This study investigates the high-temperature flexural properties and damage mechanisms of three-dimensional five-directional (3D5d) braided Al2O3/mullite ceramic matrix composites(CMCs). The flexural tests were carried out at room temperature and at 900 degrees C, 1000 degrees C, 1100 degrees C, and 1200 degrees C. The results show that the loaddisplacement curve of the 3D5d material evolves with temperature in a two-stage manner: they continuously improves from room temperature to 1000 degrees C, and reaches its peak at 1000 degrees C; while in the high-temperature range from 1000 degrees C to 1200 degrees C, its curve rapidly declines, and the curve becomes relatively flat at 1200 degrees C. At 1000 degrees C, the 3D5d composite achieved a flexural strength of 135 MPa and a modulus of 12.2 GPa. Microstructural characterization revealed that the damage mechanism evolves with temperature. As the temperature rises, the failure mechanism of the material undergoes a significant transformation: at room temperature, the failure mode mainly consists of matrix cracking, interface debonding and fiber bridging; at high temperature, it gradually transitions to mode controlled by matrix cracks, interface degradation and fiber strength attenuation. Among them, the failure within the temperature range of 900 degrees C-1100 degrees C is mainly dominated by interface weaking; while when the temperature reaches 1200 degrees C, the matrix undergoes significant softening, becoming the key factor leading to a drastic decline in performance.

This study systematically investigates the evolution of bending properties and damage mechanisms of threedimensional angle-interlock woven alumina/mullite ceramic matrix composites (3DAW Al2O3-CMCs) under room-temperature (RT) and various high-temperature environments. The experimental temperature range was set from RT to 1200 degrees C. The results indicate that the bending properties of 3DAW Al2O3-CMCs exhibit a nonmonotonic temperature dependence, initially increasing before decreasing, with optimal performance observed at 1000 degrees C where the maximum bending strength and bending modulus reach 128 MPa and 15.20 GPa, respectively. The damage mechanism shifts with increasing temperature: from localized fiber fracture and matrix cracking, to ductile fracture characterized by matrix softening and enhanced interfacial bonding, and finally to the state of matrix degradation coupled with fiber bundles collapse.

This study investigates the damage mechanisms and associated interlaminar toughness (GIC) increase in Mode I delamination with large-scale fibre bridging for a carbon fibre/epoxy composite. Using Acoustic Emission (AE), Wavelet Packet Transform (WPT), and scanning electron microscopy, four damage modes were identified: matrix cracking, interface debonding, fibre pullout and fibre breakage. These modes are combined in the fibre bridging process. Cluster analysis of AE signals correlated each mode to a specific AE signature. The AE energy rate (AEER), defined as the cumulative AE energy per unit of crack propagation length, revealed that fibre pullout, with an AEER at least an order of magnitude higher than other modes, is the dominant toughening mechanism for GIC increase. Matrix cracking and interface debonding have a moderate effect, whereas fibre breakage has little effect on the GIC increase. The magnitude of GIC during delamination propagation also correlates with the instantaneous cumulative absolute energy per AE counts (d(AEE)/d(Counts)), defined as the ratio of the differential of cumulative AE absolute energy to the differential of cumulative counts. This ratio increases with delamination growth and finally stabilizes. These correlations provide a basis for evaluating damage mechanisms and designing composite toughening strategies.

Fine-woven needled C/C composites typically served in thermal structural components under high-load impact, where their service environment involved complex dynamic loads. In this study, the effects of strain rate and loading mode on impact performance and damage evolution mechanisms were investigated by combining experimental and finite element methods. Results showed that the impact performance of the composites demonstrated significant strain rate dependence and anisotropy. The X-impact strength and modulus increased moderately with strain rate, which was attributed to the dense in-plane yarn. In contrast, the Z-impact exhibited a more prominent strain rate strengthening effect: at low strain rates, its performance was limited by interfacial microcracks; while at high strain rates, the straight Z-yarns enabled rapid stress transmission, leading to Z-impact performance surpassing that of the X-impact. Strain rate and loading direction jointly influenced the damage evolution. For X-impact, damage initiated at in-plane yarn intersections and accelerated with increasing strain rate. For Z-impact, interfacial microcracks were dominant at low strain rates, whereas Z-yarn failure and shear damage became the main modes at high strain rates. The revealed strain rate dependence and anisotropy of impact performance, and the proposed finite element method provided theoretical guidance for the service application of C/C composites.

