CERES :: Browsing by Author "Skordos, Alexandros A."

Browsing by Author "Skordos, Alexandros A."

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Open Access
Assessment of the benefits of 3D printing of advanced thermosetting composites using process simulation and numerical optimisation
(Elsevier, 2023-01-18) Struzziero, Giacomo; Barbezat, Michel; Skordos, Alexandros A.
3D printing of continuous fibre reinforced thermosetting matrix composites is set to revolutionise composite manufacturing practice. The potential of curing additively is anticipated to bring significant improvement in terms of increasing process speed, producing geometries that are inaccessible with current processing routes and eliminating detrimental exothermic effects during the process. This study presents a comparison between the curing stage of the 3D printing and standard batch processing for carbon fibre/epoxy components of varying thickness and size. An optimisation methodology links simulation of the cure using Finite Element solver Abaqus with a Genetic Algorithm capable of dealing with multi-objective problems. Optimal cure cycles to minimise both process time and temperature overshoot in 3D printing and batch processing are identified and the optimal trade-offs compared. The results highlight that temperature overshoot reduction up to 85 % is possible and that the intrinsic additive nature of the 3D printing allows eliminating the dependence of temperature overshoot on thicknesses and producing components with thicknesses that are very difficult to manufacture conventionally. A simplified procedure for the estimation of 3D printing process duration is proposed based on the results of finite element simulation. This is used for exploration of the limits of the process with respect to part size and for a generic comparison of process applicability against batch processing. The analysis shows that 3D printing is highly advantageous for small components, is efficient for mid-size components and can – on the basis of its scalability – offer a feasible route for producing large and very large components.
Open Access
Conductive Carbon Nanotube Thermosetting Polyester Nanocomposites
(Cranfield University, 2009) Battisti, Andrea; Partridge, Ivana K.; Skordos, Alexandros A.
A commercial unsaturated polyester resin has been used in combination with commercial multiwalled carbon nanotubes (MWNTs) to study the effects of this nanofiller on the electrical properties of the mix in the liquid state, during the cure and in the solid state. The level of addition of the nanotubes ranged from 0.05 to 0.3 wt%. The dispersion of the filler particles in the matrix was carried out combining triple roll milling, horn sonication and high shear mixing. Qualitative optical and electronic microscopy characterisation supports the development of novel techniques for real-time quantitative assessments of dispersion quality. Fitting of shear dependent viscosity, measured between 0.1 and 100 s-1, to Carreau's model has been shown to provide an indicator of the state of nanotube dispersion in the mixture. Additionally, liquid electrical conductivity measurements offer the option of on-line monitoring, providing a promising tool for process optimisation. The formation of an effective conductive network of nanotubes during the cure was investigated by combining impedance spectroscopy measurements and equivalent circuit modelling with two parallel RC circuit in series with each other. This allows in-situ observation of the key phenomenon responsible for the electrical conductivity of the nanocomposite, namely the filler re-aggregation during cure. Optimisation of dispersion and cure parameters results in a nanocomposite showing conductive behaviour in the solid state, achieving DC conductivity of 0.13 S/m at 0.30 wt% loading. The percolation threshold was estimated to occur at 0.026 wt% filler loading. The conductivity achieved is comparable to state-ofthe-art epoxy thermosetting nanocomposites based on use of carbon nanotubes of equivalent quality. Successful laboratory scale trials demonstrated the suitability of the materials in copper electroplating and resistance heating. An industrial scale up trial of a 40 kg batch was carried out, using the dispersion and the monitoring techniques developed in the study.
Open Access
Conductivity grading in epoxy graphene oxide composites using thermal reduction.
(Cranfield University, 2014-02) Chatzi, Panagiota; Skordos, Alexandros A.
