Publications
168 results found
Fujita Y, Noda S, Takahashi J, et al., 2023, Initiation and propagation fracture toughness of injection-moulded short fibre composites under different environmental conditions, Composites Science and Technology, Vol: 233, Pages: 1-16, ISSN: 0266-3538
Injection-moulded short-fibre composites combine lightweight and manufacturability; however, their fracture behaviour and how it is affected by the microstructure and environmental conditions are yet to be fully characterised. The initiation and propagation fracture toughnesses of injection-moulded short glass-fibre reinforced polyamide 6.6 composites were characterised through compact tension testing under the combined effect of fibre orientation, moisture level and temperature. Full R-curves were calculated using either an FE-based compliance calibration method, or the J-integral method based on full-field measurements from Digital Image Correlation; both data reduction methods provided consistent propagation values for the fracture toughness, although only the J-integral method can characterise the initiation toughness and the shape of R-curves reliably. This work revealed that the material became tougher with increasing fibre orientation along the loading direction, increasing moisture content, and/or increasing temperature; the corresponding increase in toughness was related to changes in failure and toughening mechanisms, identified through fractography. FE simulations of the compact tension tests have demonstrated the need to consider both initiation and propagation values of fracture toughness to accurately predict the response of notched specimens. The thorough characterisation of fracture toughness presented in this paper can contribute to design safer and more efficient damage-tolerant IM-SFRP components.
Alves M, Martulli LM, Kerschbaum M, et al., 2023, A 3D finite element stochastic framework for the failure of tow-based discontinuous composites, COMPOSITES SCIENCE AND TECHNOLOGY, Vol: 232, ISSN: 0266-3538
Anthony D, Woodgate C, Shaw C, et al., 2023, Hierarchical solutions to compressive problems in fibre-reinforced composites, ECCM20 - The 20th European Conference on Composite Materials, Publisher: Composite Construction Laboratory (CCLab), Pages: 1512-1517
Currently, the useable compressive properties of a composite are restricted by set design limits well below the expected intrinsic performance of the materials contained within. The next generation of high-performance fibre-reinforced polymer composites will need to address the challenge of improving the absolute performance of composites in compression. This task requires a rethink of the whole system; not only to address practical limitations of current materials, but their combination, interface, and their architecture. The mechanisms involved do not simply act over the nano-, macro-, or meso-level independently, but are mutually related at the system level, complicating the approach.
Yu B, Katafiasz TJ, Nguyen S, et al., 2023, Characterising and predicting the relationship between translaminar fracture toughness and pull-out length distributions under distinct temperatures, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 381, ISSN: 1364-503X
The translaminar fracture toughness reflects the damage tolerance of a fibre-reinforced composite under longitudinal tension, which often governs the final failure of structures. One of the main energy-dissipation mechanisms that contributes to the translaminar toughness of composites is the fibre pull-out process. The present study aims to quantify and model the statistical distribution of fibre pull-out lengths formed on the translaminar fracture surface of composites, for the first time in the literature; this is done under different temperatures, so that the relationship between pull-out length distributions, micromechanical properties and the translaminar fracture toughness can be established. The fracture surfaces of cross-ply compact tension specimens tested under three different temperatures have been scanned through X-ray computed tomography to quantify the extent of fibre pull-out on the fracture surfaces; the distribution of pull-out lengths showed alarger average and larger variability with an increase in temperature, which also lead to an increase in translaminar fracture toughness. A similar trend has been captured by the proposed analytical model, which predicts the pull-out length distribution based on the analysis of quasi-fractal idealizations of the fracture surface, yielding an overall accuracy of more than 85%.This article is part of the theme issue 'Ageing and durability of composite materials'.