This research explores the influence of interfacial debonding between a broken fiber and matrix on stress redistribution surrounding a fiber break within a unidirectional (UD) impregnated fiber bundle, accounting for misalignment of fibers and fiber diameter distribution in randomly packed fiber configurations. Finite-element modelling is conducted on carbon-reinforced epoxy UD bundles with one fiber broken for different combinations of the bundle parameters: aligned/misaligned fibers and constant/randomly distributed fiber diameters. Two definitions of stress concentration factor (SCF) are examined, based on average and maximum stress over the fiber cross-section. The study reveals a statistically significant difference for average SCF for misaligned bundles with both constant and variable diameters of fibers compared to the case of aligned bundles of fibers with constant diameter. When the calculated SCFs are incorporated in a bundle strength model, the failure strain of unidirectional composites can be more realistically predicted.

In the collaborative effort towards standardisation of out-of-plane permeability measurement, an international benchmarking exercise was carried out whereby 19 participants worldwide were instructed to measure the out-of-plane permeability following a number of strict guidelines, informed by the outcomes of the first international benchmarking exercise completed in 2021. This paper presents the results of the exercise and an assessment of the reproducibility of the data and the suitability of the proposed test method. The data returned were subjected to a number of statistical analysis methods, which showed that adherence to the test guidelines resulted in a high likelihood of a participant not being an outlier and therefore providing evidence that the test method proposed in this paper is a suitable way forward for a standardised test method.

During Liquid Composite Molding, a fibrous reinforcement is impregnated with liquid resin. Process design requires knowledge of the reinforcement permeability for fluid flow, but until recently, there has been no standard available for its measurement. In 2023, following decades of benchmarking activities and a standardization project, an ISO standard for the experimental characterization of in-plane permeability of fibrous reinforcements for liquid composite molding was finally published. It focuses on the experimental characterization of unsaturated in-plane permeability and specifies the requirements for test equipment, methods and data analysis. Given the deficiency of standardized procedures within the composites industry, this paper intends to provide an example of the steps towards standardization and summarizes lessons learned. It illustrates the research milestones that led to the establishment of the standard, promotes the standard by detailing its general content and notable features and finally gives explanations and reasoning behind the developed guidelines.

Machine learning allows fast nano-scale defect detection in polymer-impregnated aligned carbon nanotube (CNT) nanocomposites. Digital twins were populated by TEM-validated geometry; considered defects were flat cracks and close-to-spherical voids. Finite-element analysis of piezoresistive response was conducted by embedment of CNT network into matrix. Identification of a defect by change in CNT network piezoresistivity was challenged by: (1) randomness of CNTs’ shapes and placement, ML training happened on random realisations; (2) high strength of CNTs leading to the preservation of conductive paths along CNTs and changes only in conductivities of tunnelling contacts. “Artificial approximation“ was introduced to economise computer time multi-fold: ML was trained on cases with artificially degraded tunnelling conductivities within the defect. Three ML models: XGBoost, fully connected, and convolution neural networks were employed. All models managed the task for near-spherical voids, but performed poorly for flat cracks, due to the limited number of tunnelling contacts in crack volume. When trained on the mixed set of voids and cracks, both neural networks demonstrated the ability to learn the difference and detected even cracks, while XGBoost was not up to the challenge. By metrics, the convolutional neural network demonstrated the highest accuracy of predictions.

Carbon fibre-reinforced thermoplastics sheet moulding compounds demonstrate significant potential for cost-effective, mass production applications in lightweight structures. However, the material's complex internal morphology poses substantial challenges for mechanical property prediction. To elucidate the failure mechanisms of sheet moulding compounds, in-situ tensile X-ray computed tomography experiments were conducted in conjunction with digital volume correlation analysis, marking the first application of this method to sheet moulding compounds. Detailed correlations between strain distribution, pore density, strand orientation, and microcrack formation were clarified. A strong correlation was identified between microcrack initiation and areas of high pore density. Strain concentrations were predominantly observed in regions with overlapping strands and high pore density, factors which contribute to accelerated microcrack propagation. These observations reveal that minimising internal morphological irregularities and enhancing interface properties can reduce microcrack propagation, thereby improving the mechanical performance of sheet moulding compounds.

