Publications
269 results found
Molker H, Gutkin R, Pinho ST, et al., 2019, Hot spot Analysis in complex composite material structures, Composite Structures, Vol: 207, Pages: 776-786, ISSN: 1879-1085
In this paper, failure initiation in composite structures due to high out-of-plane load components is predicted. The predictions are based on finite element models built with shell elements, intended for global models. The full 3D stress state is estimated through stress recovery by the extended 2D FEM approach. Failure initiation is predicted with state of the art failure criteria for transversely isotropic composite materials. The approach is validated for a range of geometries with different modelling resolutions. Finally, the methodology is verified on a complex composite structure. With the proposed approach, using shell elements, efficient modelling strategies of large structures can be pursued using hot spot analyses to identify critical locations.
Pascoe JA, Pimenta S, Pinho ST, 2019, TIGR nacre: Damage tolerance through damage diffusion
© 2019 by DEStech Publications, Inc. and American Society for Composites. All rights reserved. We present a new hybrid CFRP concept, aimed at increasing the energy absorption capacity of the material by diffusing damage through-out the laminate. To achieve this, we combine thin-ply CFRP layers, cut into interlocking tiles, with titanium interlayers. Uniaxial tension tests and bending tests on a preliminary design iteration showed that damage initiation and initial propagation were successfully achieved, but damage propagation localized during further loading. Final failure was triggered by strain concentrations in the titanium layers, which caused the titanium failure strain to be locally exceeded. The key to achieving higher laminate ultimate strain is to prevent or ameliorate these strain concentrations in the titanium layers.
Pascoe JA, Pimenta S, Pinho ST, 2019, TIGR nacre: Damage tolerance through damage diffusion
© 2019 by DEStech Publications, Inc. and American Society for Composites. All rights reserved. We present a new hybrid CFRP concept, aimed at increasing the energy absorption capacity of the material by diffusing damage through-out the laminate. To achieve this, we combine thin-ply CFRP layers, cut into interlocking tiles, with titanium interlayers. Uniaxial tension tests and bending tests on a preliminary design iteration showed that damage initiation and initial propagation were successfully achieved, but damage propagation localized during further loading. Final failure was triggered by strain concentrations in the titanium layers, which caused the titanium failure strain to be locally exceeded. The key to achieving higher laminate ultimate strain is to prevent or ameliorate these strain concentrations in the titanium layers.
Pascoe JA, Pimenta S, Pinho ST, 2019, TIGR nacre: Damage tolerance through damage diffusion
© 2019 by DEStech Publications, Inc. and American Society for Composites. All rights reserved. We present a new hybrid CFRP concept, aimed at increasing the energy absorption capacity of the material by diffusing damage through-out the laminate. To achieve this, we combine thin-ply CFRP layers, cut into interlocking tiles, with titanium interlayers. Uniaxial tension tests and bending tests on a preliminary design iteration showed that damage initiation and initial propagation were successfully achieved, but damage propagation localized during further loading. Final failure was triggered by strain concentrations in the titanium layers, which caused the titanium failure strain to be locally exceeded. The key to achieving higher laminate ultimate strain is to prevent or ameliorate these strain concentrations in the titanium layers.
Pascoe JA, Pimenta S, Pinho ST, 2019, TIGR nacre: Damage tolerance through damage diffusion
We present a new hybrid CFRP concept, aimed at increasing the energy absorption capacity of the material by diffusing damage through-out the laminate. To achieve this, we combine thin-ply CFRP layers, cut into interlocking tiles, with titanium interlayers. Uniaxial tension tests and bending tests on a preliminary design iteration showed that damage initiation and initial propagation were successfully achieved, but damage propagation localized during further loading. Final failure was triggered by strain concentrations in the titanium layers, which caused the titanium failure strain to be locally exceeded. The key to achieving higher laminate ultimate strain is to prevent or ameliorate these strain concentrations in the titanium layers.
Häsä R, Pinho ST, 2019, Hybrid composites with a crossed-lamellar microstructure for structural integrity
The damage tolerance of Carbon Fibre Reinforced Polymers (CFRPs) can be enhanced by bioinspired microstructural designs. Amongst molluscs, the crossed-lamellar microstructure is the toughest microstructure, despite consisting almost entirely of brittle ceramic. In this work, we explore original microstructures for CFRPs and metal/CFRP hybrid composites inspired by this crossed-lamellar microstructure. We demonstrate that composites with a crossed-lamellar microstructure exhibit extensive damage diffusion, while preserving their structural integrity up to record large curvatures. This makes them attractive for engineering applications where structural integrity is paramount, such as containment structures.
