Publications
177 results found
Henry J, Pimenta S, 2017, Semi-analytical simulation of aligned discontinuous composites, Composites Science and Technology, Vol: 144, Pages: 230-244, ISSN: 0266-3538
Aligned-discontinuous-fibre reinforced polymers have the potential to combine (i) the high specific stiffness and strength and light weight of conventional continuous-fibre composites with (ii) increased damage tolerance, improved manufacturability, and the ability to close the life-cycle loop of composites by using recycled fibres. However, predicting the mechanical response of discontinuous composites is a challenge for which no universally accepted and computationally-efficient solution exists yet. This paper presents a model for aligned discontinuous-fibre reinforced composites considering (i) a generic constitutive law for the matrix, (ii) stochastic fibre failure under non-uniform stress fields due to the presence of fibre-ends, and (iii) unstable final failure from a critical cluster of damage. Results show good agreement with experiments from the literature, and the model also stresses the importance of considering the stochastic nature of both the fibre-end locations and the fibre-strengths to model aligned discontinuous composites. Parametric studies suggest that failure of aligned discontinuous composites depends on (i) the overlap length between fibres for short-fibre composites, and (ii) the fibre strength for long-fibre composites; intermediate-length fibres would result in discontinuous composites with maximum stiffness, strength, and failure strain simultaneously.
Kaboglu C, Pimenta S, Morris A, et al., 2017, The effect of different types of core material on the flexural behavior of sandwich composites for wind turbine blades, Journal of Thermal Engineering, Vol: 3, Pages: 1102-1109, ISSN: 2148-7847
In this study, three differently-configured sandwich structures were manufactured with three different core materials: Balsa wood, Tycor and Polyethylene terephthalate (PET). Glass-Fibre Reinforced Polymer (GFRP) skins were used to understand the effects of different types of core materials on the flexural behavior of sandwich composites under four point bending (4PB) condition, using digital image correlation (DIC). DIC is one of the most outstanding techniques to understand the mechanical behavior of the structure during the test, thus defining any problematic regions in the structures. The failure mechanisms of the structures were observed by using strain maps of the structures. The results show that the sandwich structure with Balsa wood as a core material has the highest stiffness; however, catastrophic failure appeared in the early stages of the test. The sandwich structure with PET and Tycor exhibited very similar behaviour under load.
Bullegas G, Pinho ST, Pimenta S, 2016, Engineering the translaminar fracture behaviour of thin-ply composites, Composites Science and Technology, Vol: 131, Pages: 110-122, ISSN: 0266-3538
Bio-inspired patterns of micro-cuts perpendicular to the fibre direction in thin-ply CFRP laminates have been used to increase the translaminar fracture toughness of the material. An analytical model to predict the probability of bundle pull-out during translaminar crack propagation was developed and validated through an experimental parametric study. The model was used to design three hierarchical patterns of micro-cuts and the patterns have been tested using Compact Tension specimens. The increase in fracture toughness for the three patterns was +15%, +60% and +214% when compared with the baseline material, thereby demonstrating the potential of engineering the fracture surface in CFRPs through well-designed patterns of micro-cuts to improve the damage tolerance of the material.
Anthony DB, Grail G, Bismarck A, et al., 2016, Exploring the tensile response in small carbon fibre composite bundles, ECCM17 - 17th European Conference on Composite Materials
Small composite bundles, AS4 carbon fibre epoxy, with a restricted number of reinforcing fibres, ca. 20, showed a progressive failure when tested in tension. In-situ acoustic emission observations under tensile load reveal that numerous fibres fail before ultimate failure of the small composite bundle, suggesting that isolated and individual fibre failures occur without compromising the integrity of the neighboring fibres or the small composite bundle’s overall mechanical performance. The average strength of the carbon fibres in small composite bundles was 9.6% higher than in standard lab-scale composite specimens using the same fibre type.
