Imperial College London

Dr Soraia Pimenta

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 3784soraia.pimenta Website

 
 
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Location

 

521City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

152 results found

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

© 2020 Tow-based discontinuous composites consist of carbon-fibre tows dispersed in a polymeric matrix, resulting in a complex microstructure with inherent 3D features such as tow waviness, non-uniform tow thickness, and fibre content variations; however, most models for these materials are based on 2D formulations, or in very computationally-expensive FE models. This work proposes a computationally-efficient microstructure generator able to recreate the 3D features of tow-based discontinuous composites, coupled with an analytical stiffness model able to quantify the effect of tow waviness on the modulus of these materials, for the first time in the literature. The features of the microstructures generated are validated against 3D measurements obtained through micro-CT scans and optical micrographs. The results of the stiffness model are successfully validated against experimental results from the literature, showing that tow waviness can trigger a knock-down of over 20% on the stiffness of tow-based discontinuous composites. It is also shown that current testing standards can lead to uncertainties over 20% on the measurement of the Young's modulus of tow-based discontinuous composites, and alternatives to define suitable experimental campaigns are proposed.

Journal article

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.

Journal article

Alves M, Carlstedt D, Ohlsson F, Asp LE, Pimenta Set 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.

Journal article

Martulli LM, Muyshondt L, Kerschbaum M, Pimenta S, Lomov SV, Swolfs Yet 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.

Journal article

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.

Journal article

Bullegas G, Moledo Lamela J, Pimenta S, Pinho STet 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.

Journal article

Finley J, Henry J, Shaffer M, Pimenta Set 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.

Journal article

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.

Software

Schuffenhauer K, Nothdurfter SK, Li Y, Pimenta Set 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.

Conference paper

Martulli LM, Alves M, Pimenta S, Hine PJ, Kerschbaum M, Lomov SV, Swolfs Yet 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.

Conference paper

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.

Conference paper

Finley JM, Henry J, Pimenta S, Shaffer MSPet al., 2020, The influence of defects and variability in discontinuous composite materials

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

Deng X, Kinloch AJ, Pimenta S, Schueneman GT, Sprenger S, Taylor AC, Teo WSet al., 2020, Toughening epoxy composites using nano- And microcellulose modifiers

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

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.

Conference paper

Li Y, Pimenta S, 2020, Development of a FE design framework to predict the response of discontinuous composite structures with heterogeneous microstructures

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

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.

Conference paper

Pimenta S, Li Y, Alves M, Gaudron F, Tahreem, Nothdurfter SK, Schuffenhauer Ket al., 2020, Quantifying and predicting the effect of heterogenenous microstructures on the performance of discontinuous composites

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The heterogeneous microstructure of high performance Tow-Based Discontinuous Composites (TBDCs) leads to non-uniform stochastic fields of local mechanical properties; consequently, TBDC structures present a non-deterministic response, and may fail at locations that can only be predicted with stochastic methods. We have quantified the effect of the heterogeneous microstructure on the strength of unnotched and notched tensile specimens, using a range of hole sizes; we have then used an in-house developed FE Monte-Carlo framework to predict stochastic strain fields, critical locations, and critical loads in the specimens. The results from the FE Monte-Carlo analysis provide a good qualitative agreement with the experimental data, suggesting that our framework can be used for structural design of TBDC structures.

Conference paper

Schuffenhauer K, Nothdurfter SK, Li Y, Pimenta Set 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.

Conference paper

Alves M, Pimenta S, 2020, Analysing the effect of fibre waviness on the stiffness of tow-based discontinuous composites

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. Tow-based discontinuous composites (TDBCs) are a particular type of discontinuous composites and are becoming a growing class in high-performance materials. These materials consist of carbon-fibre tows dispersed in a polymeric matrix, resulting in a microstructure that has an inherent 3D nature, with the tows presenting waved configurations. This work aims to develop a microstructure generator able to recreate the characteristic features of the microstructure of TBDCs, such as tow waviness and local thickness and fibre content variations; in addition, flexibility in terms of in-plane tow orientations is also considered, as the distributions of the latter are defined by orientation tensors. The microstructure generator is able to recreate representative cross-sections of these materials, and also out-of-plane angle distributions, for any type of in-plane tow orientations considered. Furthermore, the effect of the tow waviness on the stiffness of these materials is assessed, and a parametric study on the effect of the tow geometry on the in-plane stiffness reduction is carried out.

