rapid consolidation of tailored thermoplastic composites by automated lay-up and stamp forming - a study on the consolidation mechanisms
Tjitse Slange is a PhD student in the Department of Mechanics of Solids, Surfaces & Systems (MS3). His supervisor is prof.dr.ir. R. Akkerman from the Faculty of Engineering Technology (ET).
Lightweight aircraft design is key to reducing the cost and environmental impact of flying. The high specific stiffness and strength make fiber reinforced polymer composites an attractive material for aircraft design. With the growing demand for aircraft and the increasing use of composite materials, there is a need for cost-effective composite manufacturing processes. Thermoplastic composites are potentially ideal for automated high-rate low-cost manufacturing as they can be repeatedly melted, shaped and solidified in short cycle times, which allows for forming, fusion bonding and recycling. However, despite the potential, existing thermoplastic composite manufacturing still relies mostly on slow processes like press or autoclave consolidation. Moreover, the tailorability of the mechanical performance by optimizing the location and orientation of the fibers is often not fully exploited, leading to a sub-optimal performance over weight ratio. Hence, further development is required for the high-rate manufacturing of load carrying thermoplastic composite structural components.
This thesis proposes rapid automated lay-up followed by stamp forming as a novel processing route. Automated lay-up, for example automated fiber placement (AFP) or automated tape laying (ATL), provides the ability to manufacture flat blanks with tailored and near net-shape lay-ups, which improve the performance over weight ratio of the part and reduce production scrap. Shaping of the blank takes place during a short stamp forming step. The main challenge is to achieve a high consolidation quality at the end of this process cycle, which is required for good mechanical performance. The lay-up of flat blanks is performed at high rates in order to achieve short cycle times, which results in a low degree of consolidation compared to in-situ lay-up at lower rates. This means that most consolidation has to take place during stamp forming, where the available time for consolidation is also short. Void content is considered as one of the most important measures for consolidation quality in this thesis. The main objective is to develop an understanding of the physical mechanisms that govern the evolution of void content during stamp forming and of the interrelation between material properties, processing parameters and final consolidation quality. This knowledge is then used to develop material, processing and design guidelines for consolidation using the proposed processing route.
Three key phenomena were found to govern the evolution of void content during stamp forming, namely i. deconsolidation, ii. the elimination of blank thickness variations and iii. the filling of voids at ply-drops.
Firstly, a study showed that deconsolidation, which is the undesired growth of voids and delaminations in a blank during heating, is governed by two mechanisms. The contribution of each mechanism depends on the blank manufacturing method. Expansion of dissolved moisture dominates deconsolidation in press-consolidated blanks, while the release of interal stresses, present in the prepreg tape, drive deconsolidation of fiber placed blanks. The influence of several heat treatments on moisture content and degree of deconsolidation is investigated and guidelines for minimizing deconsolidation are proposed.
In a further study it is shown that blank thickness variations rather than the initial interlaminar void content, dominate the removal of interlaminar voids during stamp forming. Flow transverse to the fiber direction of the composite plies is responsible for the redistribution of material and development of interlaminar bonding. A model based on transverse squeeze flow of the plies is proposed to study the influence of the prepreg thickness distribution and processing parameters on interlaminar void content. This model is exploited further for developing prepreg design guidelines and lay-up strategies aimed at optimizing the consolidation process.
Finally, transverse flow is also identified as the main mechanism for the consolidation of ply-drops, which are inherent to tailored blanks. The large voids in the pockets next to a ply-drop are filled by transverse flow of the dropped ply and surrounding plies. It is shown that the ability to fill the ply-drop is sensitive to lay-up accuracy during AFP and to blank-tooling alignment during stamp forming. With this knowledge, guidelines are proposed to optimize consolidation.
The work presented in this thesis shows that the final consolidation quality is a complex function of the entire processing chain, where each step has a critical function in the consolidation process. Material, design and processing guidelines are provided to support process development. Finally, the processing route is demonstrated on a tailored spar, which confirms that good consolidation can also be achieved in realistic parts. However, the need for an improved understanding of the forming and consolidation of more complex tailored parts was highlighted. Altogether, this thesis provides a fundamental basis for the further development of the rapid manufacturing route for lightweight tailored composite components.
