Evaluation and impact of the degree of impregnation of uncured out-of-autoclave prepreg

Out-of-autoclave prepreg contains partially impregnated fibre tows which act as paths for gas evacuation. The degree of impregnation of resin into the dry fibre bed is a quantitative measure of the size of these dry fibre pathways and can affect part quality and rejection rates. There is no well-defined and accurate standard method to measure this quantity. The current state-of-the-art is to manufacture discriminator panels, which are panels containing complex features such as tight radius corners, internal ply drop-offs, sandwich regions, etc. to determine the producability of specific components by specific material. That is, it is an empirical method used to determine if the initial properties of the prepreg fall within certain boundaries. This technique is time consuming and does not necessarily account for variations within specific rolls of material. The aim of this thesis is twofold: examine different techniques to measure the degree of impregnation of out-of-autoclave prepreg and perform an experimental investigation into the impact of degree of impregnation on the breathability of the material.Firstly, different methods to quantitatively measure the degree of impregnation of out-of-autoclave prepreg are examined. Three methods are compared: X-ray computed tomography, the water uptake test, and active infrared thermography. The first method serves as a baseline for accuracy but is expensive both in terms of cost and time. The second method is characterized by its simplicity but is destructive and only gives local information over a small sample area. The latter method shows promise as a quick, non-destructive evaluation technique. A further investigation into active infrared thermography highlights some limitations of the technique with regards to its implementations in an industrial setting.Secondly, the impact of degree of impregnation on the gas transport phenomena in composite materials is evaluated by manufacturing single skin sandwich panels in an instrumented fixture with a simple design of experiments. Under room temperature conditions, as the degree of impregnation increases, the material's ability to transport gas is reduced, resulting in higher core pressure. However, at elevated temperatures, this impact is lessened and final laminate quality in all panels manufactured is similar.Both material and process variability is inherent in composite materials. Currently, it is important to use robust material systems and process cycles; however, through the use of non-destructive material inspection it would be possible to measure this variability and mitigate any problems caused by it through process modification or material rejection.Les pré-imprégnés hors autoclave comportent des régions sèches qui permettent l'évacuation des gaz. Le degré d'imprégnation de la résine dans les fibres sèches est une mesure quantitative de la taille de ces régions et peut affecter la qualité des pièces et le taux de rejet. Il n'existe pas de méthode définie et précise pour mesurer cette quantité. La pratique actuelle consiste à fabriquer des panneaux discriminants, qui contiennent des éléments complexes tels que des rayons serrés, des fins de plis internes, des zones renforcées par une âme, etc. pour déterminer la faisabilité de ces composantes spécifiques. Autrement dit, il s'agit d'une méthode empirique utilisée pour déterminer si les propriétés initiales du pré-imprégné sont conformes dans les limites de la pièce à fabriquer. Cette technique prend beaucoup de temps et ne tient pas nécessairement compte des variations à l'intérieur des rouleaux de matière. L'objectif de cette thèse est double: examiner différentes techniques pour mesurer le degré d'imprégnation de pré-imprégnés hors autoclave et effectuer une étude expérimentale sur l'influence du niveau d'imprégnation sur la respirabilité du matériau.Premièrement, différentes méthodes de mesure quantitative du degré d'imprégnation de pré-imprégnés hors autoclave sont examinées. Trois méthodes sont comparées: la tomodensitométrie par rayon X, le test d'absorption d'eau, et la thermographie infrarouge. La première méthode sert de base de référence pour la précision mais elle se révèle coûteuse en termes de coût et de temps. La deuxième méthode se caractérise par sa simplicité, mais est destructive et se limite à de petites surfaces de l'échantillon, fournissant uniquement une information locale. La dernière méthode est rapide, non destructive et prometteuse comme technique d'évaluation. L'étude plus approfondie de la thermographie infrarouge met en évidence certaines limites de cette technique concernant son implémentation dans un milieu industriel.Deuxièmement, l'influence du degré d'imprégnation sur les phénomènes de transport de gaz dans les matériaux composites est évaluée par la fabrication de panneaux sandwich dans un moule instrumenté avec un simple plan d'expériences. Dans des conditions de température ambiante, lorsque le degré d'imprégnation augmente, la capacité du matériau à transférer des gaz est réduite, entraînant une pression de l'âme à nid d'abeille élevée. Cependant, à des températures élevées, cet effet est réduit et la qualité finale des peaux de tous les panneaux fabriqués est similaire.La variabilité des matériaux et des processus est inhérent dans les matériaux composites. Actuellement, il est important d'utiliser des systèmes de matériaux et cycles de procédés robustes; toutefois, grâce à l'utilisation d'inspection non destructive, il serait possible de mesurer cette variabilité et d'atténuer les problèmes qu'il cause par des modifications du procédé ou rejet des matériaux

TOWARDS CONTINUOUS RESISTANCE WELDING FOR FULL-SCALE AEROSPACE COMPONENTS

This paper presents the preliminary stages of the collaboration between the National Research Council Canada (NRC) and the German Aerospace Center (DLR). A study demonstrating the current status of continuous resistance welding and the efforts to increase the process maturity is presented. Furthermore a continuous resistance welding end-effector concept developed at the DLR’s Center for Lightweight Production Technology (ZLP) Augsburg is discussed in this work

BENCHTOP CONTINUOUS RESISTANCE WELDING OF STRUCTURAL THERMOPLASTIC COMPOSITE JOINTS

The objective of this paper is to present a continuous resistance welding benchtop setup capable of performing large scale welded joints. In this process, a continuous conductive implant (e.g.: metal wire mesh) is placed between the two substrates to be welded. A mobile end-effector moves along the weld length, locally heating the conductive implant via Joule heating while compacting the joint locally throughout the melt and solidification stages of the thermoplastic material. The performance of the joint has been shown to be highly dependent on the process temperature at the weld interface; however, it cannot be measured directly during the process in a non-invasive manner (i.e., without placing thermocouples at the weld interface). Thus, additional process feedback variables that can reliably relate the weld temperature will be discussed and demonstrated as potential process control variables. This paper will demonstrate the continuous resistance welding process as a viable method for joining large thermoplastic aerospace structures

Towards in-line Control of Continuous Resistance Welding for joining structural Thermoplastic Composites

The continuous resistance welding (CRW) process consists of an end-effector which moves along the length of a weld seam, heating a conductive implant while compacting the joint locally throughout the melt and solidification stages of the thermoplastic material. The performance of the joint has been shown to be highly dependent on the process temperature at the weld interface; however, this cannot be measured directly during the process in a non-invasive manner. Other parameters such as boundary conditions, substructure properties, or part geometry may vary along the length of the weld. As such, a physics-based simulation is developed founded upon an “MSTEP” framework which defines how the materials (M), shape (S), tooling (T), and equipment (E) interact to determine the process (P). Detailed finite element (FE) models are developed for thermal analysis based on the weld geometry, boundary conditions, and previously developed and validated melt/crystallization models for the thermoplastic matrix. Experimental CRW tests are presented to validate simulations and calibrate suitable control variables