The aim of this project is the development of a method for the production of conductivity graded polymeric materials through the in-situ reduction of Graphene Oxide (GO). These types of composites can be used for the reduction of electrical field concentrations observed in several applications, such as cables, cable joints, microelectronic components and bushings. The evolution of electrical conductivity of GO was monitored during thermal reduction under isothermal and dynamic heating thermal conditions and compared with the kinetics of the reduction reaction as observed using calorimetry. These results were replicated in epoxy/GO composite to prove the feasibility of in-situ reduction and capabilities for controlling this process. In addition, reduction was achieved in epoxy/GO composites using infrared continuous and pulsed laser. It was shown that thermal reduction is possible in the cured epoxy composite at relatively low temperatures up to 26ᴼC. The kinetics of reduction follows a double mechanism which can be represented using a double n-th order process. Electrical measurements showed that conductivity increased significantly up to 0.12 S/m for dry particles and 0.09 S/m for the composite. Heat transfer by conduction and irradiation on specific areas resulted in selective reduction, and a conductive graded material.
Open Access
Consolidation of continuous fibre reinforced composites in additive processes: A review
(Elsevier, 2021-11-03) Struzziero, Giacomo; Barbezat, Michel; Skordos, Alexandros A.
Additive manufacturing of continuous reinforced polymer is currently a focus topic in the composite manufacturing industry as it represents a viable solution to satisfy the requirements of high volume production and automation that could facilitate expanding the use of composite materials and meet sustainability goals. Nevertheless, several challenges need to be addressed to increase the quality standards to match those of parts manufactured by standard composite processing routes. Specifically, consolidation issues appear to be the determining factor which hold the technology back. The present review paper analyses current consolidation techniques utilised in additive processing of composites and identifies the most promising current and future manufacturing technologies capable of complying with stringent sustainability, quality and cost standards.
Open Access
Cure kinetics, glass transition advancement and chemo-rheological modelling of an epoxy vitrimer based on disulphide metathesis
(Elsevier, 2023-11-03) Anagwu, Festus Ifeanyi; Skordos, Alexandros A.
This study develops cure and chemo-rheological models for a vitrimeric system appropriate for use as matrix in continuous fibre composites based on a disulphide-endowed amine/epoxy formulation. The focus is on the processing conditions relevant to liquid composite moulding. Kinetics is investigated using Differential Scanning Calorimetry. An autocatalytic model is developed, simulating the reaction with an average reaction rate error of 5.2 % and degree of cure error of 3 %. The viscosity development is modelled at relatively low degrees of cure, focusing on the liquid moulding infusion window, yielding results with an average relative error of 4.8 %. The glass transition temperature advancement is represented by the Di Benedetto equation, whilst the topological transition temperature of the system is determined using stress relaxation experiments and found to be 149.5 °C. These results set the scene for the development of composites based on the matrix system investigated in this work.
Open Access
Design selection methodology for composite structures
(Elsevier Science B.V., Amsterdam., 2008-01-01T00:00:00Z) Monroy, Aceves C.; Skordos, Alexandros A.; Sutcliffe, Michael P. F.
This paper presents a methodology to help designers select a shortlist or optimum design of composite structure from a large number of alternatives, taking into account conflicting design objectives or constraints (e.g. weight and cost). The methodology is based on creating a database containing results from an exhaustive search of a wide range of possible solutions. These results can be viewed using a commercial software selection package, originally written for materials selection. The designer then has freedom to change the selection criteria and required design constraints, to allow interactive selection of the data. The design methodology is illustrated by way of a case study, the design of a reinforced dogbone specimen.
Open Access
Developing cure kinetics models for interleaf particle toughened epoxies
(Society for the Advancement of Material and Process Engineering (SAMPE), 2016-12-31) Kratz, James; Mesogitis, Tassos; Skordos, Alexandros A.; Hamerton, Ian; Partridge, Ivana K.
