Imperial College London

ProfessorSilvestrePinho

Faculty of EngineeringDepartment of Aeronautics

Professor in the Mechanics of Composites
 
 
 
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Contact

 

+44 (0)20 7594 5076silvestre.pinho Website

 
 
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Location

 

314City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

245 results found

Yu B, Katafiasz TJ, Nguyen S, Allegri G, Finlayson J, Greenhalgh ES, Pinho ST, Pimenta Set al., 2021, Hygrothermal effects on the translaminar fracture toughness of a highly toughened aerospace CFRP: Experimental characterisation and model prediction, Composites Part A: Applied Science and Manufacturing, Vol: 150, Pages: 1-12, ISSN: 1359-835X

The translaminar fracture toughness and its dependence on the environmental condition are key considerations in designing aerospace-grade composites with a high damage tolerance to severe service conditions in terms of temperature and moisture. The present work characterises and models the hygrothermal effects on the translaminar fracture toughness of an interlaminar toughened aerospace carbon/epoxy composite under six environmental conditions: −55 °C, 23 °C, and 90 °C, for both ‘dry’ (i.e. moisture free) and ‘wet’ (fully moisture-saturated) specimens. Cross-ply compact-tension experiments show that the translaminar fracture toughness increases with the rise of temperature for both dry and wet conditions with the latter exhibiting a much greater increase. A model to predict the effect of moisture and temperature on the translaminar fracture toughness is here proposed and developed. This approach yields good agreement with experimental results, and it allows an improved understanding of the complex synergistic effects of interfacial properties on the overall translaminar toughening mechanisms.

Journal article

Plocher J, Mencattelli L, Narducci F, Pinho Set al., 2021, Learning from nature: Bio-Inspiration for damage-tolerant high-performance fibre-reinforced composites, Composites Science and Technology, Vol: 208, ISSN: 0266-3538

Over millions of years Nature has attained highly optimized structural designs with remarkable toughness, strength, damage resistance and damage tolerance - properties that are so far difficult to combine in artificial high-performance fibre-reinforced polymers (HPFRPs). Recent studies, which have successfully replicated the structures and especially the toughening mechanisms found in flora and fauna, are reviewed in this work. At the core of the manufacturing of damage-tolerant bio-inspired composites, an understanding of the design principles and mechanisms is key. Universal and naturally-inherent design features, such as hierarchical- and organic-inorganic-structures as well as helical or fibrous arrangements of building blocks were found to promote numerous toughening mechanisms. Common to these features, the outstanding ability of diffusing damage at a sub-critical state has been identified as a powerful and effective mechanism to achieve high damage tolerance. Novel manufacturing processes suitable for HPFRP (such as tailored high-precision tape placement, micro-moulding, laser-engraving and additive manufacturing) have recently gained immense traction in the research community. This stems from the achievable and required geometrical complexity for HPFRPs and the replication of subtly balanced interaction between the material constituents. Even though trends in the literature clearly show that a bio-inspired material design philosophy is a successful strategy to design more efficient composite structures with enhanced damage tolerance and mechanical performance, Nature continues to offer new challenging opportunities yet to be explored, which could lead to a new era of HPFRP composites.

Journal article

Katafiasz T, Greenhalgh ES, Allegri G, Pinho ST, Robinson Pet al., 2021, The influence of temperature and moisture on the mode I fracture toughness and associated fracture morphology of a highly toughened aerospace CFRP, Composites Part A: Applied Science and Manufacturing, Vol: 142, ISSN: 1359-835X

This paper addresses the characterisation of the mode I interlaminar fracture toughness of a carbon fibre/epoxy composite material, toughened with thermoplastic particles in the ply interlayers. The characterisation is undertaken at −55 °C, 19 °C, and 90 °C, on both dry and fully moisture saturated coupons. Fractographic observations of the delamination surfaces allows identification of the failure mechanisms. The mode I propagation fracture toughness tested at wet/90 °C exhibits a 176% increase compared to the dry/19 °C specimens, due to enhanced plastic deformation of the interlayers and more prominent fibre bridging. Moisture-saturated coupons tested at −55 °C suffered a 57% reduction of mode I fracture toughness compared to those under dry/19 °C conditions. This is due to the dis-bond and consequent plucking of the thermoplastic particles from the surrounding matrix. This observation points to the fact that wet/cold conditions may represent the worst-case scenario for the interlaminar fracture performance of composite systems toughened with thermoplastic interleaves.