Mencattelli L, Tang J, Swolfs Y, et al., 2019, Enhancing the damage tolerance of SRPP/CFPP hybrid composites via a bio-inspired design
In this paper, we examine the fracture toughness of a self-reinforced polypropylene/carbon fibre polypropylene cross-ply hybrid laminate and design techniques to enhance two damage tolerance requirements: (i) the energy dissipation capability via promoting sub-critical diffused damage and (ii) the damage tolerance to impact. For this purpose, we micro-engineered the architecture of the composite via tailoring two patterns of discontinuities in the form of laser-cuts across the fibres of the carbon-fibre plies to meet each specific requirement. We performed double edge notched tensile and low velocity impact tests on specimens with engineered microstructures and compared the results against non-engineered specimens. We used the essential work of fracture method to investigate the volumetric energy dissipation capability and the fracture toughness of the hybrid laminates. The DEN-T tests show that hybridising PP tapes with continuous carbon fibres results in a tough material - fracture toughness = 213 kJ/m2. The use of tailored pattern of discontinuities managed to successfully delocalise strain concentrations hence greatly increasing the extent of diffused damage. The activation of highly-dissipative sub-critical failure mechanisms, such as pull-outs of CF bundles, has greatly enhanced the energy dissipation of the microstructure, leading to a 90% increase in the slope of the specific work of fracture with respect to the baseline. Engineering the microstructure of impact samples resulted in a 42.7% increase in energy dissipation at a sub-critical level and a 26.6% delay in critical failure, therefore enhancing the impact damage tolerance of the composite.
Wehrkamp-Richter T, Vieira De Carvalho N, Pinho ST, 2018, Predicting the non-linear mechanical response of triaxial braided composites, Composites Part A: Applied Science and Manufacturing, Vol: 114, Pages: 117-135, ISSN: 1359-835X
In this paper, the non-linear mechanical response of triaxial braided composites under multiple loading conditions was investigated with a meso-scale simulation strategy. Numerical predictions made by three-dimensional finite element unit cells with a realistic internal geometry in two nesting configurations correlated well with experimental stress–strain curves and damage mechanisms. Although the investigated braid topologies exhibited considerable geometric variability, the unit cell modelling approach with a compacted geometry model built from average input parameters was capable of correctly predicting the homogenised constitutive response, localisation, and damage evolution. Further, the mechanical response was predicted under variable uni-axial off-axis load cases and the effect of the textile topology on the ultimate strength of the material was investigated. Aside from providing a valuable insight into how damage propagation is affected by the meso-structure, the predicted stress–strain curves can be used to calibrate macroscopic material models suitable for large-scale crash simulations of textile composites.
Narducci F, Pinho ST, 2018, Interaction between nacre-like CFRP mesolayers and long-fibre interlayers, Composite Structures, Vol: 200, Pages: 921-928, ISSN: 1879-1085
In this paper, a carbon-fibre/epoxy (CF) composite with nacre-inspired tiled micro-structure is designed and synthesised. The aim is to investigate the interaction between the CF discontinuous micro-structure and continuous glass-fibre/epoxy (GF) layers, which are intended to act as crack stoppers, similarly to the organic interlayers that separate layers of ceramic tiles in natural nacre. Firstly, we use a GF skin to trigger unstable failure in nacre-like mesolayers, and show how the damage mode in the latter changes from pull-out to brittle fibre fracture due to the interaction with the GF skin. Secondly, we demonstrate how continuous GF interlayers can succeed in arresting unstable crack propagation in the nacre mesolayers. Furthermore, we show that they can also change the morphology of damage in the nacre, promoting a transition from brittle tile fracture to more damage-tolerant tile pull-out.
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.
