Colloquia

Summary

The push for reducing carbon footprint in the aerospace industry has highlighted the potential of exploring multi-material designs assembled with composite-oriented joining approaches. This thesis aims to stimulate the application of thermoplastic composites (TPCs) by exploring the fusion bonding-based joining of hybrid assemblies with thermoset composites (TSCs). The joining process explored in this work comprises two steps: first, a C/Epoxy TSC prepreg is co-cured with a PEKK-DS thermoplastic film and an epoxy-based adhesive; then, a thermo-stamping process is used to join the C/LMPAEK TPC laminate and the TSC system through the PEKK-DS film interface at elevated temperature and pressure. The interphase formed between the components of the coupling complex during co-curing a compatible interface to perform fusion bonding with the TPC laminate without prior surface treatment. 

Characterization experiments were carried out on a material level to establish thermo-stamping guidelines leading to an optimized joining process. Generally, developed temperatures and pressing times should be above a certain threshold delimited by chain diffusion kinetics and thermal stability of the thermoset-based materials. Based on these experiments, a series of thermo-stamping experiments were conducted, followed by both single-lap shear and mandrel peel tests. 

The processing parameters for bonding aligned with PEKK crystallization kinetics, indicating optimal processing times of 60 s to 450 s for a lower mold temperature of 225 ◦C and 90 s to 300 s for a mold set point of 250 ◦C. Single-lap shear tests showed cohesive failure in the TPC laminate for 90 s processing times, with a shear strength of ca. 20 MPa. Fracture toughness of the TPC-PEKK interface yielded an average of 1.6 kJ/m² on mandrel peel testing, with a plateau reached for both mold temperatures of 225 ◦C and 250 ◦C at pressing times above 180 s. Interestingly, shorter processes led to higher fracture toughness of 2.02 kJ/m² at 225 ◦C but lower values of 0.58 kJ/m² at 250 ◦C. The discrepancy is ascribed to the initial crystallinity development at the PEKK interface due to a conditioning (pre-heating) phase in which the co-cured TSC system is placed on the heated mold before thermo-stamping. Temperatures in the cold crystallization range prompted early annealing of the TP film, resulting in a partially crystallized surface that temporarily limited interdiffusion rates and, consequently, the adhesion quality. At prolonged pressing periods, sufficient time under temperature was given to erase crystals at the PEKK-DS surface and enhance fracture toughness.