Medeau V, Kazemi ME, Greenhalgh E, et al., 2022, Helicoidal layups and interleaved hybrids: a novel design methodology for impact-resistant composite structures, ECCM20 - The 20th European Conference on Composite Materials
Kazemi M, Medeau V, Greenhalgh E, et al., 2022, Implementing structural fuses in CFRP components via microstructurally-engineered crack paths, 20th European Conference on Composite Materials, ECCM20. 26-30 June, 2022, Lausanne, Switzerland, Publisher: Composite Construction Laboratory (CCLab)
This study aims to develop and implement actual carbon fibre-reinforced polymer (CFRP) solutions for realising structural fuses in real components. To this end, we have developed various concepts for structural fuses, applied to generic idealised components and aimed at engaging different in-plane and through-the-thickness damage propagation mechanisms. Micro-cut patterns (MCPs) / crack path combinations have been engraved on thin-ply CFRP prepregs (by using a laser cut machine) for manufacturing CFRP specimens. Afterwards, we have carried out a series of experimental studies to evaluate the fracture properties of various MCPs under three-point bending (3PB). Then, 3PB results were used to refine and down-select ourconcepts, for use in our generic idealised component design to test them under indentation test using a cantilever beam rig. The test results demonstrated that MCPs can provide significant control over the fracture locus and path, additionally allowing the failure initiation load and energy dissipation to be tailored.
Fujita Y, Noda S, Takahashi J, et al., 2022, ANALYSING AND PREDICTING FAILURE OF INJECTION-MOULDED SHORT-FIBRE COMPOSITE COMPONENTS, Pages: 312-317
Injection-moulded short-fibre composites are lightweight materials suitable for highvolume applications; however, current simulation methods for these materials cannot yet predict failure accurately. This work proposes a methodology to predict failure of injection-moulded short-glass-fibre reinforced PA66 composite components, based on experimentally measured properties. The material's fracture toughness was characterized for different fibre orientations, and these values were used as the input for cohesive zone modelling in Finite Element analyses of the components, coupled with simulations of the injection-moulding process. The coupled process/structural simulations using cohesive zone modelling presented excellent agreement with the experimental data of the component tests, highlighting the importance of accounting for the finite fracture toughness of the material to accurately predict the ultimate failure of injection-moulded short-fibre reinforced PA66 composite components.
Breite C, Melnikov A, Turon A, et al., 2021, A synchrotron computed tomography dataset for validation of longitudinal tensile failure models based on fibre break and cluster development, DATA IN BRIEF, Vol: 39, ISSN: 2352-3409
Yu B, Katafiasz TJ, Nguyen S, et al., 2021, Hygrothermal effects on the translaminar fracture toughness of a highly toughened aerospace CFRP: Experimental characterisation and model prediction, Composites Part A: Applied Science and Manufacturing, Vol: 150, Pages: 1-12, ISSN: 1359-835X
The translaminar fracture toughness and its dependence on the environmental condition are key considerations in designing aerospace-grade composites with a high damage tolerance to severe service conditions in terms of temperature and moisture. The present work characterises and models the hygrothermal effects on the translaminar fracture toughness of an interlaminar toughened aerospace carbon/epoxy composite under six environmental conditions: −55 °C, 23 °C, and 90 °C, for both ‘dry’ (i.e. moisture free) and ‘wet’ (fully moisture-saturated) specimens. Cross-ply compact-tension experiments show that the translaminar fracture toughness increases with the rise of temperature for both dry and wet conditions with the latter exhibiting a much greater increase. A model to predict the effect of moisture and temperature on the translaminar fracture toughness is here proposed and developed. This approach yields good agreement with experimental results, and it allows an improved understanding of the complex synergistic effects of interfacial properties on the overall translaminar toughening mechanisms.
Breite C, Melnikov A, Turon A, et al., 2021, Detailed experimental validation and benchmarking of six models for longitudinal tensile failure of unidirectional composites, COMPOSITE STRUCTURES, Vol: 279, ISSN: 0263-8223
Mulakkal M, Castillo Castillo A, Taylor A, et al., 2021, Advancing mechanical recycling of multilayer plastics through finite element modelling and environmental policy, Resources, Conservation and Recycling, Vol: 166, ISSN: 0921-3449
Plastics are attracting negative publicity due to the scale of current pollution levels, yet they are irreplaceable in several applications such as food packaging, where different types of plastics are combined in laminate form to produce multilayered packaging (MLP) materials which extend the life of food items packaged within them. Increased plastic recycling is urgently needed, however for MLP it is particularly difficult. For the first time, this study combines engineering tools with environmental policy towards developing solutions for current single use plastic packaging. This study investigates recycling challenges for MLP and emerging melt-blending based mechanical recycling solutions as this is the main current method for material recovery of conventional plastics. Melt-blending of MLP with compatibilisers is explored, and the current lack of models addressing the influence of compatibilisers is identified. This gap in knowledge is addressed using novel engineering models based on the finite element (FE) micromechanical modelling technique to estimate the mechanical properties of recycled blends. Our model output is compared with experimental data available in literature and the good agreement highlights its predictive ability, providing a fast and cost-effective novel method for optimising recycled plastics. The policy aspect proposes the introduction of twenty policies based on mission-oriented innovation strategy to enable deployment of the recycling technologies studied whilst improving the viability of recycling of material currently not recycled. Implementation of these measures by the stakeholders will enable adoption of new MLP recycling techniques, create demand for recycled materials from MLP and incentivise MLP collection to mitigate pollution.