The processing condition of the semi-crystalline thermoplastic composites can affect their mechanical performance. This study investigated the mechanical behavior of CF/PA6 thermoplastic tape processed into laminates at different conditions. A temperature range starting from the melting point of PA6 at 220 degrees C to 260 degrees C is investigated, together with a holding time of 5 to 30 seconds and two cooling approaches: water-cooled plates and cold plates The influence of processing conditions varied depending on the specific mechanical property being evaluated. Longitudinal tensile strength and modulus were found to be relatively insensitive to processing conditions. A slight increase for the higher cooling rate was obtained with the highest average tensile strength of 1180.2 +/- 117.6 MPa. Peel strength was highest for laminates processed at higher temperatures (240 degrees C and 260 degrees C) and with cold plates for cooling, reaching a maximum of 3.9 +/- 0.1 N/ mm. These findings highlight the importance of optimizing processing conditions to balance thermal exposure and mechanical performance.

Natural fibre composites are increasingly explored for structural applications due to improvements in mechanical performance. For this, damage prognostics are crucial. We integrate acoustic emission (AE) and deep learning techniques to predict the remaining useful life of a flax fibre composite under long-term creep load. Derivatives of cumulative AE features with respect to time, such as cumulative hit and count rates, are introduced to reflect the performance degradation rate of the materials. These proposed features seem more relevant for creep lifespan than traditional AE features. Long short-term memory networks and temporal convolutional networks are adopted to estimate the composite’s remaining useful life. The two models' normalized root mean square errors are below 0.11, less than 20% of the error of a statistical Weibull-distribution benchmark model. Our study demonstrates that AE-based data-driven models can predict the performance degradation of composite materials subject to sustained load.

Due to the layer-by-layer printing nature of fused filament fabrication of short fiber reinforced thermoplastics, the microstructural properties of these materials, such as fiber volume fractions and fiber orientations will be different across different cross-sections or sub-volumes. This study investigates the variability in the internal structure of a glass fiber-reinforced thermoplastic composite by scanning different-sized specimens under X-ray computed tomography. Scanned images are divided into equal-sized sub-volumes that cover the entire volume, either with an exact fit or with overlaps, depending on the size of the sub-volume. Fiber volume fractions and fiber orientations are analyzed over these sub-volumes. Results show that analyzing only 42.2% of the scanned volume provides an estimation of fiber volume fraction around 1% of error. Fiber orientation distributions are determined to be similar between layers of the same printing orientation.

The cooling rate of the manufacturing process can significantly alter the performance of the semi-crystalline polymers and composites by influencing their degree of crystallinity (DOC). To this end, the research studies the cooling rate effect on the crystallization of injection-molded polyamide-6 (PA6) and carbon fiber-reinforced PA6 (CF/PA6) compression-molded laminates. DSC, mDSC, and XRD techniques are used and compared for DOC characterization while highlighting the challenges and possible erroneous results when using DSC and XRD. mDSC results after careful drying and storage and due to the capacity of the technique to separate the reversing and non-reversing signals showed consistent results, while the XRD underestimated the DOC. Based on mDSC, different cooling methods of the compression molding process induced DOCs in the 36-50 % range for the CF/ PA6 laminates, whereas changing the mold temperature of the injection molding process resulted in PA6 films with DOC of 28-35 %.

High-performance C-SMCs typically consist of long carbon fibre reinforcement with high fibre content and can be compression moulded at elevated temperatures and pressures to form parts with complex geometries. Squeeze flow testing has been increasingly adopted for experimentally characterising the flow behaviour of C-SMCs, but the reliability and repeatability of the testing results are low due to the lack of standardised methods. This paper presents a benchmarking exercise on C-SMC squeeze flow testing, jointly delivered by 14 European research institutes, with the objective of quantifying the variabilities in the testing results and identifying their sources.

This study leverages the inspiration from bamboo and the design freedom of 3D printing to develop a novel continuous fiber-reinforced vascular composite. After analyzing the microstructural characteristics of bamboo vascular bundles, a fiber distribution pattern was derived and further implemented using a hollow channel pre-placement strategy in conjunction with 3D printing technology. Scanning electron microscopy and X-ray micro-computed tomography were employed to analyze the fiber and hole distribution characteristics. Charpy impact tests demonstrated that bamboo-inspired vascular composites exhibited enhanced specific impact strength compared to traditional fiber arrangement patterns, effectively dissipating more impact energy due to complex fracture modes. These findings suggest that bamboo-inspired design and 3D printing can be used to create advanced composite materials with improved mechanical performance.

