Simulation of bleeder flow and curing of thick composites with pressure and temperature dependent properties

A two-dimensional transient heat transfer, one-dimensional compaction and two-dimensional resin flow analysis of a thick laminated composites fabrication assembly including the bleeders and vacuum bagging is carried out in a fully coupled manner. Resin distribution within the laminate and the bleeders is controlled by their respective compaction behavior as well as permeability of the fiber network. Compaction behavior of dry bleeders is obtained from compression experiments carried out in-house and the derived relevant empirical parameters are used in the numerical simulation. The variations in the resin volume fraction within the laminate due to the resin outflow to the surrounding bleeder were tracked and updated as a function of time in the cure simulation. Four case studies were performed with temperature dependent as well as independent resin properties and pressure dependent and also pressure independent resin volume fractions. These simulations were carried out to understand the behavior of resin flow through the bleeder and its impact on the local variations in the resin content of the laminate. The results obtained show a significant disparity in the thermal overshoot at the center of the laminate and the pressure distribution within the laminate and the bleeder, when a complete coupling and the parametric changes as in the real situation are ignored in numerical simulation. It is shown that the complete coupling of various real-time phenomena helps in accurate prediction of temperature, pressure, resin viscosity, bleeder and fiber compaction, resin flow and associated changes in fiber volume content, and degree of cure distributions for thick composites as they are cured

Void growth mitigation in high heating rate Out-of-Autoclave processing of composites

Increased pressure on the transport industry to reduce greenhouse gas emissions has hastened the adoption of high performance composites, particularly in the aerospace industry where the value of weight saving is very high. However, the current method of choice for manufacturing high performance composites (autoclave processing) is not cost effective for processing large (greater than 5m2) structural composite components. Developments in Out-of-Autoclave (OoA) prepreg systems have facilitated the use of vacuum only consolidation pressure to process laminates with autoclave level mechanical properties. However, owing to the low consolidation pressure, the process is heavily dependent on de-bulk quality and low cure temperatures; leading to reduced margin for error as well as long cycle times. In parallel, developments in high heating rate OoA processes have been shown to enable short cure cycle times and autoclave-level mechanical properties; albeit with a high tendency towards porosity. To date, studies on high heating rate OoA processing have been limited and the processes are not well understood. The main objectives of this self-funded study were to understand the mechanism of void growth mitigation in high heating rate OoA processes and to study the feasibility of achieving further reduction in cycle time and cost, whilst maintaining high mechanical properties. The primary mechanism of void growth was identified and an analytical model was used to predict the propensity for void growth during a given cure cycle. The model outcome highlighted a window within the cure cycle during which void growth takes place. It was hypothesised that a reduced time to resin gelation in high heating rate processes can reduce the window for void growth, leading to lower laminate porosity. A novel high heating rate pressurised tooling system (the Pressure Tool) was developed to process laminates at 15oC/min combined with the application of up to 7 Bar hydrostatic pressure. The Pressure Tool was used to verify the hypothesis that reduction in size of the window for void growth, facilitated by high heating rate, can lead to lower laminate porosity. Good agreement was observed between the model outcome and the experimental results. Studies have claimed that the reduction in resin minimise viscosity due to high heating rate can lead to gains in mechanical properties; sometimes even higher than that of autoclave cured laminates. OoA prepregs cured using up to 15oC/min heating combined with up to 3 Bar hydrostatic pressure did not result in the claimed additional gain in mechanical properties. The study confirmed earlier suggestions that additional factors such as void geometry and location within the laminate have to be taken into consideration. The final part of this thesis addresses the physical limitations to high heating rate processes; such as, the effect of tooling material, process ancillaries, laminate thickness and resin kinematics on reducing cure cycle time. The poor thermal characteristics of commonly used process ancillaries limit the dissipation of energy released by the laminate during cure. Due to which, laminate core temperature can exceed by up to 5oC, even if the laminate is processed on a highly conductive tooling material. The optimum tooling material to achieve reductions in cure cycle time whilst minimising laminate core thermal overshoot was found to have a combination of high thermal conductivity and low thermal mass. However, currently used tooling systems are not optimum for achieving further reductions in cycle time, due to unfavourable combination of thermal mass and thermal conductivity. Furthermore, the high reactivity of current resin systems and the inherently poor thermal conductivity of the polymer matrix limits the gains in cure cycle times that can be achieved

Etude multi-échelle du couplage matériau-procédé pour l'identification et la modélisation des variabilités au sein d'une structure composite