In this study, we investigated the cure kinetics behaviour of the commercial Hexply® M21 thermoplastic interleaf epoxy resin system. Dynamic, isothermal, and cure interrupted modulated differential scanning calorimetry (mDSC) tests were used to measure the heat flow of the system, and semi-empirical models were fitted to the data. The cure kinetics model describes the cure rate satisfactorily, under both dynamic heating and isothermal conditions. The glass transition temperature was described using the DiBenedetto equation and showed that heating rate can influence formation of the network; therefore cure schedule must be controlled carefully during processing.
Open Access
Development of leak-before-break filament wound composite structures.
(Cranfield University, 2019-11) Loukodimou, Vasiliki; Skordos, Alexandros A.; Kazilas, Mihalis C.; Yazdani Nezhad, Hamed
A leak-before-break (LBB) concept was developed for composite pressure vessels (CPVs) to achieve a safe, predictable and controllable way of failure preventing the consequences of a catastrophic rupture. Artificial defects were introduced in the structure in prearranged patterns, acting as weak areas and enforcing failure initiation and propagation from these locations. A continuum damage constitutive model was developed through testing and simulation of tensile and compression specimens at [0°]8, specimens under cyclic in-plane shear at [±45°]2s, [+45°]₈ and [±67.5°]₂s as well as out-of-plane specimens at [0°]₁₀ and [0°]₁₂. A methodology was established for the introduction of artificial defects in the composite material for its failure control considering fibre cuts and interfacial defects. The LBB concept was investigated through the simulation of the behaviour of CPVs including defects under internal pressurisation. The assessment of the LBB behaviour was based on the ability to discern between the occurrence of two leakages; the first associated to the leakage phenomenon for pressure relief in the case of over-pressurisation and the second corresponding to ultimate failure. The influence of size and degree of damage induced through the defects was investigated, as well as the use of local reinforcing patches to enhance the LBB behaviour. The most suitable design for the optimal function of the LBB behaviour involves a circular fibre cut defective area of 87.5% fibre cut damage which results in a clear separation between leak and damage by a pressure difference of about 280 bar. The selected case was used for the manufacturing of a closed-end loaded composite pipe to validate the concept. The results of the testing showed that leakage did not occur from the introduced weak points due to manufacturing defects; however, the prediction of the damage initiation from the introduced defects was accurately identified with a difference of 2% compared to the simulation results.
Open Access
Dielectric flow sensing in resin transfer moulding of carbon fibre reinforced composites.
(Maney Publishing, 2000-08-01T00:00:00Z) Skordos, Alexandros A.; Partridge, Ivana K.
An in situ dielectric technique for sensing flow is outlined and shown to be suitable for use in moulding of composites from liquid resins containing continuous conducting fibres. This work builds upon the recent development of a sensor for monitoring flow and cure in thermosetting composites with (non- conductive) glass reinforcements. The linear relationship between the position of the flow front in the mould and the electrical admittance of the sensor system, which is observed in resins with non-conductive reinforcements, was found to extend to resins containing conducting fibres. The performance of the sensor in detecting the position of the flow front was evaluated by comparison with visual observations. The capability of the sensing system to monitor the cure of the resin after mould filling was also investigated. The results demonstrate that the sensor can both locate the flow front accurately and monitor the cure, identifying vitrification in a way that is similar to conventional microdielectrometry. Data from flow measurements are output in complex number format, which offers the potential to produce a method that can be used to evaluate the performance of the sensor system quantitatively.
Open Access
Dielectric monitoring of carbon nanotube network formation in curing thermosetting nanocomposites
(Iop Publishing Ltd, 2009-01-01T00:00:00Z) Battisti, Andrea; Skordos, Alexandros A.; Partridge, Ivana K.