Narducci F, Lee K, Pinho ST, 2018, Realising damage-tolerant nacre-inspired CFRP, Journal of the Mechanics and Physics of Solids, Vol: 116, Pages: 391-402, ISSN: 0022-5096
Inthiswork,anacre-inspiredCarbonFibreReinforcedPolymer(CFRP)compositeis designed,synthesisedandtested. Analyticalandnumericalmodelsareusedtodesign a tiled micro-structure, mimicking the staggered arrangement of ceramic platelets in nacreandexploitinggeometricalinterlocksforcrackdeflectionanddamagediffusion. The designed pattern of tiles is then laser-engraved in the laminate plies. In order to increase the damage-spreading capability of the material, a thin layer of poly(lactic acid) (PLA) is film-cast on the interlaminar region, both as a continuous film and as a pattern of fractal-shaped patches. Three-point bending tests show how the nacre-like micro-structure succeeds in deflecting cracks, with damage diffusion being significantly improved by the addition of PLA at the interface between tiles. It is observed that a texture of discontinuous fractal-shaped PLA patches can increase damage diffusion, by promoting the unlocking of tiles whilst preserving the interface strength.
Mencattelli L, Pinho ST, 2018, Low velocity impact and compression after impact of thin-ply CFRP Bouligand structures, 18th European Conference on Composite Materials (ECCM 18), Publisher: ECCM
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 thinply technology has allowed for the first time to explore the effect of very small inter-ply angles, downto 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.
Tagarielli V, Matos MAS, Pinho S, et al., 2018, Predictions of the electro-mechanical response of conductive CNT-polymer composites, Journal of the Mechanics and Physics of Solids, Vol: 114, Pages: 84-96, ISSN: 0022-5096
We present finite element simulations to predict the conductivity, elastic response and strain-sensing capability of conductive composites comprising a polymeric matrix and carbon nanotubes. Realistic representative volume elements (RVE) of the microstructure are generated and both constituents are modelled as linear elastic solids, with resistivity independent of strain; the electrical contact between nanotubes is represented by a new element which accounts for quantum tunnelling effects and captures the sensitivity of conductivity to separation. Monte Carlo simulations are conducted and the sensitivity of the predictions to RVE size is explored. Predictions of modulus and conductivity are found in good agreement with published results. The strain-sensing capability of the material is explored for multiaxial strain states.
Wehrkamp-Richter T, Carvalho N, Pinho ST, 2018, A meso-scale simulation framework for predicting the mechanical response of triaxial braided composites, Composites Part A: Applied Science and Manufacturing, Vol: 107, Pages: 489-506, ISSN: 1359-835X
In this paper, we propose a novel simulation framework for accurately predicting the mechanical response of highly compacted triaxial braided composites using meso-scale finite element models. Unit cells with a realistic internal geometry are generated within an automated simulation work-flow. Local volumetric interpenetrations are removed from a nominal geometry in a fictitious thermal simulation step. A compaction simulation of a single textile layer is performed to the desired target fibre volume fraction while implicitly considering multiple plies in different nesting configurations through periodic boundary conditions. For mechanical simulation, a matrix pocket mesh is created from a reconstruction of the deformed textile. A novel meshing methodology incorporates branching cohesive yarn-to-yarn and yarn-to matrix interfaces for modelling delamination. The framework was validated by detailed comparison with experimental results for three braid architectures. The excellent correlation of the internal geometry and the elastic properties underlines the framework’s potential for future damage modelling.
Narducci F, Lee KY, Pinho ST, 2018, Interface micro-texturing for interlaminar toughness tailoring: a film-casting technique, Composites Science and Technology, Vol: 156, Pages: 203-214, ISSN: 0266-3538
In this work, we developed a film-casting technique to deposit thin (13 μm) layers of poly(lactic acid) (PLA) on the interface of carbon/epoxy prepregs, with the aim of increasing the interlaminar toughness. PLA patches with fractal shape were explored, based on preliminary results showing that the toughening effect increases when PLA is deposited at multiple scales simultaneously. Double Cantilever Beam (DCB) and 4-point End-Notched Flexure (4ENF) tests showed an increase in interlaminar toughness of, respectively, up to 80% for Mode I and 12% for Mode II. This is specially remarkable because the interface thickness is only 13 μm. Moreover, it was demonstrated that this technique can promote interaction between neighbouring layers where PLA has been cast, thus triggering fibre bridging and leading to a further enhancement of toughness.
