Consolidation Problems in Composite Processes

The use of composite materials in structural applications is limited by the high costs associated with the production of the components. The research of cost-effective solutions leads to the exploration of innovative manufacturing approaches. One of those is the combined prepreg / liquid composite molding (LCM) process. This manufacturing technique consists in the combination of prepreg materials with dry fabrics, subsequently impregnated by the injection of a liquid resin. The main ad-vantage of the combined process is the possibility to exploit the characteristics of the two manufacturing approaches in different regions of the same component. This shall enable the manufacturing of structures with a high level of integration and increase the effectiveness of the production. New challenges arise when combining different manufacturing techniques. A characteristic phenomenon of the combined prepreg/LCM process is the bleeding of the prepreg resin into the dry fabric. Such consolidation problem has a relevant influence on the properties of the cured laminate and needs to be understood to guarantee a good reliability of the process. The main objective of this thesis is the description and study of the mechanisms driving the consolidation of a stack of reinforcement plies. The first part of the work aims at establishing the manufacturing procedures for the combined out-of-autoclave (OOA) prepreg / vacuum assisted resin transfer molding (VARTM) process. Following an experimental approach, the main characteristics of the process are described and the influence of the process parameters on the composite quality is assessed. The derived procedures are then validated with the production of a representative aerospace demonstrator component. In the second part of the thesis, the material properties sup-porting the modeling of the consolidation process, i.e. fiber bed through-thickness compaction and permeability, are investigated. A new approach for the characterization of the quasi-static and dynamic (time-dependent) compaction response of fiber beds is proposed and validated for different conditions. Furthermore, an experimental setup and procedure for the derivation of the “true” fiber bed through-thickness saturated permeability is established. The characterization method is validated exploiting a numerical model to assess the deformation the fiber bed sample during the experiment. Finally, the derived material properties are implemented in coupled finite element models describing the consolidation of reinforcement plies and the resin flow in composite manufacturing processes. The simulation of a saturated consolidation is validated against experimental data measured in a dedicated setup. The observations made in this thesis contribute to a better understanding of consolidation phenomena in composite manufacturing processes. The insight shall support the definition of manufacturing and design guidelines, aiming at increasing the reliability and robustness of the production of composite components