Interlaminar Fracture Toughness of Carbon Fiber Reinforced Thermoplastic In-situ Joints

Due to their ability to form fusion bonds, composites with thermoplastic matrix offer new opportunities to assemble complex structures with a multitude of single components. By combining the laser-assisted Thermoplastic Automated Fiber Placement process (TP-AFP) with the thermoforming technology, a time-efficient process chain for producing hollow skin-stiffener structures was developed. Removable core systems are inserted into the cavity of thermoformed omega profiles for subsequent TP-AFP processing of skin layers to the top of the flanges. This enables in-situ joining between the flanges of the thermoformed profile and the skin layers without the need for other joining technologies or supplementary bonding materials. Mode I interlaminar fracture toughness tests were conducted to evaluate the performance of this joining strategy. Partially fiber-placed and thermoformed test specimen were produced by the combined TP-AFP/thermoforming process transferring the developed joining strategy to coupon level. In addition, fiber-placed specimens were manufactured with and without additional processing in a press to act as a reference. The effects of different cooling rates during production of the test panels on the crystallinity were determined by Differential Scanning Calorimetry (DSC). The mode I interlaminar fracture toughness test results show that partially fiber-placed and thermoformed test specimen have a comparable level of resistance against delamination as entirely fiber-placed or thermoformed test specimen. The similar amount of crystallinity of these test panels, determined by an introduced crystallinity ratio (CR), correlates with the mode I interlaminar fracture toughness test results and indicates a low influence of the developed joining strategy on the crystallinity amount at the joint zone.