Narducci F, Pinho ST, 2017, Exploiting nacre-inspired crack deflection mechanisms in CFRP via micro-structural design, Composites Science and Technology, Vol: 153, Pages: 178-189, ISSN: 0266-3538
In this paper, a bio-inspired carbon-fibre/epoxy composite with nacre-like tiled microstructure is designed, synthesised and tested. Analytical models are developed to predict the energy dissipation and crack deflection properties of such composite, and the predictions for the stress-strain response during tile pull-out are validated against direct numerical simulation. Suitable configurations for tile geometry with interlocks are then identified (with dimensions of the order of 0.6 mm) and used for the subsequent prototyping. In-situ three-point bend tests are then carried out in a SEM environment, showing the capability of the interlocking micro-structure in deflecting the crack, avoiding sudden failure in the most highly loaded cross-section of the specimen. The results suggest that further damage diffusion could in principle be achieved by additionally modifying the interface between tiles.
Swolfs Y, Geboes Y, Gorbatikh L, et al., 2017, The importance of translaminar fracture toughness for the penetration impact behaviour of woven carbon/glass hybrid composites, Composites Part A: Applied Science and Manufacturing, Vol: 103, Pages: 1-8, ISSN: 1359-835X
The impact resistance of fibre-reinforced composites is vital in many applications, and can be improved by exploiting synergies in fibre-hybridisation. These effects are however not sufficiently well understood in the literature. Penetration impact tests were hence performed on carbon/glass hybrids, and the results were linked to the flexural behaviour and translaminar fracture toughness. The results revealed large synergetic effects of up to 40% compared to the linear rule-of-mixtures. The results are also the first to reveal that creating a translaminar fracture surface can strongly contribute to the energy absorbed during penetration impact: 56% for an all-carbon fibre composite and 13% for an all-glass fibre composite. These results prove that strategies for maximising the translaminar fracture toughness can also be exploited to maximise the penetration impact resistance of fibre-hybrids. In carbon fibre composites in particular, ply blocking, using larger yarns and introducing micro-cuts should therefore increase the penetration impact resistance.
Häsä R, Pinho ST, 2017, Designing and prototyping a microstructure for CFRP inspired by the strombus gigas shell, 21 st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. Damage tolerance of composite materials can be improved significantly by nature-inspired microstructural design. Among the mollusc family, the Strombus gigas has the toughest shell although it consists almost entirely of brittle aragonite. The toughness of the shell arises from its microstructure. Despite its outstanding damage tolerance, few attempts have been made to exploit such microstructure in synthetic composites, and none using carbon fibre reinforced polymers (CFRP). In this work, we design a Strombus gigas inspired microstructure for CFRP using a parametric FE model, develop a process to synthesise this microstructure in CFRP, and will test it in a SEM environment. The optical observations demonstrate that the designed microstructure was successfully obtained experimentally and with good accuracy, and that the microstructure holds the potential to successfully create diffuse damage.
Narducci F, Pinho ST, Lee KY, 2017, CFRP composite with nacre-inspired micro-structure and fractal-textured interface, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. In this work, a nacre-inspired carbon/epoxy composite is designed, prototyped and tested. A tiled micro-structure, designed via analytical and numerical models, is laser-engraved in the laminate plies, in order to mimic the staggered arrangement of ceramic platelets in nacre. Geometrical interlocks are also exploited to reproduce a nacre-like crack deflection behaviour. In order to increase the damage spreading capability of the composite, the interface between tiles is modified with the addition of a thin layer of PLA via film-casting, both as a continuous film and as discontinuous fractal-shaped patches. Results from 3-point bend tests show that the nacre micro-structure succeeded in deflecting damage. Furthermore, with respect to the pure epoxy interface, the added PLA layer is demonstrated to facilitate the progressive unlocking of tiles, thus increasing damage diffusion without significantly increasing the material thickness.
Tang J, Aslani A, Swolfs Y, et al., 2017, Exploring discontinuities and hybridization to design gradual failure in unidirectional carbon fiber/self-reinforced polypropylene composites, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. Both discontinuous fiber-reinforced composites and hybrid composites show the potential to overcome the stiffness-toughness dilemma that conventional fiber reinforced polymer composites suffer from. To fully utilize the advantages of discontinuities and hybridization, careful design of the configuration of discontinuities is required. In this paper, high-precision fiber cuts with a staggered pattern were introduced in the unidirectional carbon fiber layer of hybrid carbon fiber/self-reinforced polypropylene composites. This aimed to control the failure mechanisms, hence targeting a gradual failure in tension. The effect of the geometrical parameters of the patterns, including cut length, cut fraction and step length, were investigated. The tensile behavior of the hybrids was influenced by a combined effect of these three parameters. The hybrids exhibited three types of the failure behavior: 1) single fracture of the carbon fiber layer followed by single delamination growth, 2) carbon fiber layer fragmentation accompanied by multiple delamination growth and 3) multiple delamination growth without carbon fiber layer fracture. In the regime of carbon fiber layer fragmentation accompanied by multiple delamination growth, a gradual failure with relatively high damage resistance was achieved.