This paper focuses on monitoring of carbon nanotube (CNT) network development during the cure of unsaturated polyester nanocomposites by means of electrical impedance spectroscopy. A phenomenological model of the dielectric response is developed using equivalent circuit analysis. The model comprises two parallel RC elements connected in series, each of them giving rise to a semicircular arc in impedance complex plane plots. An established inverse modelling methodology is utilized for the estimation of the parameters of the corresponding equivalent circuit. This allows a quantification of the evolution of two separate processes corresponding to the two parallel RC elements. The high frequency process, which is attributed to CNT aggregates, shows a monotonic decrease in characteristic time during the cure. In contrast, the low frequency process, which corresponds to inter-aggregate phenomena, shows a more complex behaviour explained by the interplay between conductive network development and the cross-linking of the polymer.
Open Access
A dielectric sensor for measuring flow in resin transfer moulding.
(Iop Publishing Ltd, 2000-01-01T00:00:00Z) Skordos, Alexandros A.; Karkanas, Panagiotis I.; Partridge, Ivana K.
The development, analysis and experimental validation of a novel flow and cure sensor for use in the resin transfer moulding of composites are presented. A linear relationship is established between the flow front position in the mould and electrical admittance measurements gathered using the sensor setup, allowing accurate flow front location. The sensor performance as an indicator of flow front position is evaluated using visual verification. Its efficiency for monitoring of the curing stage is assessed by comparison of the measurements with data obtained from more conventional microdielectrometry. Experimental results demonstrate that the sensor can locate the flow front accurately. The measurement output is in the form of a complex number; this suggests a potential qualitative self-assessment method. The monitoring of the cure process using the new sensor shows performance similar to that of the established microdielectrometric techniques.
Open Access
Drape optimization in woven composites manufacture.
(2005-07-01T00:00:00Z) Skordos, Alexandros A.; Monroy, Aceves C.; Sutcliffe, Michael P. F.
This paper addresses the optimisation of forming in manufacturing of composites. A simplified finite element model of draping is developed and implemented. The model incorporates the non-linear shear response of textiles and wrinkling due to buckling of tows. The model is validated against experimental results and it is concluded that it reproduces successfully the most important features of the process. The simple character of the model results in low computational times that allow its use within an optimisation procedure. A genetic algorithm is used to solve the optimisation problem of minimising the wrinkling in the formed component by selecting a suitable holding force distribution. The effect of regularisation is investigated and the L-curve is used to select a regularisation parameter value. Optimised designs resulting from the inversion procedure have significantly lower wrinkling than uniform holding force profiles, while regularisation allows force gradients to be kept relatively low so that suggested process designs are feasible.
Open Access
Effect of tufting on the response of non crimp fabric composites.
(2007-09-01T00:00:00Z) Colin de Verdiere, Mathieu; Pickett, Anthony K.; Skordos, Alexandros A.; Witzel, Volker
This paper examines the effectiveness of tufting in enhancing the through the thickness properties of non crimped fabrics. The response of tufted and untufted carbon/epoxyn on crimped fabric composites produced using resin infusion is investigated in axial tension and compression and cyclic [±45] loading. The out- of-plane response is studied in mode I and mode II delamination tests. Tufted material has significantly higher delamination resistance and lower in plane properties in tension and compression. Tufted and untufted non crimp fabric composites have similar response in shearin terms of both plastic strain and strength. Optical strain measurements allowed monitoring of the two dimensional damage field incyclic experiments. Damage tends to be localised in untufted specimens, where as it becomes more uniform across the width of specimens with tufts. Material models for untufted and tufted non crimped fabric composites simulating both non-linear in-plane behaviour and delamination damage were developed. Material parameters were estimated from experimental results, and both material submodels were implemented in a commercial explicit finite element code which allows the in corporation of delamination behaviour using cohesive interface elements. A comparison of experiments and finite element model results simulating coupons verified the implementation of the models
Open Access
Electrical and mechanical behaviour of copper tufted CFRP composite joints
(SAMPE, 2021-09-29) Asareh, Mehdi; Pouchias, A.; Zitoun, A.; Yasaee, Mehdi; Kazilas, M.; Skordos, Alexandros A.
