Browsing by Author "Maistros, G. M."

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    Dielectric monitoring during the cure of epoxy resin blends
    (Cranfield University, 1991-10) Maistros, G. M.; Partridge, Ivana K.; Block, H.
    Dielectric monitoring and supporting techniques (differential Scanning calorimetry, infra-red spectrosoopy, viscometry, dynamic mechanical thermal analysis and light transmittance) were used to study the isothermal cure reaction of the CTBN rubber modified DGEBA resin/amine hardener blends. The neat system was also examined for the required knowledge of the matrix properties. The complexity of the cure kinetics was demonstrated by the use of a rapid technique for kinetic parameters evaluation. The utility of the dielectric cure monitoring is focused at the observation of evidence o phase separation, gelation and vitrification. The phase separatlon which the blends underwent during the cure was detected by the dielectric »monitoring through a permittivity increase at the low frequency response. The onset of the rapid viscosity increase leading to gelation was also indicated by the sharp decrease o the dielectric constant atlhigh frequencies. The frequency dependence of the times reach the dielectric loss peaks was used to predict successfully the vitrification times during the isothermal reactions o the blends. The in-situ nature o the technique and the basic understanding o the features appearing in the dielectric signal during the cure reaction provide the basis for the use of dielectric monitoring in the process of composite materials, manufacture.
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    Materials modelling and process simulation of the pultrusion of curved parts
    (Elsevier, 2021-02-11) Struzziero, Giacomo; Maistros, G. M.; Hartley, J.; Skordos, Alexandros A.
    The present paper addresses the simulation of a concept for the manufacturing of aerospace quality carbon/epoxy composite curved parts using pultrusion. In this approach, the part is first partially cured in a pre-former followed by final curing in a curved post-former. An aerospace epoxy resin system has been fully characterised and the corresponding constitutive material models, incorporating dependence on both temperature and degree of cure, developed. A 3D Finite Element model of the process, to manufacture a T-stiffener, involving impregnation, curing and forming of the curvature was developed and implemented. The simulation results show that a degree of cure of around 62% -close to the gelation point of the resin system considered - at the exit of the pre-former stage is appropriate for the success of the subsequent stage. In the post-former the cure is completed reaching a final degree of cure of about 87%. The stresses generated in post-forming reach a maximum of 54 MPa in compression in the transverse direction and of 200 MPa in tension in the fibre direction showing that the process is feasible without inducing defects linked to micro buckling or rupture.
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    Optimisation of an in-process lineal dielectric sensor for liquid moulding of carbon fibre composites
    (Elsevier, 2020-11-04) Mesogitis, T. S.; Maistros, G. M.; Asareh, M.; Lira, C.; Skordos, Alexandros A.
    A dielectric sensor appropriate for process monitoring of carbon fibre composites manufacturing has been optimised and implemented in Resin Transfer Moulding (RTM). The sensor comprises a pair of twisted insulated copper wires and can be adapted to monitor both flow and cure. To simulate the dielectric response of the sensor, an electric field model was developed. The model was coupled with a multi-objective optimisation genetic algorithm to optimise the sensor design. The optimisation showed that increasing wire radius and decreasing coating thickness increases sensor sensitivity. Different sensor designs were implemented and used in a series of RTM trials to validate the technology in industrial conditions. The sensor operated successfully at pressures up to 7 bar and temperatures up to 180°C. A low diameter sensor using copper wire coated with polyimide showed the best response monitoring flow with an accuracy of 95%, whilst also following the cure and identifying vitrification