Malgioglio F, Pimenta S, Matveeva A, et al., 2021, Microscale material variability and its effect on longitudinal tensile failure of unidirectional carbon fibre composites, COMPOSITE STRUCTURES, Vol: 261, ISSN: 0263-8223
Breite C, Melnikov A, Turon A, et al., 2021, Blind benchmarking of seven longitudinal tensile failure models for two virtual unidirectional composites, COMPOSITES SCIENCE AND TECHNOLOGY, Vol: 202, ISSN: 0266-3538
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- Citations: 7
Henry J, Pimenta S, 2021, Bio-inspired non-self-similar hierarchical microstructures for damage tolerance, Composites Science and Technology, Vol: 201, Pages: 1-12, ISSN: 0266-3538
Natural composites achieve damage tolerance through a complex microstructure with several hierarchical levels; this has inspired the development of synthetic composites with hierarchical microstructures, in order to overcome their inherent brittleness. However, while most natural composites have non-self-similar hierarchical microstructures, most synthetic hierarchical designs in the literature use self-similar features. The aim of this work is therefore to investigate whether non-self-similar hierarchical composites could be more damage tolerant than their self-similar equivalent. Non-self-similar hierarchical composites are designed, manufactured and tested; results show that releasing the self-similar constraint can increase the design space of composite microstructures and lead to materials with better damage tolerance, with a clearer warning before failure, and with more stable failure mechanisms.
Alves M, Pimenta S, 2020, The influence of 3D microstructural features on the elastic behaviour of tow-based discontinuous composites, COMPOSITE STRUCTURES, Vol: 251, ISSN: 0263-8223
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- Citations: 4
Finley JM, Shaffer MSP, Pimenta S, 2020, Data-driven intelligent optimisation of discontinuous composites, Composite Structures, Vol: 243, Pages: 1-19, ISSN: 0263-8223
Fibre composites, and especially aligned discontinuous composites (ADCs), offer enormous versatility in composition, microstructure, and performance, but are difficult to optimise, due to their inherent variability and myriad permutations of microstructural design variables. This work combines an accurate yet efficient virtual testing framework (VTF) with a data-driven intelligent Bayesian optimisation routine, to maximise the mechanical performance of ADCs for a number of single- and multi-objective design cases. The use of a surrogate model helps to minimise the number of optimisation iterations, and provides a more accurate insight into the expected performance of materials which feature significant variability. Results from the single-objective optimisation study show that a wide range of structural properties can be achieved using ADCs, with a maximum stiffness of 505 GPa, maximum ultimate strain of 3.94%, or a maximum ultimate strength of 1.92 GPa all possible. A moderate trade-off in performance can be achieved when considering multi-objective optimisation design cases, such as an optimal ultimate strength & ultimate strain combination of 982 MPa and 3.27%, or an optimal combination of 720 MPa yield strength & 1.91% pseudo-ductile strain.
Martulli LM, Muyshondt L, Kerschbaum M, et al., 2020, Morphology-induced fatigue crack arresting in carbon fibre sheet moulding compounds, International Journal of Fatigue, Vol: 134, Pages: 1-10, ISSN: 0142-1123
Carbon Fibre Sheet Moulding Compounds (CF-SMCs) are tow-based composite materials. Interrupted fatigue tests, combined with computed tomography, were performed here to investigate the damage mechanisms in high in-mould flow CF-SMC. The tow-based microstructure created obstacles for fatigue damage propagation, increasing the CF-SMC’s resistance against cyclic loading. Failure is shown to nucleate inside the tows, but inter-tow crack propagation tends to be hindered by the presence of the other tows. Tows oriented perpendicularly to the initial fatigue crack stop the crack itself, showing an intrinsic crack arrest mechanism. Additionally, pre-existing manufacturing cracks or voids do not propagate at all. As a result, flatter slopes of the SN diagrams were observed for CF-SMC than for other carbon or glass fibre composites with short, long and even continuous fibres.
