrometre barium titanate particles to act asdisplacement trackers (i.e.fiducial markers). For the novel materials systems we have developed, measurementnoise is considered along with the spatial filtering intrinsic to DVC data processing. Compared to volume imagesacquired through Micro-focus Computed Tomography (μCT), hold-at-load artefacts are mitigated through scantimes on the order of seconds using SRCT, as opposed to hours. Instances of individually fractured fibres evolvinginto clusters of breaks are presented, together with the associated strain redistribution (imaged at a voxel re-solution of 0.65 μm). It is shown that the distance over which strain is recovered in the broken fibres not onlyincreases with the applied force, but also with the number of broken fibres, delineating aspects of the loadshedding phenomenon. The study demonstrates that unprecedented, mechanistically-consistent three-dimen-sional (3D) strain measurements may be made in relation to fibre failure events, that can be used to validatemicromechanical models for predicting UD tensile failure. We believe this work presents the first application ofDVC to the SRCT imaging of failure in CFRPs, achieving significantly higher resolution than reported previouslywithin the literature. ispartof: Composites Part A - Applied Science and Manufacturing vol:137 status: published

Reliable investigation of damage in fibre reinforced composites requires concurrent in- and ex-situ application of multiple instruments at different scale: digital image correlation, acoustic emission registration, optical/electron microscopy, C-scan, X-ray imaging and micro-computed tomography. The multi-instrument experimental mechanics allows detailed damage monitoring and inspection.

ispartof: Proceedings of the American Society for Composites Thirty-Fifth Technical Conference ispartof: American Society for Composites 2020 Thirty-Fifth Technical Conference location:Virtual Conference status: Published online

We apply Digital Image Correlation (DIC) to X-ray Computed Tomography (CT) slices for characterization of individual fibers misalignment in continuous fiber composites. This is an atypical application of DIC since the input is slices of a CT tomogram acquired at one time step, whereas the input of the typical deformation analysis application is 2D images captured over consecutive loading steps. The methodology is demonstrated and validated on synchrotron CT images of a glass/epoxy laminate, where individual fibers can be clearly identified. It is found that the average/standard deviation of the DIC displacement field in every CT slice is a good estimation of the average/standard deviation of the actual displacement of all fibers’ cross-sections in that slice. This provides a shortcut to the measurement of the moments of fiber orientation distribution, compared to the algorithms that reconstruct the fiber center lines. Using this shortcut, the fiber misalignment was also characterized in synchrotron CT images of a carbon/epoxy laminate, where individual fibers were not clearly observed and where, fiber segmentation tools failed to characterize the fibers. Furthermore, being applied to synchrotron images acquired during in-situ tension, the methodology is employed to quantify the evolution of misalignment during tension. The investigated 3D volumes are provided in [1] and described in [2]. ispartof: Composites Part A - Applied Science and Manufacturing vol:142 status: published

We have performed synchrotron computed tomography on two different fiber-reinforced composites while they were being continuously in-situ loaded in 0° tension. One material is a glass/epoxy laminate and the other is a carbon/epoxy laminate. The voxel size is 1.1 µm, which allows clear recognition of the glass fibers, but not distinct individual carbon fibers. For each material, four loading steps are selected with approximately 0, 40, 73, and 95% of the failure load, and the 3D images of the four volumes from each material are overlaid. A volume of interest in the middle 0° ply is chosen and located in the 3D image of each loading step (Fig. 1). The cropped volumes of interest for each material are presented in this publication and are publicly available on Mendeley Data [1]. As examples of two frequently-used type of unidirectional fiber-reinforced composites, the presented data can be used for different microstructural analyses, including investigation of the 3D variability in fiber distribution and orientation, and their evolution during tensile loading. For example, we have performed fiber orientation analysis on this data, using our digital image correlation-based technique, in [2]. Moreover, real-time formation of fiber breaks with tensile loading can be investigated in the data. ispartof: Data in Brief vol:34 ispartof: location:Netherlands status: published