Journal article

Kocaman ES, Chen BY, Pinho ST, 2020, A floating connector element formulation for multi-level modelling of composite structures, Composite Structures, Vol: 251, Pages: 1-13, ISSN: 0263-8223

Design and optimisation of large structures, including the positioning of lower-level components, typically require extensive user involvement and sequential mechanical analysis/optimisation iterations. This paper presents an original method that enables adaptive positioning of lower-level models (such as components) within higher level-models (such as large structures), and that achieves a combined mechanical/optimisation problem for the design of structures with various hierarchical levels (such as the positioning of stiffeners within a wingbox). As the position of the lower-level model evolves, our proposed method does not require re-generating of the geometry, remeshing or modifying the stiffness matrix of the elements corresponding to the various hierarchical levels. Instead, we achieve the adaptive positioning via an original concept that we propose here: Floating Connector (FC) elements. In this paper, we validate the FC elements against reference purely-mechanical solutions, show that they can be combined with gradient-descent method and genetic algorithms, and that they can be applied to optimise the positioning of a stiffener runout taking into account a debonding manufacturing defect.

Journal article

Häsä R, Pinho ST, 2020, Bio-inspired armour: CFRP with scales for perforation resistance, Materials Letters, Vol: 273, Pages: 1-4, ISSN: 0167-577X

This paper proposes a novel biomimetic Carbon Fibre Reinforced Polymer (CFRP) with a microstructure inspired by fish scales. The aim of the novel microstructure is to improve the perforation resistance of CFRP without compromising its flexibility. To this end, we prototype the first ever CFRP laminate with scales and test it in quasi-static indentation on a soft backing material. We compare the mechanical behaviour of the CFRP with scales to two baseline configurations with conventional lay-ups. The results presented in this paper suggest that the CFRP with scales significantly outperforms the two baseline configurations in terms of force and displacement at penetration, while being flexible. This makes CFRPs with scales an attractive alternative for applications where perforation resistance is paramount, such as body armour against low velocity strikes.

Journal article

da Costa ROSS, Pinho ST, 2020, A novel formulation for the explicit discretisation of evolving boundaries with application to topology optimisation, Computer Methods in Applied Mechanics and Engineering, Vol: 367, Pages: 1-30, ISSN: 0045-7825

Evolving boundaries are an intrinsic part of many physical processes and numerical methods. Most efforts to model evolving boundaries rely on implicit schemes, such as the level-set method (LSM). LSM provides the means to efficiently model the evolution of a boundary, but lacks the ability to transmit information or provide information directly at the boundary. Explicit alternatives based on remeshing or partial-remeshing are often computationally expensive and inherently complex to implement. This work proposes a solution to this dichotomy: a novel finite element method (FEM) based formulation capable of explicitly discretising moving boundaries in an accurate and numerically-efficient way. It couples the floating node method (FNM) with LSM for the first time, which yield a methodology suitable for implementation as user-element in a generic FEM package. The explicitly discretised boundary allows for a new velocity-extension methodology, and a new LSM-reinitialisation procedure, which show benefits in accuracy and efficiency. The potential of this formulation is showcased within topology optimisation, showing greater geometrical accuracy and improvements in the optimum solution attained when compared to implicit methods.

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

Mencattelli L, Pinho ST, 2020, Herringbone-Bouligand CFRP structures: A new tailorable damage-tolerant solution for damage containment and reduced delaminations, Composites Science and Technology, Vol: 190, Pages: 1-13, ISSN: 0266-3538

In this work, we design, prototype, test and analyse the first high-performance Herringbone-Bouligand microstructure (with Carbon Fibre Reinforced Plastic (CFRP)) inspired to the high-impact-resistant mantis shrimp's dactyl club. To this end, we devised the first prototyping procedure to manufacture point-by-point tailorable Herringbone-Bouligand CFRP microstructures; this was based on the micro-moulding of uncured CFRP prepreg, and led to mimicking features of the club microstructure never achieved before with CFRPs. We investigated the damage tolerance of the prototyped Herringbone-Bouligand CFRP laminates, compared against ‘classical’ Bouligand CFRP laminates, using quasi-static indentation tests. Our test results show that the Herringbone-Bouligand microstructure resulted in delayed onset of delaminations, reduced in-plane spreading of damage, increased energy dissipation capability, and in the containment of damage within the tailored Herringbone-Bouligand region. We conclude that Herringbone-Bouligand CFRP microstructures offer an excellent tailorable damage-tolerant solution with great potential for composite applications where resistance to through-the-thickness loads is paramount.

