The unique sensing features of the tilted Fibre Bragg Grating (TFBG) as a single three-parameter optical sensor are demonstrated in this work, to monitor the manufacturing process of composite materials produced using Vacuum Assisted Resin Transfer Moulding (VARTM) process. Each TFBG sensor can measure simultaneously and separately strain, temperature and refractive index (RI) of the material where the optical fibre is embedded. A TFBG embedded in a 2 mm glass-fibre/epoxy composite plate was used to measure the thermomechanical variations induced during the curing process. At the same time, the RI measurements, performed with the same TFBG sensor, can estimate the degree of cure of the resin. The TFBG sensor shows to be a valid and promising technology to improve the state of art of sensing and monitoring in composite material manufacturing.

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The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.@en

The paper tackles the multi-objective optimisation of the cure stage of the Vacuum Assisted Resin Transfer Moulding (VARTM) process to manufacture the root insert of wind turbine blades. The aim of the study is to compare the Pareto front obtained from a pure heat transfer analysis, where temperature overshoot is an objective, with the one achieved when a coupled thermo-mechanical analysis is used, in which spring-in deflection is directly addressed. The optimisation methodology links the finite element solution of a heat transfer and a coupled thermo-mechanical problem of the cure process with a genetic algorithm suitable for multi-objective problems. The paper highlights that whilst minimising overshoot temperature is effective in reducing the spring-in, optimal design points might remain hidden. Furthermore, by showing the evolution of transverse residual stresses the paper suggests that designing cure cycles that introduce compressive stresses before vitrification is beneficial to counteract the subsequent tensile stresses generation.

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The impetus for higher performance, robustness and efficiency in the aerospace, automotive and energy industries has been reflected in more stringent requirements which the composite manufacturing industry needs to comply with. The process design challenges associated with this are significant and can be only partially met by integration of simulation in the design loop. The implementation of numerical optimisation tools is therefore necessary. The development of methodologies linking predictive simulation tools with numerical optimisation techniques is pivotal to identify and therefore develop optimal design conditions that allow full exploitation of the efficiency opportunities in composite manufacturing. Numerical and experimental results concerning the optimisation techniques and methodologies implemented in literature to address the optimisation of thermoset composite manufacturing processes are presented and analysed in this study.

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The present paper investigates the generation of cure induced residual stresses during the cure stage of the Vacuum Assisted Resin Transfer Moulding (VARTM) process for the fabrication of ultra-thick components (i.e. 105 mm) for wind turbine blades manufacturing (i.e. root insert). The viscous-elastic material characterisation of the Airstone 780E epoxy resin mixed with the 785H Hardener has been undertaken and the corresponding coupled thermo-mechanical simulation has been implemented using a commercial FE solver. The finding points out that the level of residual stresses generated during the cure stage leads to a spring-in of about 1.3 cm when Manufacturer Recommended Cure Cycle is applied and that improvements in both spring-in and cure time can be obtained by applying different cure cycles.@en

The present paper addresses the multi-objective optimization of the filling stage of the Resin Infusion manufacturing process. The optimization focuses on the selection of an optimal temperature profile which addresses the tradeoff between filling time and the risk of impeding the flow of resin due to excessive curing. The methodology developed combines a numerical solution of the coupled Darcy’s flow and heat conduction problem with a Genetic Algorithm (GA). The methodology converges successfully to a final Pareto set for the case of a C-stiffener which is 130 mm high, 60 mm wide and lies on a skin 280 mm wide. The results highlight the efficiency opportunities available compared to standard industrial manufacturing practice. Reductions in filling time up to 66% and up to 15% in final degree of cure are achieved compared to standard solutions.@en

The paper deals with the influence of the convection coefficient and laminate thickness on multi-objective optimisation of the vacuum assisted resin transfer moulding cure stage for the manufacturing of wind turbine components. An epoxy resin system widely used in the wind turbine industry has been chemically characterised and the correspondent finite element implementation validated. The optimisation methodology developed links the finite element solution with a genetic algorithm and identifies a set of optimal cure cycles for a range of thicknesses (10–100 mm) able to minimise cure time (t cure ) and the maximum degree of cure gradient developed through thickness (Δα max ) during the cure stage as a measure of quality of the product. The results highlight that, by adding convection coefficient as design parameter of the process, significant benefits could be obtained when insulation is applied at the vacuum bag side for all thicknesses. @en

The paper addresses the multi-objective optimization of the cure process of a Vacuum Assisted Resin Transfer Molding for components ranging from 40 to 100 mm thickness and aims to investigate the effect of thickness on the identification and quantification of a set of optimal cure profiles that minimize temperature overshoot and process time. Optimal cure solutions are sought among three dwells temperature profiles and are compared to the manufacturer’s recommended cure cycle (MRCC). The methodology successfully approximates the efficient fronts for the three different cases under study (40, 70 and 100mm) and points out the efficiency opportunity available compared to MRCC. In the case of 70 and 100 mm thick component temperature overshoot reductions of about 75% are achievable and 67% reduction in process time. The results also suggest a change in the objectives’ landscape for the higher thicknesses in the vertical region of the Pareto.@en

A stochastic multi-objective cure optimisation methodology is developed in this work and applied to the case of thick epoxy/carbon fibre laminates. The methodology takes into account the uncertainty in process parameters and boundary conditions and minimises the mean values and standard deviations of cure time and temperature overshoot. Kriging is utilised to construct a surrogate model of the cure substituting Finite Element (FE) simulation for computational efficiency reasons. The surrogate model is coupled with Monte Carlo and integrated into a stochastic multi-objective optimisation framework based on Genetic Algorithms. The results show a significant reduction of about 40% in temperature overshoot and cure time compared to standard cure profiles. This reduction is accompanied by a reduction in variability by about 20% for both objectives. This highlights the opportunity of replacing conventional cure schedules with optimised profiles achieving significant improvement in both process efficiency and robustness.

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This work reports the development of a methodology for the measurement of thermal conductivity of thermosetting polymers during their cure. The study addresses the reliability and robustness of the method through FEA modeling and testing using a noncuring material with known thermal conductivity. The thermal conductivity and its evolution during the cure has been measured for three widely used aerospace epoxy resins, namely, RTM6, 890RTM, and the XU3508/XB3473 system as function of cure temperature. A constitutive model expressing the dependence of thermal conductivity on the degree of cure and temperature has been established. The device developed here can measure thermal conductivity of epoxy resin with accuracy up to 3%.@en

This paper addresses the multi-objective optimisation of the cure stage of composites manufacture. The optimisation aims to minimise the cure process duration and maximum temperature overshoot within the curing part by selecting an appropriate thermal profile. The methodology developed combines a finite element solution of the heat transfer problem with a Genetic Algorithm. The optimisation algorithm approximates successfully and consistently the Pareto optimal front of the multi-objective problem in a variety of characteristic geometries of varying thickness. The results highlight the efficiency opportunities available in comparison with standard industrial cure profiles. In the case of ultra-thick components improvements of up to 70% in terms of overshoot and 14 h in terms of process time, compared to conventional cure profiles for ultra-thick components, can be achieved. In the case of thick components reduction up to 50% can be achieved in both temperature overshoot and process duration.@en