Barely visible impact damage (BVID) can cause serious issue for composite structures, due to sub-surface damage seriously reducing the strength of the material without showing easily detectable surface signs. Dark-field imaging measures ultra-small angle scattering caused by microscopic features within samples. It is sensitive to damage in composite materials which would otherwise be invisible in conventional radiography. Here we demonstrate BVID detection with speckle-based dark-field imaging, a technique requiring only sandpaper (to create the speckle-pattern) in addition to a conventional X-ray imaging setup to extract the dark-field imaging. We demonstrate that the technique is capable of detecting both matrix cracking and delaminations by imaging materials susceptible to these failure mechanisms.

In fiber-reinforced polymer composites, the fiber-matrix interface controls stress transfer mechanisms, thereby affecting mechanical performance. Interfacial properties are often extracted via single-fiber composite tests. In these tests, the load is transferred from the polymer to the fiber through interfacial shear stresses, necessitating the evaluation of interfacial shear properties. To adopt these properties in the design of industrially relevant composites, one must assume that the damage mechanisms in single-fiber composites are representative of those in multi-fiber composites, consisting of highly aligned, unidirectional plies with high fiber volume fractions. That assumption, however, has never been validated. In this paper, the real-time damage development is monitored in single-fiber and multi-fiber composites using in situ X-ray holo-tomography at 150-nm pixel size. The technique enables the first-ever 3D detection of longitudinal interfacial debonding in carbon and glass single-fiber composites. This mechanism is not detected in multi-fiber composite specimens, suggesting that single-fiber composites are intrinsically unrepresentative of realistic composite behavior.

A methodology to develop a 3D Finite Element (FE) model of a full metal-lined Composite Overwrapped Pressure Vessel (COPV) was developed and is presented in this paper. The model is intended for prediction of the metal-composite delamination and residual dent depth developed in the metal liner as a result of quasi-static indentation loading. Cohesive elements are used to model the composite and the metal-composite interfaces. Experimental and numerical comparisons of force–displacement curves, de- lamination area and residual dent depth are presented. Numerical results are in good agreement to the experimental data.

As a building block towards improved understanding and design of Composite Overwrapped Pressure Vessels (COPV), this paper presents simulations and experimental validation of a 3D finite element model for a metal–composite ring structure subjected to quasi-static indentation, used as a proxy for low velocity impact (LVI). The focus of the work is to model composite ply delamination as well as metal–composite separation, using cohesive elements. A methodology is presented to determine the parameters used for the traction-separation law that controls the cohesive elements. The model was calibrated and validated using a hybrid metal–composite ring at reasonable engineering length-scales, corresponding to structures with 159 mm outer diameter and 50 mm length. Each ring specimen was loaded in displacement controlled compression up to 20 mm, i.e. the point at which composite delamination and plastic deformation of the metallic layer has occurred. Validation is performed by comparing experimental force–displacement curves, strain fields and damage mechanisms to results obtained from the finite element (FE) model. Results from the numerical modelling are in good agreement with experimental values.

The heterogeneity of the microstructure and the anisotropy of the mechanical properties of carbon fiber reinforced polymer composites (CFRP) are associated with complex damage mechanisms that make failure prediction highly challenging. Typical damage mechanisms for CFRP include: (i) interlaminar cracking (delamination), (ii) matrix cracking, and (iii) fiber breakage. X-ray computed tomography (CT) has been utilized recently to identify the complex damage in progressive failure of CFRP in 3D and 4D (3D spatial and 1D temporal). However, these scans contain a large amount of data (≈10 GB/mm3), which makes it challenging to extract mechanistic insights in a reasonable time frame. In previous work, we presented the first deep learning (DL) convolutional neural network (CNN) segmentation of multiclass microscale matrix damage in CFRP. The trained CNN segments complex and sparse composite matrix damage mechanism to ≈ 99.99% agreement, with nearly 100% of human time eliminated, providing higher accuracy compared to human segmentation. In this work, we developed CNN-based method for the automated segmentation of fiber breaks using synchrotron radiation-based CT (SRCT) scans that have been human-labeled. The model employed a U-Net full CNN as the high-level encoder (feature extraction)-decoder (feature localization) structure, and VGG16 as the deep CNN backbone via the publicly available Segmentation Models GitHub repository. An unbiased evaluation was performed comparing the machine vs. the human-labeled ground truth. Finally, active learning was presented for the machine-assisted relabeling of datasets containing fiber breaks. The deep learning-based automated segmentation method demonstrated accurate identification (> 90%) of fiber breakage for CFRP.

