Colloquia

Summary

Laser-Assisted Fiber Placement (LAFP) is an automated additive manufacturing technology that employs a laser to melt thermoplastic composite tapes such that they can be welded to previously placed layers. This technology promises to enable In-Situ Consolidation (ISC) which is economically attractive since it does not require an expensive post-consolidation step. However, despite several decades of research, ISC has not yet matured to a point where it can meet the high consolidation quality demanded by the aerospace industry. The consolidation quality is mainly determined by the physical mechanisms of intimate contact development and polymer healing which combined, determine the interlaminar bond strength. However, the bond strength is a typical shortcoming of the components made through ISC. The main challenge is that the short typical processing time, combined with high viscosity of the matrix material, inhibits establishment of full intimate contact and subsequent healing.

This thesis aims to assess the ISC capability of the LAFP machine installed at GKN Fokker. The assessment is performed through three stages. First, the underlying physical mechanisms and parameters that affect the consolidation quality, and their dependence on process settings, are identified in a literature review. Second, using this information, suitable process settings are configured to set-up the machine. Finally, assessment of the process is performed through experimentation. Two material systems are assessed, namely carbon fiber reinforced LM-PAEK and PEKK.

In the assessment of the process, the consolidation quality was evaluated through a range of destructive and non-destructive tests, of which the most notable are mentioned here. The bonding strength, that was evaluated through Interlaminar Shear Strength (ILSS) testing, ISC specimens obtained 50-65% of the autoclave reference ILSS values. The void content was found in between 0.5-1.5% for both material systems. The curvature resulting from residual stress-induced deformations was measured, showing that the curvature of PEKK specimens reduced with increased processing temperature, whereas, the curvature of LM-PAEK specimens did not show a similar decrease with temperature. This points to a possible stress relaxation during processing which is facilitated by the resin-rich layer that encapsulates the fibers in PEKK material.

Based on the results presented, the consolidation quality is inferior to the autoclave process as a result of poor healing. Further research could be directed into the effect on consolidation quality of increasing the processing temperature and processing time for these material systems.