113 results found
Martulli LM, Muyshondt L, Kerschbaum M, et al., 2019, Carbon fibre sheet moulding compounds with high in-mould flow: Linking morphology to tensile and compressive properties, Composites Part A: Applied Science and Manufacturing, Vol: 126, ISSN: 1359-835X
© 2019 Elsevier Ltd 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.
Pascoe J-A, Pimenta S, Pinho ST, 2019, Interlocking thin-ply reinforcement concept for improved fracture toughness and damage tolerance, Composites Science and Technology, Vol: 181, ISSN: 0266-3538
An original concept for improving the delamination resistance and damage tolerance of a composite laminate is proposed. The concept is to insert interlocked thin-ply reinforcement units between the laminae. Each reinforcement unit consists of two thin-ply layers with tabs cut into one layer, and slits cut into the other layer. The slits, and the long axis of the tabs, are parallel to the fibre direction in their respective layers. The two thin-ply layers are placed together, and the tabs are inserted through the slits, creating an interlocked reinforcement unit.The effect of the reinforcement units was quantified via mode I (DCB) and mode II (4ENF) fracture toughness tests, as well as compression after impact tests. Mode I propagation fracture toughness was increased by 77.6%, while mode II fracture toughness was not affected. In the compression after impact tests, an 11.4% reduction in delamination area was achieved but this only resulted in a 5.1% increase in CAI strength.
Finley J, Henry J, Shaffer M, et al., The influence of variability and defects on the mechanical performance of tailorable composites, Journal of Composite Materials, 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.
Li Y, Pimenta S, 2019, Development and assessment of modelling strategies to predict failure in tow-based discontinuous composites, Composite Structures, Vol: 209, Pages: 1005-1021, ISSN: 0263-8223
Tow-based discontinuous composites (TBDCs) are a growing class of materials that combine manufacturability, light-weight, and high performance. This study proposes multi-scale modelling approaches to predict the tensile strength and failure envelopes of tow-based discontinuous composites, by representing the actual composite (with randomly-oriented tows) as an equivalent ply-by-ply laminate. Several modelling approaches are considered for the different scales, including (i) a stochastic bi-linear shear-lag formulation accounting for the random location of tow-ends and matrix cracking, (ii) a novel failure criterion for a discontinuous uni-directional ply accounting for the interaction between tow pull-out and transverse failure, and (iii) a ply-discount method or a maximum strain energy criterion for the final failure of the composite. The model computes full failure envelopes for ply-by-ply laminates equivalent to TBDCs within minutes, and the results show good agreement with experimental data.
Alves M, Pimenta S, 2018, A computationally-efficient micromechanical model for the fatigue life of unidirectional composites under tension-tension loading, International Journal of Fatigue, Vol: 116, Pages: 677-690, ISSN: 0142-1123
Failure of fibre-reinforced composites is affected by fatigue, which increases the challenge in designing safe and reliable composite structures. This paper presents an analytical model to predict the fatigue life of unidirectional composites under longitudinal tension-tension. The matrix and fibre-matrix interface are represented through a cohesive constitutive law, and a Paris law is used to model fatigue due to interfacial cracks propagating from fibre-breaks. The strength of single-fibres is modelled by a Weibull distribution, which is scaled hierarchically though a stochastic failure analysis of composite fibre-bundles, computing stochastic S-N curves of lab-scaled specimens in less than one minute. Model predictions are successfully validated against experiments from the literature. This model can be used to reduce the need for fatigue testing, and also to evaluate the impact of constituent properties on the fatigue life of composites.
Bullegas G, Benoliel J, Fenelli PL, et al., 2018, Towards Quasi Isotropic laminates with engineered fracture behaviour for industrial applications, Composites Science and Technology, Vol: 165, Pages: 290-306, ISSN: 0266-3538
Carefully placed patterns of micro-cuts have been inserted in the microstructure of Cross-Ply (CP) and Quasi-Isotropic (QI) thin-ply CFRP laminates to engineer their translaminar fracture behaviour with the purpose of increasing their damage resistance under different loading conditions. A novel Finite Fracture Mechanics model has been developed to predict the translaminar crack propagation behaviour and to guide the microstructure design. This technique led to a 68% increase in the laminate notched strength, and a 460% increase in the laminate translaminar work of fracture during Compact Tension tests for CP laminates. It also allowed to achieve a 27% increase in the laminate notched strength, and a 189% increase in the translaminar work of fracture during Compact Tension tests for QI laminates. Furthermore, an increase of 43% in the total energy dissipated, and of 40% in maximum deflection at complete failure was achieved during quasi-static indentation tests on QI laminates. Given the significant improvements in the mechanical performance under different loading conditions, and the industrial relevance of QI laminates and the increasing industrial interest in thin-ply laminates, these results demonstrate that microstructure design can be used effectively to improve the damage tolerance of CFRP structures in industrially-relevant applications.