In order to control the magnetic properties and electronic structures of black phosphorene (BP) monolayer, the structures, electronic and magnetic properties of non-metallic elements doped BP monolayer without or with defects including P vacancy (VP) have been studied by density functional theory (DFT). Defective BP appears ferromagnetic metallicity, and the magnetic moment is 0.086 μB. The magnetism mainly comes from the spin polarization of P atoms near the defect point. For non-metallic elements doped intrinsic BP, system doped with B and N shows P-type semiconductor. C-doped shows non-magnetic metal properties. O doped exhibits magnetic P-type semiconductor. Si and S doped shows ferromagnetic metal properties. The magnetism mainly comes from the spin polarization of P atoms near the defect point, and a small part comes from doped atoms. In the case of non-metallic elements doped defective BP, the results show that flaw-b-C and flaw-s-Si exhibit non- magnetic metallic properties. The flaw-b-S shows P-type semiconductor with indirect band gap of 0.712 eV. Other systems exhibit ferromagnetic metallicity, and the magnetism mainly comes from the spin polarization of P atoms near defect point. Non-metallic elements doped BP monolayer without or with point defects can effectively adjust magnetic properties and electronic structures.

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.

Composite materials are increasingly used to help in reducing the carbon footprint of transportation and upscaling renewable energy infrastructure that provides clean energy for future cities. However, the inherent susceptibility of carbon fibre reinforced polymers to impact damage results in knock-down in design and is linked to the micro-mechanistic response of the material to damage. In situ experimental and high-resolution imaging techniques using X-ray computed tomography (X-ray CT) have been used to gain a mechanistic understanding of the key factors controlling crack path — and hence macro-scale toughness within a composite. Multiscale Synchrotron Radiation Computed Tomography (SRCT) and lab-based micro-focus X-ray CT are used to investigate different material systems toughness response from standard Double Cantilever Beam tests. The crack transition to the weaker ply region of the composite is identified as a controlling factor across a scale of mm’s, and ‘trigger’ regions are reported on and investigated. The ‘trigger’ regions were identified as gaps in the ply adjacent to the interlayer. This work feeds directly into delamination growth predictions, a better understanding of material response, and enabling informed manufacture and design, allowing for reduced material usage, longer life and more sustainable vehicles and infrastructure.

In situ synchrotron radiation computed tomography (SRCT) was used to compare the fibre damage progression in five configurations of (90 2 /0 2 ) s carbon-epoxy coupons loaded to failure. The effects of different sizing types, surface treatments and fibre diameters on the macroscopic properties, for example, ultimate tensile strength (UTS), and on the damage accumulation at a microscopic scale, for example, fibre break accumulation, were assessed. A semi-automated approach was adopted to process the large amount of data obtained from the SRCT scans and further method applicability areas can be envisaged. Single fibre break accumulation was seen to be influenced by the fibre type, while the formation of interacting fibre break groups by the surface treatment and the sizing type. For the materials presented, it can be suggested that an increased defect tolerance can be obtained by moving from stronger to weaker fibre-matrix adhesion, with sub-critical multiplet behaviour emerging as independent of the average UTS value.

View Video Presentation: https://doi.org/10.2514/6.2022-0375.vid In this study, the effects on 3D strengthening and toughening mechanisms of interlaminar nanoreinforcement (termed 'nanostitch' here, achieved by embedding highly dense forests of vertically aligned carbon nanotubes in polymer-rich ply/ply interfaces) are studied qualitatively and quantitatively via 4D progressive damage in carbon (micro) fiber reinforced plastic/polymer (CFRP) composite laminates by implementing in situ synchrotron radiation computed tomography (SRCT) of delamination-prone cross-ply double edge-notched tension (DENT) configurations (scaled-down specimen geometry) via semi-automatic (human-driven) damage segmentation. A 20°-canted loading rig fixture was also designed, fabricated, and employed here to enable clear imaging of features that are typically blurred due to their alignment with the X-ray beam (e.g., 90° lamina-based features). SRCT here was performed at beamline 47XU (BL47XU) of the Super Photon ring-8 GeV (SPring-8) facility in Japan. Intermediate-thickness-ply laminates (2× thicker ply vs. thin-ply, similar to conventional aerospace-grade unidirectional plies) exhibit no change in DENT ultimate tensile strength for baseline vs. nanostitched configurations, explained mechanistically by an observed progressive damage mode transition from notch-blunting inter- and intra-laminar matrix damage-dominated (typical of thicker-ply laminates in literature) to brittle fiber breakage- and diffuse matrix damage-dominated (typical of thinner-ply laminates in literature). Thin-ply and thick-ply laminates have been tested similarly, showing significant strength increase in the nanostitched thick-ply (3× thicker ply vs. thin-ply) configuration, which will be the subject of future work. These findings contribute new CFRP failure insights, which can guide and inform mechanical enhancement approaches fundamental to eliciting synergistic latency in hybrid/hierarchical laminates, as well as advance currently limited modeling.