Fiber/matrix interface stresses and micro-scale stress concentrations (SCs) under transverse loading were analyzed for hybrid composites, reinforced with glass and carbon fibers. A finite element model was implemented using parametric modeling technique to determine the stresses in the matrix and fiber/matrix interface. Several micro-scale finite element models were generated for the analyses with various combinations of fiber material, fiber size, volume ratio and fiber spatial distribution models. Models with single fiber type (non-hybrid composites) were also analyzed to understand the hybridization effect on the stresses. The results reveal the effect of the presence of different fiber types and their arrangement on the micro-scale stress distributions and fiber/matrix interface behavior. Under transverse loading, when the same type of fibers are aligned with the loading direction, the SCs on the stiffer fibers are larger than the case in the composite with the single stiffer fiber type. When these fibers are aligned with the direction perpendicular to the loading, the SCs on the stiffer fibers are lower. Fiber material type was more effective than the fiber size and fiber volume ratio enhances the effect of hybridization on the stress distributions. According to the authors' knowledge this is the first study investigating the transverse SCs in hybrid composites at the micro-scale level.Highlights Different representative volume elements (RVE) used: triple, hexagonal, random. Transverse loading, and shear loading studied and results are given. Combined effects of some parameters are investigated. Stress concentration (SC) distributions are calculated and compared in each case.

Finite element modelling is conducted to simulate the stress redistribution around a broken fibre (BF) in a bundle with experimentally measured fibre diameter distributions (FDD), followed by a parametric study of the influence of the FDD coefficient of variation on the stress concentration factor (SCF) and ineffective length (IL). Two variants of the SCF definition are considered: based on average and maximum stress in the fibre cross-section. Results demonstrate that bigger fibre diameters show higher SCF and clustering of such fibres increases SCF in nearest neighbour fibres (NNFs). Critically, maximum stress-based SCF (maxSCF_max) significantly exceeds average stress-based SCF (maxSCF_avg), with differences about 40 to 75 to FCD bundles. This emphasises the necessity of prioritizing maximum stress criteria over conventional average stress models in failure predictions. The findings challenge benchmark models that rely on averaged SCF values, offering critical insights for improving accuracy in predicting fibre break propagation and composite strength.

Woven fabrics constitute an important material class used in a broad range of the applications, from the apparel and garments industry to the fields of architecture, composites, marine engineering, aeronautics and space studies. A woven fabric is a fibrous material with multi-scale structural organization, with fibers organized into yarns and yarns organized into a weave. Their mechanical behavior is defined by structure-property relations. The present chapter starts with a description of the internal geometry of woven fabrics and analytical models for their deformation under tension, shear, bending and compaction. Subsequently, the continuum mechanics theories for fibrous yarns and woven fabrics are introduced, and their implementation as generalized media constitutive models is described. Micro-computed tomography-based internal structures and finite elements models of woven fabrics link the continuum mechanics theories with digital twins of the fabrics and a virtual identification of mechanical properties.

This work aims to reveal the damage mechanisms and evolution in unidirectional flax fibre biocomposites when subjected to creep load. X-ray micro computed tomography and acoustic emission (AE) was used to monitor the failure progress during flexural creep tests. A correlation between the event clusters and directly observed damage modes was established based on coupons with expected failure mechanisms and then validated by computed tomography observations. The damage initiated from matrix cracking in the primary creep stage, and then fibre–matrix interface debonding combined with fibre pull-out occurred sequentially during the steady creep stage. In the tertiary creep stage, the explosive fibre fracture emerged and eventually triggered catastrophic failure. Considerably more AE events were detected during creep tests compared to those in quasi-static tests, which indicates that intensive damage is generated under creep load, and therefore causes the strength degradation. A good agreement was observed between the cumulative number of AE events and the increasing damage volume fraction over time determined by X-ray micro computed tomography during multi-step creep tests.

The longitudinal tensile strength is an important basic property of unidirectional (UD) composites or plies that often governs failure in multidirectional laminates. The standard tensile testing methods almost always result in sample failure near the grip. Finite element analyses revealed the presence of multiaxial stress concentrations, including longitudinal, transverse, and shear stresses in the tabbed section of the standards’ recommended design. There are still unknowns and uncertainties about the causes of these stress concentrations and ways to eliminate them. A major challenge is obtaining acceptable failure within the gauge section and yielding the highest tensile strength of UD composites. This paper reviews the different methods for performing quasi-static and fatigue tensile tests on UD composites. The primary sources of stress concentrations and the parameters that affect them are reviewed using the available experimental and modeling investigations. We survey the effects of the different specimen and end tab designs as well as test setups on quasi-static and fatigue loading. Specific proposals are made for each of the discussed parameters for more reliable results.