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

Albuquerque Da Silva Matos M, Tagarielli V, Pinho S, 2020, On the electrical conductivity of composites with a polymeric matrix and a non-uniform concentration of carbon nanotubes, Composites Science and Technology, Vol: 188, ISSN: 0266-3538

We present a multiscale modelling approach to explore the effects of a non-uniform concentration of carbon nanotubes (CNTs) on the electrical conductivity of CNT-polymer composites. Realistic three-dimensional representative volume elements (RVEs) are generated from a two-dimensional CNT concentration map, obtained via microscopy techniques. The RVEs capture the measured probability density function of the CNT concentration and include a length-scale to represent the details of the spatial distribution of the concentration. The homogenized conductivity of the RVEs is computed via multiscale FE analyses for different values of such length-scale, and it is compared to measurements. The modelling strategy is then used to explore the effects of the microstructural features of these materials on their electrical conductivity.

Journal article

Mencattelli L, Pinho ST, 2020, Ultra-thin-ply CFRP Bouligand bio-inspired structures with enhanced load-bearing capacity, delayed catastrophic failure and high energy dissipation capability, Composites Part A: Applied Science and Manufacturing, Vol: 129, Pages: 1-15, ISSN: 1359-835X

In this work, we demonstrate for the first time that the inherent low performance to out-of-plane loading of thin-ply Carbon Fibre Reinforced Plastics (CFRPs) can be overcome with tailored bio-inspired Bouligand microstructures. To this end, we designed, manufactured ultra-thin-ply CFRP Bouligand laminates and conducted an original study which combines full-penetration quasi-static indentation tests, in-situ three-point bending tests conducted under a SEM and detailed analytical modelling. We investigated a wide range of mismatch (pitch) angles [2.5°, 5°, 10°, 20°, 45°], showing that decreasing pitch angles simultaneously achieved a larger (i) load-bearing capability, (ii) delay in catastrophic failure and (iii) total dissipated energy. We then investigated the role of the pitch angle on the activation of the highly dissipative sub-critical failure mechanisms responsible for the high mechanical performances achieved by small pitch angles laminates. Our investigation clearly shows that, with ultra-thin-ply CFRP, smaller pitch angles achieve higher damage tolerance and structural integrity.

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

Pinho ST, Narducci F, Lee KY, 2020, Damage-tolerant nacre-inspired CFRP

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. In this paper, a carbon/epoxy composite with nacre-inspired micro-structure is designed, synthesised and tested. A nacre-like discontinuous micro-structure of interlocking tiles is designed via original analytical and numerical models, and then laser-engraved in the laminate plies. Firstly, three-point bending (3PB) tests are carried out on the nacre-like carbon/epoxy composite, demonstrating its capability in deflecting cracks and avoiding localised failure in the specimen, with some amount of damage diffusion. Subsequently, the laminate interfaces are toughened by film-casting thin (0.3 µm) patches of poly(lactic acid) (PLA), in order to promote a more extensive pull-out of tiles and increase the damage-diffusion capability of the material. Finally, continuous layers of glass-fibre/epoxy, similar to the thick protein interlayers that separate layers of ceramic tiles in real nacre, are introduced in the laminate, in order to act as crack stoppers in case of unstable crack propagation in the micro-structure.

Conference paper

Mencattelli L, Pinho ST, 2020, Low velocity impact and compression after impact of thin-ply CFRP Bouligand structures

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. In this work, several bio-inspired thin-ply CFRP laminates mimicking the helicoidal architecture of the mantis shrimp's dactyl club periodic region have been modelled and tested under low velocity impact (LVI) and compression after impact (CAI), investigating the effect of the inter-ply angle (pitch angle) on the mechanical response and damage characterization of the biomimetic laminate. The use of thin-ply technology has allowed for the first time to explore the effect of very small inter-ply angles, down to 2.5°, better mimicking the microstructure of the dactyl club and achieving failure mechanisms akin those observed in the natural microstructure. Tests conducted for a wide range of pitch angles (2.5°, 5°, 10°, 20°, 45°) show that, by decreasing the pitch angle, is it possible to better mimic the failure mechanisms observed in the biological microstructure.