Henry J, Pimenta S, 2016, Prediction of mechanical properties of hybrid discontinuous composites, ECCM 2016
Henry J, Pimenta S, 2016, Mechanical properties of hierarchical discontinuous composites
This work proposes bio-inspired hierarchical brick-and-mortar microstructures-where each "brick" is itself composed of smaller platelets regularly staggered in a matrix-to add damage tolerance to conventionally brittle composites. Finite element simulations on such geometries are performed, using cohesive elements to model damage in the matrix; models with multiple repetitions of unit-cells are also used to evaluate damage dispersion at a macroscopic scale. Finally, hierarchical brick-and-mortar specimens are manufactured and tested, and showed good agreement with the model predictions. It is shown that optimised hierarchical brick-and-mortar architectures create a markedly non-linear material response, with extensive energy dissipation before failure, and also delay damage localisation in larger structures.
Kaboglu C, Pimenta S, Morris A, et al., 2016, The influence of surface finishing of core on the impact behaviour of polymer foam-cored sandwich structures, The American Society for Composites - 31st Technical Conference, ASC 2016
The interest on sandwich composite structures is increasing due to their good mechanical properties, the ability to tailor the sandwich structure to each component, and good formability. These materials are used in aerospace, wind energy and automotive industries, where minimising the weight of vehicle or components is very important. The increasing use of polymer foams in composite structures has encouraged the polymer industry to improve the mechanical properties of the core materials, by redesigning the materials and applying different types of surface finishing. In this research, the effect of surface finishing options on the impact behaviour of sandwich composite structures was investigated by performing low velocity drop weight impact tests, because impact is often critical in this kind of components. The sandwich composites used had a PVC polymer foam core with various surface finishing options (plain, flexi cut, knife cut, saw cut and mini cut), and were manufactured with Glass Fibre Reinforced Polymer (GFRP) skins through Resin Infusion under Flexible Tooling (RIFT). After the impact test, the delaminated areas at the back face of the samples with different finishes were measured and compared against each other. The surface finishing options affected the impact behaviour of the sandwich composite structure, and increased the ductility and energy-absorption capacity of the structure.
Khaleque T, Pimenta S, Taylor AC, 2016, The fracture performance and particle dispersion of rubber- and nanosilica particle-modified epoxies
© 2016, European Conference on Composite Materials, ECCM. All rights reserved.The microstructure and fracture performance of an anhydride cured epoxy polymer modified by different combinations of preformed core-shell rubber (CSR) particles and 20 nm diameter nanosilica particles are investigated. Two types of CSR particles, with diameters of 100 nm and 300 nm, are used. A quantitative study of the dispersion of CSR particles and nanosilica particles was performed using the area disorder method, and random dispersions of the CSR particles were observed for the CSR modified epoxies. No significant influence was observed on the dispersion of CSR particles with the addition of nanosilica particles in the hybrid CSR-nanosilica modified epoxies. Nanosilica particles were also found to be randomly dispersed in the hybrid modified epoxy matrix. The fracture energy increased from 78 J/m2 for the unmodified epoxy to 530 J/m2 with an addition of 9 wt% of 100 nm diameter CSR particles and to 403 J/m2 with an addition of 300 nm diameter CSR particles; this was further enhanced to 592 J/m2 by the addition of 9 wt% of nanosilica.
Pimenta S, Henry J, 2016, Hierarchical brick-and-mortar composites for damage tolerance and progressive failure
This work proposes bio-inspired hierarchical brick-and-mortar microstructures - where each "brick" is itself composed of smaller platelets regularly staggered in a matrix - to add damage tolerance to conventionally brittle composites. Finite element simulations of different geometries are performed, using cohesive elements to model damage in the matrix; models with multiple repetitions of unit-cells are also used to evaluate damage dispersion at a macroscopic scale. It is shown that optimised hierarchical brick-and-mortar architectures create a markedly non-linear material response, with extensive energy dissipation before failure, and also delay damage localisation in larger structures.
Grail G, Coq M, Guesdon C, et al., 2016, Combined FE/statistical approach for the strength of composite fibre bundles considering hierarchical failure
© 2016, European Conference on Composite Materials, ECCM. All rights reserved.This paper presents an overview of a combined FE/statistical approach for predicting the full strength distribution of unidirectional Composite Fibre Bundles (CFBs). FE models of CFBs with differently sized pre-fractured clusters of fibres were built to predict the stress concentration field around the respective broken cluster, and this was then used in an independent statistical model to predict failure of the surrounding fibres. The latter considers a hierarchical crack propagation from a single fibre break, with strength following a Weibull distribution, to its closest neighbour, hence forming a bundle of two broken fibres; the process is then repeated hierarchically, until final failure of the bundle. A key attribute of the statistical model for fibre failure is that it uses accurate full stress fields for each size of the broken cluster of fibres. Results show that, if a hierarchical approach (with separation of hierarchies) is used in both the FE and statistical models, there is no need to include detailed stress fields to obtain a good correlation with experiments. However, if no separation of hierarchies is imposed in the FE models, it is necessary to consider the full stress fields to achieve more realistic predictions and to capture the correct trends in size effects.