Henry J, Pimenta S, 2018, Increasing damage tolerance in composites using hierarchical brick-and-mortar microstructures, Journal of the Mechanics and Physics of Solids, Vol: 118, Pages: 322-340, ISSN: 0022-5096
Composites are attractive materials because of their high specific stiffness and specific strength, but their application in industry is restricted by their inherent lack of damage tolerance and stable energy dissipation mechanisms, due to the brittleness of the fibres. Nature overcomes a similar issue by arranging natural composites, made of mostly brittle constituents, in discontinuous and hierarchical microstructures. This work aims at evaluating the potential of hierarchical discontinuous carbon-fibre reinforced polymers to achieve damage tolerance, by a combination of modelling and experiments. Two different models (one analytical and the other numerical) are developed to predict the tensile response of hierarchical brick-and-mortar microstructures with two levels of hierarchies, and to design specimens with a non-linear response. Such specimens are then manufactured using laser micro-milled carbon/epoxy thin-plies, and tested under tension. The results show that the presence of discontinuities and hierarchies promotes stable energy dissipation before failure, ensures damage diffusion throughout the specimen, and delays damage localisation in otherwise brittle composites.
Bunsenll A, Gorbatikh L, Morton H, et al., 2018, Benchmarking of strength models for unidirectional composites under longitudinal tension, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING, Vol: 111, Pages: 138-150, ISSN: 1359-835X
Several modelling approaches are available in the literature to predict longitudinal tensile failure of fibre-reinforced polymers. However, a systematic, blind and unbiased comparison between the predictions from the different models and against experimental data has never been performed. This paper presents a benchmarking exercise performed for three different models from the literature: (i) an analytical hierarchical scaling law for composite fibre bundles, (ii) direct numerical simulations of composite fibre bundles, and (iii) a multiscale finite-element simulation method. The results show that there are significant discrepancies between the predictions of the different modelling approaches for fibre-break density evolution, cluster formation and ultimate strength, and that each of the three models presents unique advantages over the others. Blind model predictions are also compared against detailed computed-tomography experiments, showing that our understanding of the micromechanics of longitudinal tensile failure of composites needs to be developed further.
Pimenta S, Mersch A, Alves M, 2018, Predicting damage accumulation and fatigue life of UD composites under longitudinal tension, 39th Risø International Symposium on Materials Science, Publisher: IOP Publishing, ISSN: 1757-8981
Unidirectional composites under cyclic longitudinal tension develop damage through the accumulation and clustering of fibre-breaks, and through fibre-matrix interface debonding growth; these processes lead to a reduction of the material's load-carrying ability with increasing loading cycles, which raises a challenge to predict the fatigue response of composite structures. This paper proposes the first model in the literature to predict the kinetics of fibre-breakage and their effect on the macroscopic response of unidirectional composites under cyclic longitudinal tension. The model couples (i) a statistical hierarchical scaling law to predict fibre failure with (ii) a Paris law to predict interfacial fatigue damage propagating from broken fibres; due to its analytical formulation, the model predicts the response of composite bundles up to virtually any size and for their entire fatigue life in less than one minute. Model predictions for the accumulation and clustering of fibre-breaks show a good correlation with experiments from the literature; the model also predicts that, although the critical cluster size does not vary significantly between static, low-cycle/high-stress fatigue, and high-cycle/low-stress fatigue, the material can withstand the highest amount of softening under high-cycle/low-stress fatigue.
Anthony DB, Bacarreza Nogales O, Shaffer M, et al., Pseudo-ductile failure mechanism introduced into finger jointed thermoplastic PES interleaved CFRC, ECCM18 - 18th European Conference on Composite Materials
Pre-cut unidirectional carbon fibre prepreg composites, with an overlapped finger-joint architecture, were modified through the addition of polyethersulfone (PES) interleaves. The properties arising from these finger-jointed configurations were strongly dependent on the interply overlap region. When the tough thermoplastic interleaves spanned only the central portion of the overlap, a crack arresting failure mechanism was observed in tension. A pronounced plateau region or pseudo-ductile response was shown in conjunction with a strain hardening response after crack arrest. The local strain-to-failure of PES interleaved samples was ~3.2%, an increase of 85% compared to the pre-cut baseline (strain-to-failure 1.6%, pre-cut specimens without interleaves).