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.
Matos MAS, Tagarielli VL, Pinho ST, 2017, Simulation of the electromechanical repsonse of self-sensing carbon nanotube polymer nanocomposites, 21st International Conference on Composite Materials
© 2017 International Committee on Composite Materials. All rights reserved. A novel finite element approach to simulate the electromechanical properties and strain sensing capabilities of carbon nanotube polymer composites is presented. The models capture the nanoscale tunneling effect and its sensitivity to the imposed strain field. The approach is based on mechanical and electrical simulations of a representative volume element constructed based on measurable statistical descriptors of the microstructure. 2D and 3D approaches are described and resulting homogenized properties compared. Predictions are found in good agreement with previously published data.
Wehrkamp-Richter T, Hinterholz R, Pinho ST, 2017, Damage and failure of triaxial braided composites under multi-axial stress states, COMPOSITES SCIENCE AND TECHNOLOGY, Vol: 150, Pages: 32-44, ISSN: 0266-3538
Damage and failure of triaxial braided composites under multi-axial stress states was investigated. In order to introduce different multi-axial stress states in the material, uni-axial tensile tests were performed at different off-axis orientations. Three braid architectures, comprising braiding angles of 30°, 45° and 60° were each loaded parallel to their axial, transverse and braid yarn direction. Digital image correlation measurement techniques were used to quantify the effects of the textile architecture and its heterogeneity on the strain field, to identify and locate constituent failure mechanisms and to investigate damage initiation and development. In order to identify the driving physical mechanisms behind the material non-linearity, the evolution of the damage variable and the accumulated inelastic strain was quantified using incremental loading/unloading experiments. A high-speed camera was employed in order to study the dynamic nature of catastrophic failure. The triaxial braids within this study exhibited severe non-linearities in the mechanical response before final failure as a result of extensive matrix cracking. While we found the underlying textile architecture to slightly reduce the elastic properties compared to equivalent tape laminates, it functions as a natural crack arresting grid. As a result of this mechanism, braids under certain load conditions were capable of withstanding a higher strain to failure, even if a large portion of the specimen surface was saturated with matrix cracks. The accompanying mechanical behaviour can be desirable in the design of crash absorbing or pseudo-ductile materials. An additional failure mode intrinsic to the textile architecture was encountered for loading in the heavily undulated braid yarn direction. Due to yarn straightening and out-of-plane movements, braided composites were found to fail as a result of large scale delaminations accompanied by progressive fibre bundle pull-out.
Chen BY, Tay TE, Pinho ST, et al., 2017, Modelling delamination migration in angle-ply laminates, Composites Science and Technology, Vol: 142, Pages: 145-155, ISSN: 0266-3538
This paper presents a numerical study of the delamination migration in angle-ply laminates observed in experiments reported in the literature, where the delamination originally propagates along the lower, 0∘/60∘ interface and later migrates onto the upper, 60∘/0∘ interface. The recently-developed Floating Node Method (FNM) is used for modelling this problem. The initiation and propagation of both delamination and matrix cracks are modelled within the FNM elements. Experimentally-observed phenomena such as the numerous kinking attempts and the multiple onset locations of migration are successfully predicted. The effect of load offset on the locations of migration is captured. In addition, this work tries to shed light on the proper use of standard cohesive elements in cases where delamination migration is expected.
St-Pierre L, Martorell NJ, Pinho ST, 2017, Stress redistribution around clusters of broken fibres in a composite, COMPOSITE STRUCTURES, Vol: 168, Pages: 226-233, ISSN: 0263-8223
A key aspect of the longitudinal tensile failure of composites is the stress redistribution that occurs around broken fibres. Work on this topic has focussed mainly on the stress field surrounding a single broken fibre; however, this is an important limitation as unstable failure in carbon fibre bundles occurs when a cluster of about 16 or more broken fibres is formed. Therefore, we have developed a detailed Finite Element (FE) model to investigate how stress redistribution varies with the number of broken fibres in a cluster. The results show that both the recovery length and stress concentration factor increase significantly with increasing number of broken fibres in a cluster. We have also developed an analytical model, suitable to be included in existing or new fibre bundle models, that captures how the recovery length and stress concentration factor vary with the broken cluster size, and validated its predictions against our FE simulations. Finally, we extended our FE model to predict the survival probability of fibre bundles using Monte Carlo simulations, and found that these predictions were in good agreement with experimental and analytical results on microcomposites.