Conference paper

Pinho ST, Narducci F, Lee KY, 2020, Damage-tolerant nacre-inspired CFRP

In this paper, a carbon/epoxy composite with nacre-inspired micro-structure is designed, synthesised and tested. A nacre-like discontinuous micro-structure of interlocking tiles is designed via original analytical and numerical models, and then laser-engraved in the laminate plies. Firstly, three-point bending (3PB) tests are carried out on the nacre-like carbon/epoxy composite, demonstrating its capability in deflecting cracks and avoiding localised failure in the specimen, with some amount of damage diffusion. Subsequently, the laminate interfaces are toughened by film-casting thin (0.3 µm) patches of poly(lactic acid) (PLA), in order to promote a more extensive pull-out of tiles and increase the damage-diffusion capability of the material. Finally, continuous layers of glass-fibre/epoxy, similar to the thick protein interlayers that separate layers of ceramic tiles in real nacre, are introduced in the laminate, in order to act as crack stoppers in case of unstable crack propagation in the micro-structure.

Conference paper

Pinho ST, Narducci F, Lee KY, 2020, Damage-tolerant nacre-inspired CFRP

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. In this paper, a carbon/epoxy composite with nacre-inspired micro-structure is designed, synthesised and tested. A nacre-like discontinuous micro-structure of interlocking tiles is designed via original analytical and numerical models, and then laser-engraved in the laminate plies. Firstly, three-point bending (3PB) tests are carried out on the nacre-like carbon/epoxy composite, demonstrating its capability in deflecting cracks and avoiding localised failure in the specimen, with some amount of damage diffusion. Subsequently, the laminate interfaces are toughened by film-casting thin (0.3 µm) patches of poly(lactic acid) (PLA), in order to promote a more extensive pull-out of tiles and increase the damage-diffusion capability of the material. Finally, continuous layers of glass-fibre/epoxy, similar to the thick protein interlayers that separate layers of ceramic tiles in real nacre, are introduced in the laminate, in order to act as crack stoppers in case of unstable crack propagation in the micro-structure.

Conference paper

Häsä R, Pinho ST, 2020, Improving the damage tolerance of CFRP using a biomimetic crossed-lamellar microstructure

The damage tolerance of composites can be enhanced by bio-inspired microstructural designs. The crossed-lamellar microstructure is the toughest microstructure found in molluscs, despite consisting almost entirely of brittle ceramic. In this work, we investigate microstructures for CFRPs inspired by the crossed-lamellar microstructure in order to improve the damage diffusion capability of the former. This includes exploring two different prototyping procedures with different interface properties. The results indicate that the investigated configurations have the potential to significantly increase the damage dissipation capability of CFRP. The microstructures diffuse damage in a stable manner while preserving their structural integrity up to large curvatures under bending load. We demonstrate that composites with crossed-lamellar microstructures have very attractive properties in terms of damage diffusion.

Conference paper

Pascoe JA, Pimenta S, Pinho ST, 2020, Interlocking thin-ply reinforcements for the improvement of CAI strength

A new reinforcement concept for improving compression after impact strength of carbon fibre reinforced polymers is explored. The concept consists of manufacturing interlocked reinforcement units from thin-ply prepreg, which are inserted at the interlaminar interfaces within a regular ply-thickness laminate. Compression after impact tests following ASTM standards were performed on both reinforced and baseline specimens. The reinforced specimens showed a 11.4 % reduction in post-impact delamination area. The strength results will be presented at the conference.

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

Häsä R, Pinho ST, 2020, Improving the damage tolerance of CFRP using a biomimetic crossed-lamellar microstructure

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The damage tolerance of composites can be enhanced by bio-inspired microstructural designs. The crossed-lamellar microstructure is the toughest microstructure found in molluscs, despite consisting almost entirely of brittle ceramic. In this work, we investigate microstructures for CFRPs inspired by the crossed-lamellar microstructure in order to improve the damage diffusion capability of the former. This includes exploring two different prototyping procedures with different interface properties. The results indicate that the investigated configurations have the potential to significantly increase the damage dissipation capability of CFRP. The microstructures diffuse damage in a stable manner while preserving their structural integrity up to large curvatures under bending load. We demonstrate that composites with crossed-lamellar microstructures have very attractive properties in terms of damage diffusion.