Henry J, Pimenta S, 2018, Virtual testing framework for hybrid aligned discontinuous composites, COMPOSITES SCIENCE AND TECHNOLOGY, Vol: 159, Pages: 259-272, ISSN: 0266-3538
The inherent brittleness of conventional high-performance composites can be addressed by the use of discontinuous fibres or hybridisation of fibre-types. In this paper, we propose the first models in the literature to predict the stress-strain curve of hybrid discontinuous composites, with either a brick-and-mortar or an intermingled-fibre microstructure. The models consider a shear-lag stress-transfer between the hybrid reinforcement units, and show that this stress transfer becomes less efficient with hybridisation. The model for intermingled-fibre hybrids also considers stochastic fibre strengths and fibre fragmentation, and can therefore predict a brittle or pseudo-ductile response of hybrid discontinuous composites as observed experimentally, as well as hybrid effects. These models can be used to perform virtual testing and microstructural design of hybrid aligned discontinuous composites.
Henry J, Pimenta S, Bio-inspired non-self-similar hierarchical composites, European conference on composites materials
Finley JM, Henry J, Pimenta S, et al., 2018, The influence of variability and defects on the structural performance of discontinuous composites, Pages: 2419-2425
© 2018 by DEStech Publications, Inc. All rights reserved. Composite materials often feature defects, particularly for composites which feature a complex microstructure, such as aligned discontinuous composites. This study uses an accurate but efficient virtual testing framework, which was used to predict the influence of defects on the structural performance of both hybrid and nonhybrid aligned discontinuous composites. Fibre vacancy defects were found to cause the strongest reduction in material properties, while hybridisation was found to be an effective means to reduce the influence of defects on the structural response.
Finley JM, Yu H, Longana ML, et al., 2017, Exploring the pseudo-ductility of aligned hybrid discontinuous composites using controlled fibre-type arrangements, Composites Part A: Applied Science and Manufacturing, Vol: 107, Pages: 592-606, ISSN: 1359-835X
Pseudo-ductility presents a potential means for preventing catastrophic failure in composite materials; large deformations will prevent brittle fracture and provide warning before final failure. This work explores how the pseudo-ductility and strength of aligned hybrid discontinuous composites can be controlled by manipulating the arrangement of different fibre types. Aligned carbon/glass hybrid specimens with different fibre arrangements are manufactured and tested using a modification to the High Performance Discontinuous Fibre (HiPerDiF) method. Experimental results are complemented by an improved virtual testing framework, which accurately captures the fracture behaviour of a range of hybrid discontinuous composite microstructures. With a randomly intermingled fibre arrangement as a baseline, a 27% increase in strength and a 44% increase in pseudo-ductility can be achieved when low elongation fibres are completely isolated from one-another. Results demonstrate that the HiPerDiF method is the current state-of-the-art for maximising the degree of intermingling and hence the pseudo-ductility of hybrid composites.
Henry J, Pimenta S, 2017, Investigating the potential of hierarchical non-self-similar discontinuous composites, 21st International Conference on Composite Materials (ICCM 2017), Publisher: ICCM
This work investigates the potential and the applicability of combining hierarchical and discontinuous microstructures to overcome the brittleness of man-made composites. Finite Element simulations were developed to model hierarchical “brick-and-mortar” composites, in which each brick of the structure is itself made of an arrangement of bricks at a smaller scale. Then, optimal brick geometrieswere identified, manufactured and tested. The experimental results confirmedthe potential of such microstructures to dissipate energy stably through damage dispersion in the whole material, hence delaying damage localisation, and providing warning before failure. Finally, non-self-similar microstructureswereidentified and optimised to improve furtherthe tensile response of composites. It was found that relaxing the constraints of self-similarity could delay damage localisation even more, and increase both the strength and the damage tolerance of hierarchical discontinuous composites.
Anthony DB, Bacarreza Nogales OR, Shaffer MSP, et al., 2017, Crack arrest in finger jointed thermoplastic interleaved CFRC, 21st International Conference on Composite Materials, Publisher: Chinese Society for Composite Materials
Pre-cut unidirectional carbon fibre prepreg (M21/194/34%/T800S) composites were tested in tension with a 20 mm overlapped finger joint architectures. In between the overlapping finger jointed region the effect of introducing polyethersulfone (PES) interleaves is investigated. Samples with the addition of a thick PES interleave arrested the initial crack which formed at the pre-cut site. The strain-to-failure of the thick PES interleaved samples was over 3.2%, an increase of 85% compared to the baseline samples, and catastrophic failure was delayed in the majority of instances.