Pinho ST, Bullegas G, Pimenta S, 2016, High-toughness CFRP laminates with engineered fracture surfaces: A shark-teeth design
© 2016, European Conference on Composite Materials, ECCM. All rights reserved.Carefully placed patterns of micro-cuts have been designed and then used to increase the translaminar work of fracture in thin-ply composite laminates. These patterns of micro-cuts are able to deviate a translaminar crack and force the formation of large bundle pull-outs. The technique allowed to achieve a 68% increase in the laminate notched strength and a 460% increase in the laminate translaminar work of fracture when compared with the un-modified baseline material. Part of the improved performance was found to be due to the interaction of fracture mechanisms between contiguous plies with different orientation; this mechanism opens new possibilities for micro-structure design which will be explored in future works.
Li Y, Pimenta S, Thierry M, et al., 2016, Prediction of stiffness for tow-based discontinuous composites
Tow-based discontinuous composites are a new class of high-performance materials composed of carbon-fibre tows randomly oriented in a polymeric matrix. The discontinuous and random architecture of these materials implies that their local properties - including the stiffness - are highly inhomogeneous. Therefore, this work aims at developing a new model to predict the local stiffness field of tow-based discontinuous composites, while accounting for its intrinsic variability and implementing the method in a Finite Element environment. A multi-scale model based on a ply-by-ply laminate analogy is used to determine the stiffness of an equivalent laminate consisting of randomly-oriented and discontinuous plies. This model generates a distribution of stiffness values which are locally assigned to stiffness "seeds", mapped on the geometry of a component to be modelled using Finite Element simulations (in Abaqus). The FE model generates a heterogeneous strain field in the component, which shows good agreement with experimental observations on these materials. This model can be used to account for the variability in the mechanical properties of tow-based discontinuous composites when simulating structural components, thus contributing to a more sound design.
Kaboglu C, Pimenta S, Morris A, et al., 2016, The effect of surface finishing options for core materials on the mechanical properties of sandwich structures, ECCM 2016 - The 17th European Conference on Composite Materials
© 2016, European Conference on Composite Materials, ECCM. All rights reserved. Polymeric sandwich composite structures are widely used as lightweight and durable materials for naval, aviation and energy applications. The increasing use of polymer foams in composite structures has encouraged the polymer industry to improve the mechanical properties of the core materials by applying different types of surface finishing. In this study, sandwich composites consisting of PVC polymer foams with various surface finishing options (plain, flexi cut, knife cut, saw cut and mini cut) have been investigated, by performing four-point bend tests and digital image correlation. The strain maps and equivalent shear stresses in the core have been examined, and the influence of the different foam finishing options was determined.
Bullegas G, Pinho ST, Pimenta S, 2016, On the role of shear transfer mechanisms in the longitudinal tensile failure of cfrp composites
A three-Dimensional (3D) Fibre Bundle Model (FBM) has been developed to simulate the longitudinal tensile failure and predict the statistical strength distribution of CFRP bundles of different sizes. An original semi-analytical approach has been developed to determine the stress field inside the bundle and predict the evolution of the failure process. The present model can account for the effects of fibre-matrix debonding, as well as the effect of the size of the cluster of broken fibres on the stress recovery length and, in turn, on the bundle strength. The model results compare favourably with different sets of experimental results, thus demonstrating the suitability of the modelling approach and the importance of including the previously mentioned effects in FBMs.
