Multifaceted characterization and optimization of aerospace grade thermoplastic composites joints: advances in surface treatments, mechanical performance, and environmental resilience

This thesis is composed of four interconnected and complementary investigations into novel manufacturing processes designed to enhance the mechanical performance of carbon fiber-reinforced polyether-ketone-ketone (CF/PEKK) composite laminates and adhesively bonded CF/PEKK joints for aerospace composite structures.The first study, CF/PEKK laminates consolidated both in-situ and in an autoclave are produced by using automated fiber placement (AFP) and then investigated microstructural and thermal properties. Autoclave-consolidated CF/PEKK laminates are mechanically characterized and examined their damage behavior under tensile loading conditions. Using a multi-instrument structural health monitoring (SHM) approach, including acoustic emission (AE), digital image correlation (DIC), and infrared thermography (IRT), this study provides insights into failure mechanisms and damage progression in high-performance thermoplastic composites (TPCs). The results can help identify the appropriate damage severity index for TPCs, a valuable parameter for estimating the remaining useful life of CF/PEKK composite structures. After developing a comprehensive understanding of microstructural, thermal, and mechanical properties, as well as the failure mechanisms of CF/PEKK as an adherend, the second study demonstrates the effectiveness of atmospheric plasma activation (APA) on adherend surfaces and compares the mechanical behavior of adhesively bonded joints (ABJs) with those on non-treated (NT), peel-ply (PP), and mechanical abrasion (MA) surfaces. Mechanical performance assessments of single lap-joints (SLJs) under tensile and flexural loadings reveal that APA-treated joints exhibit superior load-carrying capacities, with a notable increase in shear and flexural strength. Post-fracture surface analyses and in-situ AE monitoring confirm enhanced interface interactions and a shift towards cohesive failure in APA-treated joints.The third study assesses the impact of surface treatments on Mode-I and Mode-II fracture toughness of adhesively bonded CF/PEKK joints, showing significant performance enhancements with APA treatment. The findings indicate substantial improvements in fracture toughness values for APA-treated samples, with AE analyses correlating with slower delamination and occurrence of cohesive failure.Lastly, the research on adhesively bonded joints is further extended by investigating the influence of environmental conditions (i.e., room temperature (RT), high temperature (HT), low temperature (LT), and cycling hygrothermal (CHT) conditions) on the mechanical and thermomechanical performances, durability, and reliability of adhesively bonded CF/PEKK joints. This approach enables a better understanding of the performance of aerospace-grade composite structures in real-world aerospace applications by simulating various service conditions. The results indicate that environmental conditions significantly alter fracture toughness and dynamic-mechanical behaviors, thus highlighting the importance of considering environmental factors in aerospace structural design.