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 composite. The voxel size was sufficiently small to recognize individual 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.

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.

In situ X-ray synchrotron radiation computed tomography (SRCT) of carbon fiber composite laminates reveals the first-ever qualitative and quantitative comparisons of 3D progressive damage effects introduced by two mechanical enhancement technologies: aligned nanoscale fiber interlaminar reinforcement and thin-ply layers. The technologies were studied individually and in combination, using aerospace-grade unidirectional prepreg standard-thickness (‘std-ply’) and thin-ply composite laminates. The relatively weak interlaminar regions of the laminates were reinforced with high densities of aligned carbon nanotubes (A-CNTs) in a hierarchical architecture termed ‘nanostitching’. Quasi-isotropic double edge-notched tension (DENT) laminates were tested and simultaneously 3D-imaged via SRCT at various load steps, revealing a progressive 3D network of damage micro-mechanisms that were segmented according to modality and extent. For load steps of 0%, 70%, 80%, and 90% of baseline ultimate tensile strength (UTS), intralaminar matrix cracking and fiber/matrix interfacial debonding are found to be the dominant damage mechanisms, common to all laminate types. For both std-ply and thin-ply, nanostitched laminates had qualitatively and quantitatively similar matrix damage modality and extent compared to the baseline laminates through 90% UTS, including relatively few delaminations, despite an ~9% increase in std-ply nanostitched UTS over the std-ply baseline. Complementary finite element-based modeling of damage predicts greater delamination extent in std-ply vs. thin-ply laminates that manifests only between 90% and 100% UTS, offering an explanation for the observed positive nanostitch effect in the std-ply, which is known to be more susceptible to delamination formation and growth than the thin-ply laminates. Thin-ply, with and without nanostitch, intrinsically suppresses matrix damage, as expected from past work and evidenced here by 6.5X less overall matrix damage surface area vs. std-ply baseline laminates averaged over all load steps. These findings contribute new insights from high-resolution experimental mapping of composite damage states that can guide and inform mechanical enhancement approaches and improved damage models.

Digital Volume Correlation (DVC), in concert with in situ Synchrotron Radiation Computed Tomography (SRCT), has been applied to Carbon-Fibre Reinforced Polymers (CFRPs) under quasi-static tensile loading. DVC represents a relatively novel tool for quantifying full-field volumetric displacements and implicit strain fields. The highly 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 fibre direction. 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 barium titanate particles to act as displacement trackers (i.e. fiducial markers). For the novel materials systems we have developed, measurement noise is considered along with the spatial filtering intrinsic to DVC data processing. Compared to volume images acquired through Micro-focus Computed Tomography (mu CT), hold-at-load artefacts are mitigated through scan times on the order of seconds using SRCT, as opposed to hours. Instances of individually fractured fibres evolving into clusters of breaks are presented, together with the associated strain redistribution (imaged at a voxel resolution of 0.65 mu m). It is shown that the distance over which strain is recovered in the broken fibres not only increases with the applied force, but also with the number of broken fibres, delineating aspects of the load shedding phenomenon. The study demonstrates that unprecedented, mechanistically-consistent three-dimensional (3D) strain measurements may be made in relation to fibre failure events, that can be used to validate micromechanical models for predicting UD tensile failure. We believe this work presents the first application of DVC to the SRCT imaging of failure in CFRPs, achieving significantly higher resolution than reported previously within the literature.