Alves M, Carlstedt D, Ohlsson F, et al., 2020, Ultra-strong and stiff randomly-oriented discontinuous composites: closing the gap to quasi-isotropic continuous-fibre laminates, Composites Part A: Applied Science and Manufacturing, Vol: 132, Pages: 1-12, ISSN: 1359-835X
Conventional randomly-oriented Tow Based Discontinues Composites (TBDCs) are materials which combine good mechanical properties, lightweight and high manufacturability, and are therefore interesting for high-volume transport industries. This paper proposes, designs and successfully demonstrates a pathway to produce TBDCs with outstanding stiffness and tensile strength, by using ultra-thin tapes of (ultra-) high modulus carbon-fibres. Numerical models are used to explore the design space of discontinuous composite materials, in order to identify the optimal microstructural design to maximise stiffness and strength. Selected microstructures are manufactured and tested under tension; the experimental results show good agreement with the numerical predictions, and demonstrate a significant increase in the tensile strength and Young’s modulus of TBDCs by reducing the tow thickness and increasing the modulus of the fibres. Strength and stiffness increases of over 100% compared with the commercially available TBDC systems are achieved, resulting in mechanical properties that match the strength and overcome the stiffness of aerospace-graded continuous-fibre laminates.
Pascoe JA, Pimenta S, Pinho ST, 2020, The effect of tab orientation on the toughening mechanisms produced by interlocked interlaminar thin-ply CFRP reinforcements, Composite Structures, Vol: 238, Pages: 1-12, ISSN: 0263-8223
The use of interlaminar reinforcement units, containing an interlocked tab-and-slit geometry, is a new concept for improving interlaminar fracture toughness. It was recently shown that such reinforcement units are capable of substantially increasing mode I fracture toughness, but mode II fracture toughness was unaffected.This paper presents an investigation into the effect of tab orientation on the toughening mechanisms, comparing the experimentally determined Mode I and II interlaminar fracture toughness for different tab orientations.The results show that the previously reported lack of mode II toughness increase was due to an unsuitable tab orientation. With a better choice of tab orientation a mode II propagation toughness increase (of 23.5%) could be obtained, while simultaneously increasing the mode I propagation toughness further than previously reported (up to a 109% improvement).Fractography was used to investigate the toughening mechanisms. It was found that the two main toughening mechanisms are crack bridging (in mode I) and deflection of the delamination path (in both mode I and II). The relationship between the tab orientation and the obtained increase of fracture toughness can be explained by the effect of tab orientation on these mechanisms.
Bullegas G, Moledo Lamela J, Pimenta S, et al., 2020, On the role of dynamic stress concentrations and fracture mechanics in the longitudinal tensile failure of fibre-reinforced composites, Engineering Fracture Mechanics, Vol: 228, Pages: 1-31, ISSN: 0013-7944
This paper investigates the role of dynamic stress concentrations, and of fracture mechanics-driven growth of critical clusters of fibres, on the longitudinal tensile failure of fibre-reinforced composites. For this purpose, we developed a semi-analytical fibre bundle model to simulate the longitudinal tensile failure of large composite bundles of continuous fibres. The model uses shear-lag to calculate the stress recovery along broken fibres, and an efficient field superposition method to calculate the stress concentration on the intact fibres, which has been validated against analytical and Finite Element (FE) results from the literature.The baseline version of the model uses static equilibrium stress states, and considers fibre failure driven by strength of materials (stress overload) as the only damage theory which can drive bundle failure. Like other models in the literature, the baseline model fails to capture the correct size effect (decreasing composite strength with bundle size) shown by experimental results.Two model variants have been developed which include dynamics stress concentrations and a fracture mechanics failure criterion respectively. To the knowledge of the authors, it is the first attempt in the literature to investigate these two effects in a fibre bundle model by direct simulation of large composite bundles. It is shown that, although the dynamic stress concentration significantly decreases the predicted bundle strength, it does not allow to predict the correct trend of the size effect. Finally, the results suggest that fracture mechanics may be the physical mechanism which is necessary to include to correctly predict the decreasing composite strength with bundle size shown by experimental results.