Conference paper

Martulli LM, Alves M, Pimenta S, Hine PJ, Kerschbaum M, Lomov SV, Swolfs Yet 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.

Conference paper

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.

Conference paper

Schuffenhauer K, Nothdurfter SK, Li Y, Pimenta Set 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.

Conference paper

Martulli LM, Alves M, Pimenta S, Hine PJ, Kerschbaum M, Lomov SV, Swolfs Yet 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.

Conference paper

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.

Conference paper

Finley JM, Henry J, Pimenta S, Shaffer MSPet al., 2020, The influence of defects and variability in discontinuous composite materials

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

Li Y, Pimenta S, 2020, Development of a FE design framework to predict the response of discontinuous composite structures with heterogeneous microstructures

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

Pimenta S, Li Y, Alves M, Gaudron F, Tahreem, Nothdurfter SK, Schuffenhauer Ket al., 2020, Quantifying and predicting the effect of heterogenenous microstructures on the performance of discontinuous composites

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The heterogeneous microstructure of high performance Tow-Based Discontinuous Composites (TBDCs) leads to non-uniform stochastic fields of local mechanical properties; consequently, TBDC structures present a non-deterministic response, and may fail at locations that can only be predicted with stochastic methods. We have quantified the effect of the heterogeneous microstructure on the strength of unnotched and notched tensile specimens, using a range of hole sizes; we have then used an in-house developed FE Monte-Carlo framework to predict stochastic strain fields, critical locations, and critical loads in the specimens. The results from the FE Monte-Carlo analysis provide a good qualitative agreement with the experimental data, suggesting that our framework can be used for structural design of TBDC structures.

Conference paper

Alves M, Pimenta S, 2020, Analysing the effect of fibre waviness on the stiffness of tow-based discontinuous composites

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. Tow-based discontinuous composites (TDBCs) are a particular type of discontinuous composites and are becoming a growing class in high-performance materials. These materials consist of carbon-fibre tows dispersed in a polymeric matrix, resulting in a microstructure that has an inherent 3D nature, with the tows presenting waved configurations. This work aims to develop a microstructure generator able to recreate the characteristic features of the microstructure of TBDCs, such as tow waviness and local thickness and fibre content variations; in addition, flexibility in terms of in-plane tow orientations is also considered, as the distributions of the latter are defined by orientation tensors. The microstructure generator is able to recreate representative cross-sections of these materials, and also out-of-plane angle distributions, for any type of in-plane tow orientations considered. Furthermore, the effect of the tow waviness on the stiffness of these materials is assessed, and a parametric study on the effect of the tow geometry on the in-plane stiffness reduction is carried out.

Conference paper

Deng X, Kinloch AJ, Pimenta S, Schueneman GT, Sprenger S, Taylor AC, Teo WSet al., 2020, Toughening epoxy composites using nano- And microcellulose modifiers

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. 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.

Conference paper

Martulli LM, Muyshondt L, Kerschbaum M, Pimenta S, Lomov SV, Swolfs Yet al., 2019, Carbon fibre sheet moulding compounds with high ine-mould flow: Linking morphology to tensile and compressive properties, Composites Part A: Applied Science and Manufacturing, Vol: 126, Pages: 1-16, ISSN: 1359-835X

In-mould flow during manufacturing of Sheet Moulding Compounds (SMCs) heavily affects the material microstructure and its mechanical properties. This influence is studied here for carbon SMCs on panels compression moulded with limited charge coverage. The high in-mould flow caused severe in-plane tow distortions, while their planarity was preserved. Flow induced fibre orientation plays a paramount role in the material failure, whereas local manufacturing defects had no discernible influence. The properties difference between specimens with preferential orientation of 0° and 90° was 150% for tensile stiffness, 260% for tensile strength, 120% for compressive stiffness and 32% for compressive strength. The compressive strength and failure strain for 45° and 90° specimens were higher than those for tension, and comparable for 0° specimens. Compressive and tensile moduli were similar for specimens with the same orientation. A clear link between SMCs manufacturing and mechanical performance is highlighted, together with its implications on structural design.

Journal article

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