Weld strength of laser-assisted tape-placed thermoplastic composites

Laser-assisted tape placement is an attractive manufacturing technology for the aerospace industry as it combines high productivity with low energy consumption. It comprises the automated deposition of fiber reinforced thermoplastic tapes to incrementally build up a structure. The process can also be used to tailor the properties of conventionally manufactured woven fabric reinforced components by locally reinforcing these with unidirectionally reinforced tapes. This thesis focuses on the weld strength between the tape and the woven fabric reinforced component. The principal objective is to develop an in situ processing strategy, combining high productivity and energy efficiency with high weld strength. For this purpose, the important bonding mechanisms, processing parameters and material properties are identified through a combination of experimental work and physical modeling. The interlaminar bonding process comprises the development of intimate contact followed by the interdiffusion of polymer chains. Both mechanisms depend strongly on the interface temperature. A thermal process model is, therefore, proposed specifically taking into account the optical aspects of laser heating. The model is validated experimentally. Based on the developed model, the important processing paramaters and material properties are identified. A mandrel peel test is introduced to quantify the interfacial fracture toughness between the tape and the laminate. The applicability and validity of the method is successfully demonstrated by comparing it to standardized fracture mechanics tests. The interfacial fracture toughness does not only depend on the degree of interlaminar bonding. The crystallinity and structural morphology of the interface also play an important role. This is demonstrated by a comparison between the (fast) tape placement process and a (slow) press-molding process. The tape-placed specimens outperform the press-molded specimens in terms of fracture toughness by almost a factor of two. This is attributed to the high cooling rates and short bonding time during the tape placement process. The former results in a low crystallinity, while the latter prevents the migration of tape fibers into the resin pockets of the laminate and thereby minimizes the fiber–fiber contact. Both the low crystallinity and the presence of resin pockets improve the interfacial fracture toughness. Finally, a processing strategy is proposed, which maximizes productivity and energy efficiency. The strategy involves the distribution of all laser power to the tape. Although the proposed strategy should be tested in practice, the work in this thesis suggests that an excellent weld strength will be achieved.