On the consolidation quality in laser assisted fiber placement.
Thijs Kok is a PhD student in the Research Mechanics of Solids, Surfaces & Systems (MS3) group. His supervisor is prof.dr.ir. R. Akkerman from the Faculty Engineering Technology (ET)
Laser assisted fiber placement (LAFP) is a promising additive manufacturing technique for large aerospace structures. The process involves the use of a robot to place thermoplastic composite prepreg tapes to build-up a component ply by ply. The prepreg tape is heated using a laser and subsequently bonded to previously placed plies under pressure applied by a roller. LAFP has the potential to reduce manufacturing time and costs by consolidating the prepreg tape in-situ, that is during placement. This way an expensive post-consolidation step in for example an autoclave can be omitted, provided that sufficient consolidation quality is obtained after placement. The consolidation quality achieved by LAFP at high placement rates does, however, not meet the high consolidation quality required by the aerospace industry yet. Especially, the void content exceeds the limit set by the aerospace industry. Voids can be classified according to their location in intralaminar and interlaminar voids. The latter are mainly originating from the LAFP processing and are the focus of this thesis. These interlaminar voids can be considered to be a lack of intimate contact between plies during the placement process. Furthermore, the presence of gaps and overlaps between adjacent tapes impedes the development of intimate contact resulting in a higher interlaminar void content.
The LAFP process has been subjected to a lot of research over the past decades. However, the effect of the heating phase on the tape has received little to no attention. This thesis aims to investigate the effect of the heating phase on the consolidation state of the incoming tape and its consequences on the governing mechanisms during consolidation. For this purpose, an experimental research has been performed to analyze the state of the tape during the process, the development of intimate contact and the deformation of the tape during LAFP.
The tape is heated with the laser above its melting temperature in milliseconds. Although this heating time is short, the tape deconsolidates before entering the nip-point. The experimental research showed that this deconsolidation resulted in a rough and fiber-rich surface and an increase in intralaminar void content. The severity of the deconsolidation has been quantified using image analysis. The matrix-poor surface of the tape hinders interlaminar bond development, which consists of intimate contact development followed by healing. Currently models describe development of intimate contact as the deformation of surface asperities. Such a description is, however, not applicable to the deconsolidated tape, which is matrix-poor. Instead a percolation flow of matrix is required to wet the surface in order to obtain intimate contact and for healing to occur. Therefore, an intimate contact model based on percolation has been developed for the deconsolidated tape. An experimental analysis on intimate contact development has been performed and the results show that the proposed model is better capable predicting the trends for varying process settings than the available intimate contact models.
Prevention of gaps and overlaps requires, apart from a consistent tape width and an accurate robot, knowledge of the tape deformation during the process. An experimental analysis was performed to show that tape spreading was, for the range of settings investigated, not dependent on the applied pressure but only on the tape temperature. A quasi static tape deformation model has been developed based on these observations. This model is capable of predicting the trends in tape deformation for various process settings.
The experimental results in this thesis help to better understand the consolidation process during LAFP. This knowledge was used to develop consolidation models which together form a valuable tool to further improvement of the tape material and optimization of the LAFP process.
