Publication: Design, manufacture and evaluation of partially-cured composite materials
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Publication date
2020-03-25
Defense date
2019-11-12
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Abstract
Normalmente, la fabricación de materiales compuestos de altas prestaciones se
realiza en autoclave mediante el uso de preimpregnados y con la aplicación de forma conjunta
de altas presiones y temperaturas, consiguiendo así reducir la porosidad del material
garantizando unas buenas propiedades mecánicas. No obstante, este sistema de fabricación
conlleva tiempos de producción largos y grandes inversiones en equipamiento, por lo que se
ha generado un aumento de la demanda de sistemas de fabricación alternativos. En esta tesis
se muestran los procedimientos experimentales para desarrollar una ruta de procesamiento
para la fabricación de piezas de material compuesto parcialmente curados capaces de ser
post-curados y consolidados con sus homólogos frescos tradicionales con el objetivo de
producir estructuras integradas de una sola vez, reduciendo así costes y tiempo. Los
materiales parcialmente curados fueron fabricados a partir de tejidos secos de carbono y
resina epoxi mediante la técnica de moldeo por transferencia de resina asistida con vacío
(VaRTM). La temperatura de transición vítrea Tg de la pieza parcialmente curada debe estar
entre dos límites. Por un lado, la temperatura de transición vítrea debe ser lo suficientemente
alta para mantener la pegajosidad y la deformabilidad dentro de un rango aceptable. Por otro
lado, la temperatura de transición vítrea debe ser lo suficientemente baja para proporcionar
una buena adaptabilidad de la pieza de material compuesto en el ciclo de post-curado
posterior. La homogeneidad de la temperatura del molde fue crucial para alcanzar los grados
específicos de curado necesarios para la integración posterior. Para este fin, se diseñaron dos
moldes, uno plano y otro en forma de T, ambos equipados con un número de resistencias
independientes para permitir así un control detallado de la temperatura de la pieza de
material compuesto. La reacción exotérmica de la resina epoxi fue controlada en función del
ciclo de curado específico. El ciclo de curado se determinó en función de algunos ensayos
realizados a muestras extraídas de la pieza parcialmente curada usando para ello calorimetría
diferencial de barrido (DSC) y análisis mecánico-dinámico (DMA). Siguiendo este
procedimiento, será posible fabricar partes finales de una manera estándar (por ejemplo, de
soporte) con un grado homogéneo de curado. Los elementos fabricados fueron almacenados
en frío como ocurre con los preimpregnados estándar antes de la integración posterior con pieles de preimpregnado frescas y curadas. Asegurar un adecuado grado de curado es la clave
de este trabajo, al igual que lograr que la pieza de material compuesto parcialmente curada
fuera capaz de adaptarse a las diferentes geometrías de la piel manteniendo la geometría de
la pieza fabricada y sin efecto del sangrado de la resina del material parcialmente curado. Con
ello nuestro objetivo de producir estructuras integradas de una sola vez, reduciendo así costes
y tiempo en el proceso de fabricación por autoclave se veía cumplido.
Typically, the manufacture of high-performance composite materials by autoclave with the use of prepregs and with the joint application of high pressures and temperatures, thus reducing the porosity of the material guaranteeing good mechanical properties. However, this manufacturing system involves long production times and large investments in equipment, resulting in increased demand for alternative manufacturing systems. This thesis shows the experimental procedures to develop a processing route for the manufacture of partially-cured composite materials capable of being post-cured and consolidated with their traditional fresh counterparts with the aim of producing integrated structures one-shot, thus reducing costs and time. Partially-cured materials were manufactured from dry carbon fabrics and epoxy resin using the Vacuum assisted Resin Transfer Moulding (VaRTM) process. The glass transition temperature, Tg, of the partially-cured material must be within two limits. On the one hand, the glass transition temperature should be high enough to keep the sticky and deformity within an acceptable range. On the other hand, the glass transition temperature should be sufficiently low to provide a good adaptability of the composite material in the later post-cure cycle. The homogeneity of the mould temperature was crucial to reach the specific degrees of curing needed for subsequent integration. For this purpose, two moulds were designed, one flat and the other in T-shape, both equipped with a number of independent resistances to allow a detailed control of the temperature of the composite material. The exothermic reaction of the epoxy resin was controlled according to the specific curing cycle. The curing cycle was determined on the basis of some tests carried out on samples taken from the partially-cured material using Differential Scanning Calorimetry (DSC) and Mechanical- Dynamic Analysis (DMA). Following this procedure, it will be possible to manufacture final materials in a standard (for example, support) way with a homogeneous degree of curing. The manufactured materials were stored cold as is the case with the standard prepregs before subsequent integration with fresh and cured prepregs skins. Ensuring an adequate degree of curing is the key to this work, as well as ensuring that the partially-cured material was able to adapt to the different geometries of the skin maintaining the geometry of the manufactured material and without effect from the resin bleeding of the material partially-cured. With this our objective of producing integrated structures one-shot, thus reducing costs and time in the autoclave manufacturing process was fulfilled.
Typically, the manufacture of high-performance composite materials by autoclave with the use of prepregs and with the joint application of high pressures and temperatures, thus reducing the porosity of the material guaranteeing good mechanical properties. However, this manufacturing system involves long production times and large investments in equipment, resulting in increased demand for alternative manufacturing systems. This thesis shows the experimental procedures to develop a processing route for the manufacture of partially-cured composite materials capable of being post-cured and consolidated with their traditional fresh counterparts with the aim of producing integrated structures one-shot, thus reducing costs and time. Partially-cured materials were manufactured from dry carbon fabrics and epoxy resin using the Vacuum assisted Resin Transfer Moulding (VaRTM) process. The glass transition temperature, Tg, of the partially-cured material must be within two limits. On the one hand, the glass transition temperature should be high enough to keep the sticky and deformity within an acceptable range. On the other hand, the glass transition temperature should be sufficiently low to provide a good adaptability of the composite material in the later post-cure cycle. The homogeneity of the mould temperature was crucial to reach the specific degrees of curing needed for subsequent integration. For this purpose, two moulds were designed, one flat and the other in T-shape, both equipped with a number of independent resistances to allow a detailed control of the temperature of the composite material. The exothermic reaction of the epoxy resin was controlled according to the specific curing cycle. The curing cycle was determined on the basis of some tests carried out on samples taken from the partially-cured material using Differential Scanning Calorimetry (DSC) and Mechanical- Dynamic Analysis (DMA). Following this procedure, it will be possible to manufacture final materials in a standard (for example, support) way with a homogeneous degree of curing. The manufactured materials were stored cold as is the case with the standard prepregs before subsequent integration with fresh and cured prepregs skins. Ensuring an adequate degree of curing is the key to this work, as well as ensuring that the partially-cured material was able to adapt to the different geometries of the skin maintaining the geometry of the manufactured material and without effect from the resin bleeding of the material partially-cured. With this our objective of producing integrated structures one-shot, thus reducing costs and time in the autoclave manufacturing process was fulfilled.
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Keywords
Composite materials, Partially-cured composite materials, Vacuum Assisted Resin Transfer Moulding, VaRTM, Fracture toughness, Mechanical properties