Deng X, Kinloch AJ, Pimenta S, et al., 2017, Homogeneous and toughened cellulose epoxy composites, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. Homogeneous and toughened cellulose-epoxy polymers were made by modifying an anhydride-cured epoxy with two green modifiers, microcrystalline cellulose (MCC) and cellulose nanocrystals (CNC). Without silane treatment, the MCC and CNC particles sedimented in the epoxy resin and formed either a gradient polymer or two distinct layers. This problem was resolved by the addition of (3-glycidyloxypropyl)trimethoxysilane (GPTMS) during the three-roll mill process, which was able to act as a coupling agent between the MCC or CNC and the epoxy, to give a modified epoxy containing homogenously dispersed cellulose particles. The addition of MCC or CNC decreased the glass transition temperature of the epoxy, but doubled the fracture energy. By comparison, the addition of 10 wt% of nanosilica only gave a 57% increase in fracture energy. The toughening mechanisms of the MCC-epoxy and CNC-epoxy were identified to be crack deflection, pull-out and debonding of the cellulose particles, which was followed by plastic void growth. The modified Halpin-Tsai model was used to predict the increase in modulus and showed good agreement with the experimental modulus values. Analytical modelling of the fracture energies showed that particle debonding and particle pull-out contributed to the increased toughness, but the main toughening contributions were due to plastic void growth for CNC-epoxy and both plastic void growth and crack deflection for MCC-epoxy. In addition, plain-weave long glass fibre (GF) composite was manufactured with MCC using resin infusion under flexible tooling (RIFT). The interlaminar fracture energy of the composite was measured and it was found that the increase in toughness in the epoxy polymer was not translated to the composite. This was thought to be due to the silane that was used to treat the MCC-epoxy system migrating to the glass fibre surface and improved the fibre-matrix adhesion.
Swolfs Y, Pimenta S, Thionnet A, et al., 2017, A benchmarking exercise for three longitudinal strength models for unidirectional fibre-reinforced composites, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. The longitudinal strength of unidirectional fibre-reinforced composites is one of the most basic strength properties of fibre-reinforced composites. Many different models have been developed, but a systematic and unbiased comparison between those models is not available in the literature. This paper therefore presents a benchmarking exercise that compares three state-of-the-art models: a hierarchical scaling law, a direct numerical simulation method and a multiscale finite element simulation method. The results reveal significant discrepancies between the predictions of the models, which can be explained by the inherent assumptions of each of the models. Experimental results were compared against blind predictions by all three models. This comparison revealed that the basic mechanisms are understood and captured, but that more experimental and modelling work is required to advance our understanding to a higher level.
Pimenta S, Henry J, Finley J, 2017, The effect of randomness at the micro-scale on failure of composites, 21 st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. It is widely accepted that the intrinsic randomness of composite microstructures creates weak regions which may trigger premature failure of the material. This paper uses a virtual testing framework to quantify how failure of aligned discontinuous composites with single or hybrid fibre-types is affected by randomness in three microstructural stochastic variables: fibre-end location, fibre-strength, and fibre-type arrangement. The results show that, by removing randomness at the micro-scale, it should be possible to improve the strength of aligned discontinuous-fibre composites by 20-30%.
Li Y, Pimenta S, Singgih J, et al., 2017, Understanding and modelling variability in modulus and strength of tow-based discontinuous composites, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. Tow-based discontinuous composites (TBDCs) are a growing class of high performance lightweight discontinuous composites, suitable for structural applications in the automotive industry. TBDCs consist of a network of randomly placed and oriented tows; this microstructure allows TBDCs to combine high specific modulus and toughness low manufacturing time and cost. However, this microstructure also leads to highly heterogeneous microstructures and properties in TBDCs, which makes it difficult to predict the mechanical response of these materials, especially in structural components with complex geometries. Therefore, this study aims to quantify the effect of the intrinsic variability of mechanical properties of TBDCs, and to develop a design framework that can be used for structural design with these materials. This study aims to understand the variability of the TBDCs, and to develop a design framework that can be used for structural design with these materials. This is achieved by (i) experimentally quantifying the effect of the intrinsic variability in microstructure on the mechanical properties of TBDCs; (ii) developing analytical models to predict the mechanical properties of TBDCs according to their local microstructure and their variability; and (iii) integrating the distribution of mechanical properties calculated from the analytical models into a finite element environment to simulate the mechanical response of a structure under load. It is found that the variability in TBDCs is so significant that the critical regions in a structure can be shifted to other locations, even with substantial stress concentrations. It is also shown that the analytical models and the FE design framework proposed in this study can be used to optimise the microstructure of TBDCs and to design structures using TBDCs, hence accelerating the design cycle and promoting the application of high-performance com
Finley J, Yu H, Longana M, et al., 2017, Grouping similar fibre types in hybrid discontinuous composite materials, 21 st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. A virtual testing framework and a new manufacturing process were developed to investigate how grouping similar fibre types within a hybrid discontinuous composite affects the strength and pseudo-ductility of the material. Both the experiments and modelling showed that increasing the level of random intermingling prevents large clusters of broken fibres from forming, which maximises the strength and pseudo-ductility of hybrid discontinuous composites.