Electrical continuity of dissimilar joints controls the current and thermal pathways during lightning strike. Tufting using carbon, glass or Kevlar fibres is a primary to introduce through thickness reinforcement for composite structures and assemblies. Replacing the conventional tuft thread material with metallic conductive wire presents an opportunity for enhancing current dissipation and deal with electrical bottlenecks across dissimilar joints. Simulation of the electro-thermo-mechanical behaviour of joints was carried out to assess the influence of metallic tufting. The finite element solver MSC.Marc was utilised. Mechanical models incorporate continuum damage mechanics (CDM) to capture progressive damage in both composite and aluminium components of the joint. The mechanical models were coupled with electrical and thermal simulations of reference and copper tufted carbon fibre epoxy composite joints to assess both the lightning strike response and mechanical robustness of the assembly as well as the improvements offered by tufting. Validation of the model is based on electrical conduction and temperature measurements alongside delamination tests.
Open Access
Enhanced DC conductivity of low volume-fraction nano-particulate suspensions in silicone and perfluorinated oils.
(Iop Publishing Ltd, 2009-03-01T00:00:00Z) Wilson, Stephen A.; Libor, Zsuzsanna; Skordos, Alexandros A.; Zhang, Qi
The contrasting DC conductivities of several different types of nanoparticles (nickel, barium titanate and magnetite) suspended in both silicone and perfluorinated oils have been measured and compared. Enhanced DC conductivity through interaction between the particles and the fluid has been demonstrated even at quite moderate fields and different types of nanoparticles have been shown to exhibit different behavioural trends. Whilst the DC enhancement is partly related to the concentration (or spatial arrangement) of the particles as expected, there is clear evidence that energy-activated (electric-field activated) processes also play a major role. It can be said that effective medium theories based solely on the electrical properties and volume fractions of the component materials have limited applicability when assessing the DC conductivity of these nanoparticle-fluid combinations at low volume fractions.
Open Access
An evaluation of large diameter through-thickness metallic pins in composites
(Elsevier, 2024-03-24) Neale, Geoffrey; Saaran, Vinodhen; Dahale, Monali; Skordos, Alexandros A.
There is increasing demand for functional through-thickness reinforcement (TTR) in composites using elements whose geometry exceeds limitations of existing TTR methods like tufting, stitching, and z-pinning. Recently, static insertion of large diameter TTR pins into heated prepreg stacks has proven a feasible and robust reinforcement process capable of providing accurate TTR element placement with low insertion forces and lower tow damage compared with existing methods for similar element sizes (>1mm diameter) like post-cure drilling. Local mechanical performance and failure mechanics of these pinned laminates are reported here. Laminates with a single statically inserted pins (1.2, 1.5, and 2.0 mm) can mostly retain their in-plane integrity alongside a local improvement in mode I delamination toughness in carbon fibre-benzoxazine laminates. Tensile strength is mostly unaffected by the pins resulting from delamination suppression, whereas there is up to a doubling of Young’s modulus. Compressive strength is significantly diminished (up to 42 %) in pinned laminates. Interlaminar toughness is improved, and peak toughness is pushed ahead of the crack as pin diameter increases. The lack of significant deterioration in in-plane tensile properties in pinned laminates produced using static insertion can expand the range and forms of materials that can be inserted compared to existing TTR.
Open Access
Evaluation of the behaviour of particulate polymeric coatings in a corrosive environment. Influence of the concentration of metal particles
(Elsevier Science B.V., Amsterdam., 1996-06-01T00:00:00Z) Kouloumbi, N.; Tsangaris, G. M.; Skordos, Alexandros A.; Kyvelidis, S.