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

Mencattelli L, Pinho ST, 2020, Low velocity impact and compression after impact of thin-ply CFRP Bouligand structures

In this work, several bio-inspired thin-ply CFRP laminates mimicking the helicoidal architecture of the mantis shrimp's dactyl club periodic region have been modelled and tested under low velocity impact (LVI) and compression after impact (CAI), investigating the effect of the inter-ply angle (pitch angle) on the mechanical response and damage characterization of the biomimetic laminate. The use of thin-ply technology has allowed for the first time to explore the effect of very small inter-ply angles, down to 2.5°, better mimicking the microstructure of the dactyl club and achieving failure mechanisms akin those observed in the natural microstructure. Tests conducted for a wide range of pitch angles (2.5°, 5°, 10°, 20°, 45°) show that, by decreasing the pitch angle, is it possible to better mimic the failure mechanisms observed in the biological microstructure.

Conference paper

Tang J, Swolfs Y, Aslani A, Mencattelli L, Bullegas G, Pinho ST, Lomov SV, Gorbatikh Let al., 2019, Engineering tensile behavior of hybrid carbon fiber/self-reinforced polypropylene composites by bio-inspired fiber discontinuities, Composites Part B: Engineering, Vol: 178, ISSN: 1359-8368

Carbon fiber layer failure is vital for the tensile behavior of interlayer hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Introducing cuts, termed here as discontinuities, into the carbon layer is a promising way to tailor its failure behavior. Inspired by structural features of biological composites, we designed and produced hybrid composites with hierarchical and polygonal arrangements of discontinuities in the carbon layer. Increasing the number of levels in the hierarchical patterns delayed the onset of carbon layer failure, hence improving the damage resistance of the hybrid composites. A progressive carbon layer failure was achieved with the polygonal patterns by creating a transition from fiber bundle pull-out to fiber bundle fracture. Spreading the polygonal patterns throughout the specimen resulted in a unique diffused delamination distribution that has not been reported in the literature. Pseudo-ductile behavior was achieved by creating dispersed fiber bundle pull-out with the fully dispersed polygonal patterns. The resulting hybrid CF/SRPP composites demonstrated a rare combination of stiffness (10 GPa) and ductility (~16% failure strain) with a pseudo-ductile strain over 14%. This paper delivers and proves the concept of utilizing discontinuities to engineer the tensile behavior of hybrid composites.

Journal article

Häsä R, Pinho ST, 2019, A three-level hybrid metal/in-plane-CFRP/crossed-lamellar microstructure concept for containment applications, Composites Part A: Applied Science and Manufacturing, Vol: 126, ISSN: 1359-835X

This paper proposes a novel hybrid metal/Carbon Fibre Reinforced Polymer (CFRP) microstructure which includes a bio-mimetic crossed-lamellar CFRP layer (with out-of-plane fibres) as well as traditional CFRP layers (with in-plane fibres). We show that this microstructure is capable of preserving its structural integrity due to the crossed-lamellar and metal layers while the in-plane fibres provides it stiffness and strength. We studied numerically and experimentally a metal hybrid/crossed lamellar block in isolation, both with aluminium and titanium for the metal. We then proceeded to create the full microstructure, including a quasi-isotropic block. Our results show that these structures can withstand very large curvatures in a pseudo-ductile way. This makes them attractive for engineering applications where structural integrity is paramount, such as for containment structures.

Journal article

Canturri C, Greenhalgh ES, Asp LE, Pinho STet al., 2019, Fractographic study to characterise the interaction between intralaminar and interlaminar fracture from embedded defects under compression loading, Composites Part A: Applied Science and Manufacturing, Vol: 125, ISSN: 1359-835X

This paper describes the fractographic observations from the study of embedded defects subject to compression. The fractographic observations aim to characterise the interaction between intralaminar and interlaminar fracture and to understand their role in the delamination growth and the delamination migration. The influence of the stacking sequence orientation on the damage modes is studied in eight different configurations. A detailed fractographic study led to the identification of the different failure modes and failure sequence. It was also possible to establish the stacking sequences more prone to delamination migration and the failure modes more critical for damage tolerance.