Narducci F, Lee KY, Pinho ST, 2017, Realizing damage-tolerant nacre-inspired CFRP, Pages: 393-399
In this work, a nacre-inspired carbon/epoxy composite is designed, synthesised and tested. Analytical and numerical models are developed to design a tiled microstructure, mimicking the staggered arrangement of ceramic platelets in nacre and exploiting geometrical interlocks for crack deflection and damage diffusion. The designed pattern of tiles is then laser-engraved in the laminate plies. In order to increase the damage-spreading capability of the material, a thin layer of Poly-Lactic Acid (PLA) is film-cast on the interlaminar region. Three-point bend tests show how the nacre micro-structure succeeds in deflecting cracks, with damage diffusion being significantly improved by the addition of PLA at the interface between tiles.
Teixeira R, Pinho ST, Robinson P, 2016, Thickness-dependence of the translaminar fracture toughness: experimental study using thin-ply composites, Composites Part A: Applied Science and Manufacturing, Vol: 90, Pages: 33-44, ISSN: 1359-835X
The concept of translaminar fracture toughness of 0° plies has enabled the development of a considerable number of ply-level numerical models for structural failure of laminated composites. Using thin-ply pre-pregs, this paper demonstrates that this translaminar toughness is not an absolute, but rather in-situ, property and depends strongly on the 0° ply-block thickness, even in situations where delamination and diffuse damage are inhibited. We used two different grades of a thin-ply carbon-epoxy system to produce four different 0° ply-block thicknesses ranging from 0.03 mm to 0.12 mm, and measured the respective translaminar fracture toughness using compact tension tests. SEM and X-ray analysis showed no delamination nor diffuse damage. Yet, the translaminar fracture toughness increased from 46 to 104 kJ/m2 (initiation), and from 49 to 160 kJ/m2 (propagation), for the thickness range above. This finding has significant implications for the development and use of ply-level numerical failure models, for structural design with thin-ply composites, and for the development of thin-ply material systems.
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.
Chen B-Y, Tay TE, Pinho ST, et al., 2016, Modelling the tensile failure of composites with the floating node method, Computer Methods in Applied Mechanics and Engineering, Vol: 308, Pages: 414-442, ISSN: 0045-7825
This paper presents the modelling of tensile failure of composites using novel enriched elements definedbased on the floating node method. An enriched ply element is developed, such that a matrix crack canbe modelled explicitly within its domain. An enriched cohesive element is developed to incorporate theboundaries of matrix cracks on the interface, such that the local stress concentrations on the interface canbe captured. The edge status variable approach allows the automatic propagation of a large number ofmatrix cracks in the mesh. A laminate element is formed, such that a fixed, planar mesh can be used forlaminates of arbitrary layups. The application examples demonstrate that the proposed method is capableof predicting several challenging scenarios of composites tensile failure, such as the large number matrixcracks, grip-to-grip longitudinal splits, widespread delamination, explosive splitting and distributed fibrebreaking in the 0 plies, etc. The complete failure process of ply-blocked composite laminates, up to the finalbreaking of the loosened 0° strips, are here firstly reproduced by modelling.
Gigliotti L, Pinho ST, 2016, Translaminar fracture toughness of NCF composites with multiaxial blankets, Materials & Design, Vol: 94, Pages: 410-416, ISSN: 0261-3069
In this paper, the translaminar initiation fracture toughness of a carbon–epoxy non-crimp fabric (NCF) composite laminate was measured using a compact tension (CT) test. The translaminar fracture toughness of the individual unidirectional (UD) fibre tows was related to that of the NCF laminate and the concept of a homogenised blanket-level translaminar fracture toughness was introduced. Using an approach developed for UD-ply prepreg composites, it is demonstrated that the translaminar fracture toughness of off-axis fibre tows/NCF blankets can be analytically related to that of axially-loaded fibre tows/NCF blankets with a difference between experimentally-measured and predicted values lower than 5%.
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