Une des problématiques liées à l'utilisation des matériaux composites dans les structures tient à la difficulté de prévoir l'effet des variabilités inhérentes à ce type de matériau sur le comportement mécanique. Les propriétés d'une structure composite dépendent non seulement du procédé, mais aussi des matières premières et des choix de conception. Dans le but d'introduire des variabilités géométriques dans le calcul numérique des pièces composites, on part dans ce travail de l'hypothèse que les variations des grandeurs géométriques ne sont pas distribuées totalement aléatoirement, mais que celles-ci suivent des évolutions spatiales continues. Pour que les valeurs d'entrée qui nourrissent le modèle numérique soient basées sur la réalité du matériau, l'identification et la quantification des plages de variabilités et de leurs évolutions sont réalisées sur la période de la fabrication de plaques composites CFRP comptant ici 16 plis avec une stratification quasi isotrope et polymérisées en autoclave. Parmi les sources de variabilité identifiées et quantifiées, l'étude de la répartition des désalignements des fibres dans le plan et de l'évolution des variations des épaisseurs des plis a mené à la proposition de lois mathématiques d'évolution spatiale basées sur la réalité du matériau dans la pièce. Ces lois mathématiques sont ensuite utilisées pour récréer numériquement plusieurs structures composites différentes de la structure observée mais qui possèdent des valeurs de dispersions des propriétés similaires aux plaques réelles. Enfin, les structures numériques sont analysées dans un modèle éléments finis pour évaluer l'impact des dites variabilités géométriques et matériaux sur les propriétés mécaniques de la structure finale au travers de plusieurs études de cas.One of the major challenges related to the use of composite materials in structural applications is the difficulty to predict the effect of their inherent variabilities on the mechanical behaviour for such materials. The structural properties do not only depend on the fabrication process, but also depend on the raw materials and design considerations. The major goal of this thesis is the introduction of geometrical variabilities into a finite element (FE) model starting from the hypothesis that geometrical variations are not completely randomly distributed, but they maintain a spatial continuous evolution. To guarantee that the input parameters of the FE model are based on the reality of the material, the identification and quantification of the variability distributions together with their spatial evolutions are performed during the fabrication of CFRP composite plates. These plates have a 16 ply quasi-isotropic stratification and are cured in autoclave. Among the identified and quantified variability sources, the study of the in plane fibre misalignments and the evolution of the ply thickness variations has conducted to the proposition of mathematical representations of the spatial evolution of these variables based on the material reality. These mathematical representations are used to recreate different sets of virtual composites structures maintaining dispersion values similar to the real plates. Finally, the virtual structures are analysed in the FE model to evaluate the impact of such geometrical and material variabilities on the mechanical properties of the final structure

Void growth mitigation in high heating rate Out-of-Autoclave processing of composites

Increased pressure on the transport industry to reduce greenhouse gas emissions has hastened the adoption of high performance composites, particularly in the aerospace industry where the value of weight saving is very high. However, the current method of choice for manufacturing high performance composites (autoclave processing) is not cost effective for processing large (greater than 5m2) structural composite components. Developments in Out-of-Autoclave (OoA) prepreg systems have facilitated the use of vacuum only consolidation pressure to process laminates with autoclave level mechanical properties. However, owing to the low consolidation pressure, the process is heavily dependent on de-bulk quality and low cure temperatures; leading to reduced margin for error as well as long cycle times. In parallel, developments in high heating rate OoA processes have been shown to enable short cure cycle times and autoclave-level mechanical properties; albeit with a high tendency towards porosity. To date, studies on high heating rate OoA processing have been limited and the processes are not well understood. The main objectives of this self-funded study were to understand the mechanism of void growth mitigation in high heating rate OoA processes and to study the feasibility of achieving further reduction in cycle time and cost, whilst maintaining high mechanical properties. The primary mechanism of void growth was identified and an analytical model was used to predict the propensity for void growth during a given cure cycle. The model outcome highlighted a window within the cure cycle during which void growth takes place. It was hypothesised that a reduced time to resin gelation in high heating rate processes can reduce the window for void growth, leading to lower laminate porosity. A novel high heating rate pressurised tooling system (the Pressure Tool) was developed to process laminates at 15oC/min combined with the application of up to 7 Bar hydrostatic pressure. The Pressure Tool was used to verify the hypothesis that reduction in size of the window for void growth, facilitated by high heating rate, can lead to lower laminate porosity. Good agreement was observed between the model outcome and the experimental results. Studies have claimed that the reduction in resin minimise viscosity due to high heating rate can lead to gains in mechanical properties; sometimes even higher than that of autoclave cured laminates. OoA prepregs cured using up to 15oC/min heating combined with up to 3 Bar hydrostatic pressure did not result in the claimed additional gain in mechanical properties. The study confirmed earlier suggestions that additional factors such as void geometry and location within the laminate have to be taken into consideration. The final part of this thesis addresses the physical limitations to high heating rate processes; such as, the effect of tooling material, process ancillaries, laminate thickness and resin kinematics on reducing cure cycle time. The poor thermal characteristics of commonly used process ancillaries limit the dissipation of energy released by the laminate during cure. Due to which, laminate core temperature can exceed by up to 5oC, even if the laminate is processed on a highly conductive tooling material. The optimum tooling material to achieve reductions in cure cycle time whilst minimising laminate core thermal overshoot was found to have a combination of high thermal conductivity and low thermal mass. However, currently used tooling systems are not optimum for achieving further reductions in cycle time, due to unfavourable combination of thermal mass and thermal conductivity. Furthermore, the high reactivity of current resin systems and the inherently poor thermal conductivity of the polymer matrix limits the gains in cure cycle times that can be achieved