In this work pretreated steel specimens were coated by a spinning process with particulate polymeric composites consisting of an epoxy resin (DOW 331) and iron powder. Applied coatings were roughly 70 Î¼m thick and the contained quantity of iron particles was varied (7.5, 15, 30% wt./wt.). The effect of the presence of iron particles in the coatings as well as the influence of their concentration on the evaluation of the coatings' behaviour in a corrosive environment (3.5% NaCI) was studied. Electrochemical impedance spectroscopy, corrosion potential, corrosion current density (Tafel) and dielectric measurements were performed. Minor differences in the anticorrosive behaviour of the coatings were observed irrespective of the iron content in the coating. Effective resistance inhibition action of the composite coatings was diminished with the increase of exposure time to the corrosive environment being in all cases very close to that of the pure epoxy resin coating
Open Access
Fibre grating refractometer sensors for composite process monitoring
(2007-06-17T00:00:00Z) Buggy, Stephen J.; Chehura, Edmon; Skordos, Alexandros A.; Dimopoulos, A.; James, Stephen W.; Partridge, Ivana K.; Tatam, Ralph P.; Osten, W.; Gorecki, C.; Novak, E. L.
Optimum performance from advanced composites requires careful control of the resin matrix during cure. This is to ensure there are no cure induced voids and to minimise the build up of internal stresses. Careful control of the process is also necessary to reduce wastage. Traditional resin inspection techniques are bulk or sample oriented and thus cannot provide data about critical component parts. Optical fibre based sensors however, allow for in-situ monitoring techniques to be deployed in components without effecting their structural integrity. In this work, two fibre optic grating techniques are demonstrated as process monitoring sensors and are compared with a Fresnel refractometric method. The change in refractive index of a resin has previously been used as a means for assessing the degree of cure. The central wavelength of an attenuation band of a long period grating (LPG) was monitored during the cure of a resin. In parallel the spectral resonances of a tilted fibre Bragg grating (FBG) are also monitored. The two techniques are shown to correlate well with the Fresnel based method in both detecting the resin and monitoring the state of cure, indicating the potential of the techniques for online production monitoring
Open Access
Finite element analysis of laminated glass
(Cranfield University, 2009-09) Watson, Chris; Walton, A.; Skordos, Alexandros A.
In the road and rail industries, annealed float glass is combined with a thin polymeric layer to form a 3-ply glass/PVB/glass structure known as laminated glass. Amongst other characteristics, the PVB acts to retain fractured glass fragments, thus minimising injury from flying glass shards. Previous attempts at predicting the behaviour of laminated glass during impact have proven difficult due to complex stress fields caused by the large differences in moduli. The following research made an attempt at modelling laminated glass using the Laminated Glass model of LS-DYNA (MAT_32). Experimental results, used for model validation, displayed a large amount of scatter that may have been due to differences in manufacturing conditions. Numerical prediction results showed the model to be highly sensitive to mesh refinement. The resultant data showed poor correlation with experimental results predicting a much softer response.
Open Access
Fluorine and nitrogen doping of zinc oxide to enhance dielectric storage of PVDF based particulate composites
(Elsevier, 2024-02-20) Chen, Shuyang; Thakur, Vijay Kumar; Skordos, Alexandros A.
Polyvinylidene fluoride (PVDF) based polymer nanocomposites with ceramics as nanofiller have been investigated as a solution for energy storage devices due to their unique and attractive combination of processability and electrical properties. This work assesses two dopants (fluorine and nitrogen) for zinc oxide (ZnO) nanoparticles PVDF matrix composites as a means of improving dielectric properties targeting capacitive storage. Fluorine doping achieves improved performance compared to pure ZnO nanocomposites increasing the decomposition temperature by 15 °C to 463 °C with 15 wt% F-doped ZnO and reducing the weight loss by 4.2 %. The highest dielectric constant that can be achieved with the addition of fluorine is about 70 at room temperature, which is more than 3 times greater than that of pure ZnO nanocomposite. Nitrogen doping also enhances the permittivity of the nanocomposites at ambient temperature but limit enhancement at high temperature due to the lower activation energy.