Finley J, Henry J, Shaffer M, et al., 2020, The influence of variability and defects on the mechanical performance of tailorable composites, Journal of Composite Materials, Vol: 54, Pages: 565-589, ISSN: 0021-9983
Aligned hybrid-fibre discontinuous composites offer the ability to tailor their mechanical response through careful microstructural design. However, with tailorability comes microstructural complexity, which in turn leads to many sources of variability and defects. A virtual testing framework was further extended to investigate the influence of variability and defects on the mechanical performance of various aligned discontinuous composite material systems. This approach identified the most critical sources of variability as (i) fibre strength, (ii) the distance between fibre ends, or (iii) the level of fibre-type intermingling, depending on the material system. Fibre vacancy defects were shown to have the most significant influence on the strength and ductility of aligned discontinuous composites, although this sensitivity can be reduced through hybridisation of the fibre types.
Pascoe J-A, Pimenta S, Pinho S, 2020, Example analysis input files for CZM analysis of delamination growth in a DCB specimens
The files in this collection accompany the paper 'How to set up a cohesive zone model for an LEFM dominated problem: a detailed analysis for first time users', submitted to Applied Mechanics Reviews. The files comprise input files that allow reproduction of the example analyses shown in that paper. The purpose of the paper is to show how to determine appropriate values of various numerical parameters required when setting up a cohesive zone model analysis of an LEFM dominated crack propagation problem. These parameters can be determined by running a series of convergence studies. The input files for those convergence studies as well as the final prediction are included in this collection.The example analyses for the paper were run using Abaqus/Standard 6.14, and the included files will work for that software version.
Pascoe JA, Pimenta S, Pinho ST, 2020, Interlocking thin-ply reinforcements for the improvement of CAI strength
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. A new reinforcement concept for improving compression after impact strength of carbon fibre reinforced polymers is explored. The concept consists of manufacturing interlocked reinforcement units from thin-ply prepreg, which are inserted at the interlaminar interfaces within a regular ply-thickness laminate. Compression after impact tests following ASTM standards were performed on both reinforced and baseline specimens. The reinforced specimens showed a 11.4 % reduction in post-impact delamination area. The strength results will be presented at the conference.
Schuffenhauer K, Nothdurfter SK, Li Y, et al., 2020, Virtual design of car components manufactured with high-performance discontinuous composites
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. High Performance Discontinuous Composites (HPDCs) combine performance and manufacturability, which makes them suitable for automotive structures. However, due to the intrinsic variability in the microstructure of HPDCs, the local modulus and strength of these materials are highly heterogeneous, which raises a challenge in designing and simulating the structural response of a HPDC component. This work analyses the mechanical response of the engine bonnet of the Lamborghini Huracan PERFORMANTE, simulated using a FE Monte-Carlo framework developed specifically for HPDC structures. A significant spread of the maximum failure index and location of the critical region of the bonnet is observed, which highlights the effect of variability in HPDC materials on their structural performance.
Martulli LM, Alves M, Pimenta S, et al., 2020, Predictions of carbon fibre sheet moulding compound (CF-SMC) mechanical properties based on local fibre orientation
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. Carbon Fibre-Sheet Moulding Compound materials (CF-SMC) are an innovative class of intermediate materials made of chopped carbon fibre strands dispersed in uncured (or partially cured) thermoset resin. These sheets are then compression moulded into the final desired shape. Currently, the more commonly used models for SMC mechanical properties were initially developed for injection moulded composites, where the reinforcement is in the form of individual fibres, rather than tows or bundles. However, the presence of an additional intermediate scale of inclusions in the SMCs is not considered by those models. Their accuracy is thus questionable, and their adoption challenging: the user should generally choose whether to consider inclusions as tows or fibres, and thus have access to different material parameters, not always available (for example, the volume fraction of the tows in the composite). This work aims to validate two different formulations of a mixed Mori-Tanaka iso-strain model: one where inclusions are considered to be fibres, one where inclusions are tows; in addition, stiffness predictions are compared with the ones obtained a shear-lag multiscale model, that involves description of both tows and fibres. The models are compared with experimental evidence.
