Visualisation of composites manufacturing

Abstract: The five papers in this thesis demonstrate five unique ways to monitor composite manufacturing. They also clarify several phenomena that take place during composite manufacturing. Of particular interest are two manufacturing methods, namely vacuum infusion (Paper A-D) and compression moulding of SMC (Paper E). The former process is, for instance, used for large surface area parts such as wind-turbine blades. The concept is that a dry reinforcement is placed on a stiff mould half and covered with a flexible and airtight bag. The bag is then sealed to the mould except at certain positions being open for resin supplies and outlets. By keeping the pressure atmospheric at the resin inlets and reducing the pressure at one or several positions in the formed cavity, liquid resin is forced to impregnate the stack. A further result of the difference between the ambient pressure and the pressure within the cavity is a compaction force and a corresponding compression of the elastic stack. In compression moulding of SMC a charge consisting of a polymer, fillers and chopped fibres is placed in a heated and open mould. When the mould is closed, the charged material will fill the mould. This is a rapid process and it is therefore suitable for parts to the automotive industry. Exclusively, this thesis presents optical measurements of the full 3D position of the flow front during vacuum infusion moulding. Equally exceptional are field measurements made with a stereoscopic digital speckle photography system of the movement of the bag during moulding by the same manufacturing process. The actual results from these two measuring techniques are also very interesting. First of all is it clarified that there can be rather large gradients in the flow front with respect to the thickness direction enabling the formation of voids. Secondly it is shown that certain permeability measurements could be used to predict the flow front position during vacuum infusion while others fail. Thirdly it is confirmed that a ditch is formed at the resin flow front and that there can be a considerable and seemingly perpetual compaction after complete filling. Special attention has also been given to the advancing flow front during compression moulding of SMC. In this case the full complexity is captured by means of continuous high resolution close-up monitoring. From these visualisations three phases are defined, namely pitch, floating, and boiling. In the initial phase, pitch, outer layers do not remain outer layers, the actual flow is very complex and air is likely to be entrapped. In the second phase, floating, the flow is stable and seemingly viscous. In the last phase, boiling, bubbles are observed in the low pressure region at the flow front, favouring the formation of void both internally and on the surface. For vacuum infusion it is also essential to develop and evaluate new numerical visualisation tools. This is rather challenging since the impregnation is characterized by a full three-dimensional flow in a porous medium having an anisotropic, spatial- and time-dependent permeability. The new approach taken here is to implement such models in an all-purpose and commercial computational fluid dynamics software through custom written subroutines. The strategy has been to first verify and validate the modifications by 2D simulations and then demonstrate the full 3D capacity through one demonstrator.