Journal article

Tang J, Aslani A, Swolfs Y, Bullegas G, Pinho ST, Lomov SV, Gorbatikh Let al., 2019, Staggered ply discontinuities for tailoring the tensile behavior of hybrid carbon fiber/self-reinforced polypropylene composites: A study of pattern parameters, Composites Part A: Applied Science and Manufacturing, Vol: 125, Pages: 1-13, ISSN: 1359-835X

This work explores the potential of discontinuities combined with hybridization in controlling the failure mechanisms of composite materials. Laser cuts, termed as discontinuities, with a predefined staggered pattern were introduced into the carbon layer of hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Three geometrical parameters were used to define the staggered pattern: cut fraction, cut length and step length. The effects of these three parameters on the failure mechanisms of the hybrid CF/SRPP composites under tensile loading were investigated. The three geometrical parameters can be successfully used to: (1) trigger more diffused damage; (2) delay onset of carbon layer fracture; and (3) promote a transition between fiber bundle pull-out and fiber bundle fracture. Comparing to hybrids with continuous fibers, hybrids with the staggered ply discontinuities exhibited a rich diversity of failure mechanisms. The enrichment of failure mechanisms is beneficial for expanding the design space for tailoring the mechanical response of interlayer hybrid composites.

Journal article

Mencattelli L, Pinho ST, 2019, Realising bio-inspired impact damage-tolerant thin-ply CFRP Bouligand structures via promoting diffused sub-critical helicoidal damage, Composites Science and Technology, Vol: 182, Pages: 1-13, ISSN: 0266-3538

In this work, we manufactured bio-inspired thin-ply Carbon Fibre Reinforced Plastic (CFRP) laminates, mimicking the helicoidal architecture of the mantis shrimp's dactyl club periodic region, with the smallest inter-ply (pitch) angle in the literature (2.5∘), thus better mimicking the actual micro-structure of the dactyl club. We conducted Low Velocity Impact (LVI) tests on a wide range of pitch angles (2.5∘, 5∘, 10∘, 20∘, 45∘), thus demonstrating that decreasing the pitch angle leads to a progressively smoother double helicoidal evolution of damage, reduces delamination areas, diffuses sub-critical damage, and enhances damage tolerance. We then conducted Compression After Impact (CAI) tests, thereby demonstrating that the residual strength and failure strain are preserved as the pitch angle is reduced, even though there is a steep decrease in the proportion of 0∘-plies (plies aligned with the loading direction) as the pitch angle decreases. Via detailed modelling, we then developed and proposed an explanation for why very small pitch angles are required to achieve the beneficial damage mechanisms exhibited by biological Bouligand structures.

Journal article

Häsä R, Pinho ST, 2019, A novel aluminium/CFRP hybrid composite with a bio-inspired crossed-lamellar microstructure for preservation of structural integrity, Composites Science and Technology, Vol: 182, ISSN: 0266-3538

In this paper, we demonstrate that a novel hybrid composite of aluminium and Carbon Fibre Reinforced Polymer (CFRP) with a microstructure inspired by a biological crossed-lamellar microstructure is an attractive alternative for applications where structural integrity is paramount. Composites with such microstructure are prototyped and tested using both standard and thin-ply CFRP prepreg. Three-point bend tests are carried out in an SEM environment, showing extensive diffuse damage in the CFRP and yielding in the aluminium. This is the first hybrid crossed-lamellar-inspired microstructure in the literature and the results demonstrate that this novel microstructure can be loaded up to record large curvatures (in comparison with other CFRPs and hybrid CFRPs) while retaining its structural integrity and dissipating energy under stable conditions.

Journal article

Swolfs Y, Pinho S, 2019, 3D printed continuous fibre-reinforced composites: bio-inspired microstructures for improving the translaminar fracture toughness, Composites Science and Technology, Vol: 182, Pages: 1-8, ISSN: 0266-3538

Translaminar fracture toughness is a vital property governing the notch sensitivity and damage tolerance of composites. Nature has shown that incorporating material transitions can increase toughness significantly. This work presents finite element models demonstrating that such transitions can indeed increase the translaminar fracture toughness. The designed microstructures were then 3D printed using continuous glass and carbon fibres. The specimens consisted primarily of glass fibres, but with local carbon fibre strips. A new compact tension specimen with a side groove was designed to ensure proper failure. When the strips were sufficiently large, toughness improvements of 20–60% were found after the crack had grown through the strips. These results reveal a powerful strategy for locally increasing the toughness in areas where it is needed the most.

Journal article

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.

Journal article

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