Khaleque T, Pimenta S, Taylor AC, 2016, The fracture performance and particle dispersion of rubber- and nanosilica particle-modified epoxies
The microstructure and fracture performance of an anhydride cured epoxy polymer modified by different combinations of preformed core-shell rubber (CSR) particles and 20 nm diameter nanosilica particles are investigated. Two types of CSR particles, with diameters of 100 nm and 300 nm, are used. A quantitative study of the dispersion of CSR particles and nanosilica particles was performed using the area disorder method, and random dispersions of the CSR particles were observed for the CSR modified epoxies. No significant influence was observed on the dispersion of CSR particles with the addition of nanosilica particles in the hybrid CSR-nanosilica modified epoxies. Nanosilica particles were also found to be randomly dispersed in the hybrid modified epoxy matrix. The fracture energy increased from 78 J/m2 for the unmodified epoxy to 530 J/m2 with an addition of 9 wt% of 100 nm diameter CSR particles and to 403 J/m2 with an addition of 300 nm diameter CSR particles; this was further enhanced to 592 J/m2 by the addition of 9 wt% of nanosilica.
Grail G, Coq M, Guesdon C, et al., 2016, Combined FE/statistical approach for the strength of composite fibre bundles considering hierarchical failure
This paper presents an overview of a combined FE/statistical approach for predicting the full strength distribution of unidirectional Composite Fibre Bundles (CFBs). FE models of CFBs with differently sized pre-fractured clusters of fibres were built to predict the stress concentration field around the respective broken cluster, and this was then used in an independent statistical model to predict failure of the surrounding fibres. The latter considers a hierarchical crack propagation from a single fibre break, with strength following a Weibull distribution, to its closest neighbour, hence forming a bundle of two broken fibres; the process is then repeated hierarchically, until final failure of the bundle. A key attribute of the statistical model for fibre failure is that it uses accurate full stress fields for each size of the broken cluster of fibres. Results show that, if a hierarchical approach (with separation of hierarchies) is used in both the FE and statistical models, there is no need to include detailed stress fields to obtain a good correlation with experiments. However, if no separation of hierarchies is imposed in the FE models, it is necessary to consider the full stress fields to achieve more realistic predictions and to capture the correct trends in size effects.
Pinho ST, Bullegas G, Pimenta S, 2016, High-toughness CFRP laminates with engineered fracture surfaces: A shark-teeth design
Carefully placed patterns of micro-cuts have been designed and then used to increase the translaminar work of fracture in thin-ply composite laminates. These patterns of micro-cuts are able to deviate a translaminar crack and force the formation of large bundle pull-outs. The technique allowed to achieve a 68% increase in the laminate notched strength and a 460% increase in the laminate translaminar work of fracture when compared with the un-modified baseline material. Part of the improved performance was found to be due to the interaction of fracture mechanisms between contiguous plies with different orientation; this mechanism opens new possibilities for micro-structure design which will be explored in future works.
Bullegas G, Pinho ST, Pimenta S, 2016, High-toughness CFRP laminates with engineered fracture surfaces: A shark-teeth design
Carefully-placed patterns of micro-cuts have been designed and then used to increase the translaminar work of fracture in thin-ply CFRP composite laminates. These patterns of micro-cuts are able to deviate a translaminar crack and force the formation of large bundle pull-outs. The technique led to a 68% increase in the laminate notched strength and a 460% increase in the laminate translaminar work of fracture when compared with the un-modi-ed baseline material. Part of the improved performance was found to be due to the generated interaction of failure mechanisms between contiguous plies with di-erent orientation; this process for generating interaction opens new possibilities for micro-structure design which will be explored in future works.
Pinho ST, Bullegas G, Pimenta S, 2016, On the role of shear transfer mechanisms in the longitudinal tensile failure of CFRP composites
A three-Dimensional (3D) Fibre Bundle Model (FBM) has been developed to simulate the longitudinal tensile failure and predict the statistical strength distribution of CFRP bundles of different sizes. The model incorporates an original semi-analytical approach to determine the stress FIeld inside the bundle and to predict the evolution of the failure process. The present model can account for the e-ects of bre-matrix debonding, as well as the effect of the size of the cluster of broken fibres on the stress recovery length and, in turn, on the bundle strength. The predictions from the model are shown to compare favourably with experimental results from the literature, thus demonstrating the suitability of the modelling approach.
