Abstract

It’s thoroughly known that sandwich structures in aeronautical sector are formed by a core material, which more usual format is honeycomb, between two facings or skins made from several prepreg material layers.  Nevertheless in order to obtain an adequate adherence between core and prepreg most of thermoset resin systems need the presence of a film adhesive placed among the core and prepreg to favour the adhesive menisci formation inside core cells. Main benefit asociate to autoadhesive prepregs usage in sandwich parts is adhesive elimination from part configuration which translates in a weight reduction of the structure. Present article is centered in autoadhesive prepreg materials 180º cure. After more than 3 years of work with several composite materials manufacturers and AIRBUS, key process parameters in an industrial environment have been defined along with basic characteristics in order to guarantee internal quality of the elements

Abstract

En la fabricación de estructuras sándwich de material compuesto, unas de las opciones más utilizadas como núcleo es el uso de espumas poliméricas. Debido al cada vez mayor esfuerzo en reducir el impacto medioambiental de los procesos industriales, existe un gran interés en incorporar materiales de origen natural que permitan disponer de fuentes renovables y que faciliten los procesos de reutilización y de reciclado. Una de las posibilidades para sustituir a las espumas poliméricas es el corcho aglomerado que presenta unas buenas propiedades mecánicas. No obstante, para que sea posible su uso como núcleo en estructuras sandwich es necesario conocer su comportamiento frente a cargas de impacto, tanto de baja como de alta velocidad.

En este trabajo se compara el comportamiento de una espuma de PVC comercial con corchos aglomerados de diferentes densidades. Se ha analizado la fuerza, desplazamiento, energía absorbida y velocidad de perforación. Se ha observado que el comportamiento del corcho aglomerado puede ser similar al de una espuma polimérica convencional, aunque con un incremento de peso. Adicionalmente se ha puesto de manifiesto que el corcho aglomerado puede ser una alternativa mejor a las espumas poliméricas en aquellas aplicaciones que puedan verse sometidas a varios impactos sucesivos.

Abstract

Sandwich structures made of composite materials, namely glass fibre/polyester, are widely used in  the naval, ground transportation, aerospace and telecommunication industries. Due to the combination of key factors: the good mechanical properties of this kind of materials together with their low specific weight and their permeability to electromagnetic waves. These structures are often subjected to impulsive loads during its service life. Therefore, in order to obtain a safer structural design, a better knowledge of the impact response must be accomplished. It has been demonstrated that the composition of the skins plays a fundamental role in the behaviour of sandwich structures against high-energy ballistic impacts. The structures studied in this work are composed by two E-glass fibre/polyester skins and a PVC core. With the objective of optimising this kind of structures when they are subjected to high-energy impact, a numerical study in the finite element code Abaqus has been carried out. In this study, the number of plies in the front and rear faces varies in order to find the configuration that presents better ballistic performance. To this aim, a VUMAT subroutine previously developed has been used for the skins. The core has been modeled with the crushable foam constitutive model available in the software Abaqus. The influence of the thickness of the skins can be tested by means of this numerical model. The results obtained show that the residual velocity disminishes the more the number of layers in the rear skin increases.

Abstract

Additive manufacturing with FFF (Fused Filament Fabrication) technology using continuous fibre, allows more complex structure designs with more technical materials than conventional manufacturing processes. The present paper is focused on analysing the bending behaviour of sandwich beams with unidirectional carbon fibre skins and honeycomb type core. The nobelty of the research is focused on printed materials (polyamide 6 reinforced with continuous carbon fibres) and the fact that the core has been manufactured directly in the printig process. The study variable consisted of the spatial arrangement of the beam; for the first type of beams the skins are positioned in parallel with the base plate, whereas for the second type of beams, the skins are placed perpendicullary with respect to the base plate. Thus, in both cases skins are obtained with the same fibre orientation, but with different manufacturing strategies. Otherwise, those configurations where skins are placed in parallel with respect to the base plate, the influence of reinforcing the hexagons of the core with carbon fibres following different strategies has been analysed. On the one hand, the hexagons are reinforced depositing continous carbon fibre following concentric deposition strategy, forming hexagonal rings in each hexagon independently. On the other hand, the overall core is reinforced toghether with continous carbon fibres following the external perimeter of the core. Results show that the beams whose skins have been manufactured in parallel with respect to the base plate have higher stiffness. In the same way, those structures with reinforced cores have a higher stiffnes compared to those with non-reinforced cores.