Pinho ST, Bullegas G, Charrier M, et al., 2017, Improving damage tolerance of carbon fibre laminates via bio-inspired micro-structural design, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. A bio-inspired micro-structure design technique was used to increase the translaminar fracture toughness and the notch strength of Quasi Isotropic carbon fibre laminates. Patterns of laser-engraved micro-cuts were inserted in the micro-structure of the laminate to promote crack deflection and force the formation of large bundle pull-outs during crack propagation. The design of the patterns of micro-cuts was defined following the predictions of a newly developed Finite Fracture Mechanics criterion. The technique allowed to achieve a 20% increase in the laminate notched strength and an 190% increase in the laminate translaminar work of fracture when compared with the un-modified baseline material.
Henry J, Pimenta S, 2017, Modelling hybrid effects on the stiffness of aligned discontinuous composites with hybrid fibre-types, Composites Science and Technology, Vol: 152, Pages: 275-289, ISSN: 0266-3538
Hybrid discontinuous composites offer the possibility to tailor the composite properties for specific applications, improve their manufacturability, and reduce cost by introducing cheaper fibres. However, the mechanical behaviour of hybrid composites often shows hybrid effects which cannot be modelled by the rule-of-mixtures and are therefore challenging to predict and explain. This paper presents models to calculate the Young's modulus of different discontinuous hybrid composites, which is affected by such hybrid effects. The models are based on shear-lag and consider two types of hybrid discontinuous architectures: (i) a deterministic “brick-and-mortar” architecture consisting of perfectly staggered platelets with two different Young's moduli and thicknesses, and (ii) a stochastic architecture of aligned fibres with two different Young's moduli and diameters, with randomly allocated fibre-ends and random or organised intermingling. The models show good agreement with numerical and experimental validations; their results show that hybrid interactions between different types of fibres or platelets reduce the Young's modulus of hybrid discontinuous composites, which justifies the negative hybrid effects observed.
Li Y, Pimenta S, Singgih J, et al., 2017, Experimental investigation of randomly-oriented tow-based discontinuous composites and their equivalent laminates, Composites Part A: Applied Science and Manufacturing, Vol: 102, Pages: 64-75, ISSN: 1359-835X
The equivalent laminate assumption is a commonly-used method to model the random architecture of discontinuous composites, but which has never been validated experimentally. This study aims to verify the equivalent laminate assumption, focusing on tow-based discontinuous composites (TBDCs), which have higher fibre-content and thus improved modulus and strength, compared to conventional discontinuous-fibre composites. This verification was achieved by manufacturing and testing (i) actual TBDCs with randomly oriented tows and (ii) their equivalent laminates (ELs), at two different tow thicknesses. The results show that ELs exhibit the same failure mechanisms as TBDCs, and are similarly weakened by an increase in tow thickness. However, ELs lack the spatial variability in local fibre-content and local tow orientations, which makes ELs stronger than TBDCs. Therefore, the equivalent laminate assumption is suitable for predicting the modulus of discontinuous composites, but cannot predict their strength without considering the local variability in their microstructure.
Kaboglu C, Pimenta S, Morris A, et al., The Influence of different types of core materials on the impact behaviour of sandwich structures, 3rd Global Conference on Materials Science
Pimenta S, 2017, A computationally-efficient hierarchical scaling law to predict damage accumulation in composite fibre-bundles, Composites Science and Technology, Vol: 146, Pages: 210-225, ISSN: 0266-3538
Unidirectional composites under longitudinal tension develop damage through the accumulation and clustering of fibre–breaks, which may lead to catastrophic failure of an entire structure. This paper uses a hierarchical scaling law to predict the kinetics of fibre–breakage and its effect on the stress–strain response of composites under longitudinal tension; due to its analytical formulation based on the statistical analysis of hierarchical fibre–bundles, the scaling law predicts the response of composite bundles up to virtually any size in less than one second. Model predictions for the accumulation and clustering of fibre–breaks are successfully validated against experiments from the literature. These results show that the present model is a much more computationally–efficient alternative to other state–of–the–art models based on Monte–Carlo simulations, without sacrificing the accuracy of predictions when compared against experiments.
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.
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.
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.
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.
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