Pimenta S, Li Y, 2016, Predicting the strength and failure envelopes of high-performance discontinuous composites
Carbon-fibre tow-based discontinuous composites are novel materials combining the high-performance of carbon-fibres with the manufacturability of metals. These composites are reinforced by a network of discontinuous and randomly-oriented fibre-tows (each composed of thousands of aligned fibres), creating a multiscale and unstructured architecture. Consequently, they can be moulded into complex 3D shapes using fully automated and high-rate processes, which makes them suitable for mass-production applications. Moreover, tow-based discontinuous composites can achieve fibre contents up to 60% in volume, and a stiffness similar to that of quasi-isotropic continuous composites. While the complex and multiscale microstructure of tow-based discontinuous composites is key for their manufacturability and good mechanical performance, it also creates a challenge for predicting their properties. This work therefore proposes a multi-scale model to predict the strength and failure envelopes of tow-based discontinuous composites, using the mechanical properties of the tows and their geometry as inputs. The model predicts full failure envelopes for any tension-tension or tension-shear in-plane stress state in less than 5 min, making the model suitable for design of structural components under complex stress states. The model is also able to predict the effect of changing the properties and geometry of the tows on the overall response of the composite, and can therefore be used to design optimal material configurations with improved performance.
Bullegas G, Pinho ST, Pimenta S, 2016, On the role of shear transfer mechanisms in the longitudinal tensile failure of cfrp composites
© 2016, European Conference on Composite Materials, ECCM. All rights reserved.A three-Dimensional (3D) Fibre Bundle Model (FBM) has been developed to simulate the longitudinal tensile failure and predict the statistical strength distribution of CFRP bundles of different sizes. An original semi-analytical approach has been developed to determine the stress field inside the bundle and predict the evolution of the failure process. The present model can account for the effects of fibre-matrix debonding, as well as the effect of the size of the cluster of broken fibres on the stress recovery length and, in turn, on the bundle strength. The model results compare favourably with different sets of experimental results, thus demonstrating the suitability of the modelling approach and the importance of including the previously mentioned effects in FBMs.
Pimenta S, 2015, Fibre failure modelling, Numerical Modelling of Failure in Advanced Composite Materials, Editors: Camanho, Hallett, Publisher: Woodhead Publishing, Pages: 193-224, ISBN: 978-0-08-100332-9
Fibre-dominated tensile failure in composites is a complex phenomenon governed by the statistical distribution of fibre strengths and the micromechanics of stress-transfer in the neighbourhood of fibre breaks. This chapter provides an overview of classical and state-of-the-art material models to simulate the fibre-dominated tensile failure process in unidirectional composites, and to predict the tensile strength and fracture toughness of these materials, as well as the associated size effects. The relevance of such material models for the simulation of composite components is discussed, and it is demonstrated that the correct representation of size effects at the material scale is fundamental to accurately predict the response of more complex structures.
Winkless L, Pimenta S, 2015, Multiscale composites, Reinforced Plastics, Vol: 59, Pages: 132-134, ISSN: 0034-3617
Pimenta S, Robinson P, 2015, Wavy-ply sandwich with crushable core: design, simulation and testing, CompTest2015
Bullegas G, Pinho ST, Pimenta S, 2015, Improving translaminar toughness of thin-ply laminates through a bio-inspired crack deflection technique, CompTest2015
Kaboglu C, Pimenta S, Morris A, et al., 2015, Strain visualisation of composite sandwich structure with different core materials for wind turbine blades, ICCM 20 - 20th International Conference on Composite Materials
Kaboglu C, Pimenta S, Morris A, et al., 2015, Failure mode of composite sandwich structures with graded core, ICAME`15 -International Conference on Advances In Mechanical Engineering
Pimenta S, 2015, Recycling of composite materials, International Meeting of Science for Recycling
Czel G, Pimenta S, Wisnom MR, et al., 2015, Demonstration of pseudo-ductility in unidirectional discontinuous carbon fibre/epoxy prepreg composites, COMPOSITES SCIENCE AND TECHNOLOGY, Vol: 106, Pages: 110-119, ISSN: 0266-3538
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Yu H, Longana ML, Grail G, et al., 2015, ALIGNED SHORT FIBRE COMPOSITES WITH NONLINEAR BEHAVIOUR, 20th International Conference on Composite Materials (ICCM), Publisher: AALBORG UNIV PRESS
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