Thermal deconsolidation of thermoplastic prepreg tapes during Laser-Assisted Fiber Placement

The automation of thermoplastic composite production and the drive towards out-of-autoclave processes, is of great relevance in the aerospace and lightweight composite structures industry. Hence, there is a crucial need for developing the current state of material and process understanding, in order to increase the technology readiness levels of automated, out-of-autoclave production processes. Laser assisted fiber placement (LAFP) is a well-researched, automated production process which has been used in developing various thermoplastic composite demonstrators. Theoretically, this process does not require an autoclave consolidation cycle. However, one of the remaining challenge in the process, is the relatively high void content in the produced laminates (>1%). This high void content is impeding the development of thermoplastic composite structures with mechanical strength comparable to structures produced through traditional processing techniques, such as an autoclave. One of the main reasons for the remaining void content in the laminates after consolidation by the roller, is thermal deconsolidation during the rapid heating phase of the process. This is a very less researched aspect of LAFP, due to which, not much is known about the changes that the incoming material undergoes, due to the rapid laser heating and which mechanisms govern these changes. Due to this, thermal deconsolidation is also not included in predictive models for the process and hence the accuracy of these models in predicting the final part quality is poor. Therefore, this research focuses on gaining a better understanding of thermal deconsolidation, in the context of rapid laser heating during LAFP, through experimental investigation.The influence of five process variables was studied in this work: heating time, heated spot length, cooling rate, nip point temperature and the polymer type in carbon-fiber reinforced thermoplastic pre-impregnated (prepreg) tapes. The deconsolidated state of prepreg tape specimens was captured after rapid laser heating and the changes were characterized. The main results revealed that thermal deconsolidation due to rapid laser heating is governed by multiple mechanisms. Some previously unreported and non-intuitive results were observed in the material response to rapid laser heating, which are suspected to have a strong influence on the quality of the laminates produced through LAFP. Based on a qualitative and quantitative study of the influence of studied process variables on thermal deconsolidation, some mechanisms were identified and later verified with confirmatory experiments. The results of this study can be used as a starting point to develop predictive models for estimating the deconsolidated state of thermoplastic prepreg tapes, at the end of the rapid heating phase, in future work. Various topics for further research prevail. These include but are not limited to: studying the influence of tool temperature on the deconsolidation response of the prepreg material, evaluation of the deconsolidated state with superior characterization techniques in order to obtain in-situ microstructural data, overcoming experimental limitations of a static laser heating setup and studying the influence of the deconsolidated tape state on intimate contact development under roller compaction pressure. By investigating these additional aspects, a robust and accurate predictive model for thermal deconsolidation can be developed in future work, which shall help improve the accuracy of a high-fidelity process model for LAFP, which is currently in development at TU Delft.Aerospace Engineering | Structures and Material