This paper presents an experimental demonstration and validation of high-resolution three-dimensional experimental strain measurement using Digital Volume Correlation (DVC) on Carbon Fibre-Reinforced Polymers, via through-thickness strain analysis under a state of pure bending. To permit the application of DVC to displacements and/or strain measurements parallel to the fibre direction in well-aligned unidirectional materials at high volume fractions, a methodology was developed for the insertion of sparse populations of 400 nm BaTiO3 particles within the matrix to act as displacement trackers (i.e. fiducial markers). For this novel material system, measurement sensitivity and noise are considered, along with the spatial filtering intrinsic to established DVC data processing. In conjunction with Micro-focus Computed Tomography, the technique was applied to a simple standard specimen subjected to a four-point flexural test, which resulted in a linear strain distribution through the beam thickness. The high-resolution, fibre-level strain distributions (imaged at a voxel resolution of ∼0.64 µm) were compared against the classical beam theory (Euler–Bernoulli) in incrementally decreasing averaging schemes and different sub-set sizes. Different sampling and averaging strategies are reported, showing that DVC outputs can be obtained that are in very good agreement with the analytical solution. A practical lower limit for the spatial resolution of strain is discerned for the present materials and methods. This study demonstrates the effectiveness of DVC in measuring local strains parallel to the fibre direction, with corresponding potential for calibration and validation of micromechanical models predicting various fibre-dominated damage mechanisms.

X-ray micro-computed tomography (μCT) is used to quantify morphology in AS4/8552 (autoclave) and IM7/M56 (Out-of-Autoclave, OoA) aerospace-grade advanced unidirectional-ply carbon fiber prepreg composites, revealing several previously unreported features. The micron-scale (1 μm voxel size) three-dimensional datasets combined with automated, objective algorithms, revealed the following previously unreported features of AS4/8552 and IM7/M56 laminates, respectively: all ply interfaces analyzed have misplaced microfibers at densities of 1–2 per mm2 of interface area that can contribute to the mean thickness of the interlaminar regions of 8.6 μm and 14.4 μm; all ply interfaces have elongated (aspect ratio > 10 and presumed to extend indefinitely) periodic resin pockets along the microfiber direction of the plies bounding the interlaminar region that we term tow-aligned resin pockets (TARPs), with typical thicknesses that are 2–3X greater than the average interlaminar thickness; overall void fractions are low at ~0.002 vol% and ~0.001 vol%, comprised primarily of newly-quantified "sub-microvoids" with an average volume of 26–31 μm3 that are equally pervasive in both materials, numbering ~300 per mm3. The new interlaminar region and void tools were also utilized to analyze laminates with aligned carbon nanotubes (A-CNTs), termed "nanostitches", incorporated between plies to reinforce the interlaminar regions. The addition of A-CNTs increased the interlaminar thickness by 2.2 μm and 8.0 μm for the AS4/8552 and IM7/M56 systems, respectively, but did not affect the quantity or distribution of voids or TARPs. These newly-identified features are relevant to the mechanical performance of such composites, as they may have positive or negative effects on damage initiation and progression.

The focus of this study is the influence of voids on the damage behaviour in quasi-static loading of resin-infused carbon fibre-reinforced polymers. Experimental results are presented for quasi-static loading in combination with high-resolution tomographic imaging and statistical analysis (homology of pores or voids and induced cracks). Three distinct mechanisms were observed to control delamination growth in the presence of sharp and blunt voids. Delamination cracks interact with the supporting yarns, especially in combination with air pockets trapped in the resin in the form of long, sharp voids. This resulted in crack growth that coalesces with delamination cracks from neighbouring yarn-voids during increased out-of-plane load–displacement, with almost no presence of intralaminar transverse cracks. This highlights the benefits and drawbacks of the supporting yarn during out-of-plane loading.

Models are still deficient in accurately reproducing the mechanisms that trigger tensile failure in unidirectional composites, highlighting a lack of direct experimental evidence. In this study, emphasis is given to the identification of connections between local fibre misorientation, packing and Weibull strength distribution in causing tensile failure. Synchrotron Radiation Computed Tomography (SRCT) and automated image processing techniques are adopted to segment individual fibres from loaded carbon fibre coupons. Subtle indications in the misorientation of local damaged sites are assessed in novel statistical detail for systematic differentiation from non-damaged sites. It is observed that the morphology of the surrounding environment of damaged sites statistically differs from that of non-damaged sites, even though locally damaged sites (containing single or coupled breaks) do not exhibit a peculiar fibre packing arrangement. For adjacent coupled breaks, the statistical nature of fibre separation distances is also reported.