Henry J, Pimenta S, 2020, Bio-inspired non-self-similar hierarchical composites
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The use of hierarchies in naturally occurring composites motivated several studies in the literature, which demonstrated that hierarchical features could be integrated in composites to achieve damage tolerance. However, while most natural composites exhibit non-self-similarity in their hierarchical microstructure, most synthetic hierarchical designs in the literature use self-similar features. The aim of this work is therefore to investigate whether non-self-similar composites could be more damage tolerant than their self-similar equivalent. Hierarchical composites were designed, manufactured and tested, and results show that releasing the self-similar constraint could increase the design space and achieve better damage tolerance and warning before failure, and provide a more stable failure mechanism.
Finley JM, Henry J, Pimenta S, et al., 2020, The influence of defects and variability in discontinuous composite materials
A virtual testing framework was further developed to investigate the influence of defects and variability on the structural performance of various aligned discontinuous composite material systems. The virtual testing framework identified the most critical sources of variability as (i) fibre strength, (ii) the distance between fibre ends, or (iii) the level of fibre-type intermingling, depending on the material system. Fibre vacancy defects were shown to also have a significant influence on the strength and ductility of aligned discontinuous composites.
Deng X, Kinloch AJ, Pimenta S, et al., 2020, Toughening epoxy composites using nano- And microcellulose modifiers
The fracture properties and toughening mechanisms of cellulose- and cellulose-rubber hybrid-modified epoxy polymers and glass-fibre (GF) composites are investigated. The cellulose modifiers used are microcrystalline cellulose (MCC) and cellulose nanocrystals (CNC), and the rubber modifiers are carboxyl-terminated butadiene-acrylonitrile (CTBN) and core-shell rubber (CSR). The toughening mechanisms of the MCC-epoxy and CNC-epoxy were identified to be crack deflection, shear band yielding, particle rupture or pull-out and debonding of the cellulose particles, which was followed by plastic void growth. An additive toughening effect is observed for the hybrid polymers. Analytical modelling of the fracture energies showed that the particle pull-out toughening contribution is negligible for CNC-epoxy, and the particle debonding and rupture toughening contributions are negligible for MCC-epoxy. The GF composites were manufactured using the wet-layup process. Cellulose modifiers did not increase the composite propagation fracture energy (GC,prop) but slight increases in GC,prop occurred for the CNC hybrids. Increases in the fibre-matrix adhesion reduced the fibre toughening mechanisms in the composites that were modified with only MCC or CNC. The crack tip deformation zone is smaller than the MCC particles, reducing their toughening ability in the GF composites.
Pascoe JA, Pimenta S, Pinho ST, 2020, Interlocking thin-ply reinforcements for the improvement of CAI strength
A new reinforcement concept for improving compression after impact strength of carbon fibre reinforced polymers is explored. The concept consists of manufacturing interlocked reinforcement units from thin-ply prepreg, which are inserted at the interlaminar interfaces within a regular ply-thickness laminate. Compression after impact tests following ASTM standards were performed on both reinforced and baseline specimens. The reinforced specimens showed a 11.4 % reduction in post-impact delamination area. The strength results will be presented at the conference.
Li Y, Pimenta S, 2020, Development of a FE design framework to predict the response of discontinuous composite structures with heterogeneous microstructures
Tow-based discontinuous composites (TBDCs) are a growing class of high performance discontinuous composites used in structural applications. These materials have significant spatial variability in their microstructure and mechanical properties, which adds complexity to structural design using TBDCs. This study proposes a FE Monte-Carlo simulation framework to predict the mechanical response of a TBDC structure, while accounting for the variability in the microstructure of TBDCs. The simulation framework calculates a stochastic distribution of the modulus and strength of TBDCs; it then defines a characteristic spacing between uncorrelated material property points, at which the stochastic distributions of modulus and strength are assigned to a structure. The structure is then analysed in a Finite Element (FE) software, whose results are mesh-independent.
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