Creating and Understanding Hybrid Interfaces of Multifunctional Composite Laminates for Extreme Environments

The primary goal of this work is to develop robust hybrid interfaces between metal/alloy and polymer matrix composites (PMC) in composite laminates for high temperature (200-300oC) applications, taking into account the consideration of the following factors: • Surface chemistry and topography • Reinforcement architecture • Damage initiation and delamination • Thermal degradation More specifically, this research is to experimentally and computationally investigate the “fracture toughness” of the hybrid metal-PMC interface as a function of temperature and interfacial architecture. In this work, hybrid composite laminates were successfully created using out-ofautoclave processing techniques. The laminates contained one single layer of metal foil sandwiched between a number (four or eight) of layers of carbon fabric reinforcement that created a symmetric layup with respect to the metal foil. In these hybrid laminates, the hybrid interfaces between metal and polymer composite were created via co-curing. Different metal foils, fabric reinforcement types and polymer matrix were investigated. In addition, a variety of surface treatment techniques were employed on the metal foil prior to placing in in the hybrid laminate layup. The fracture behavior of the fabricated hybrid interfaces was investigated at both room and elevated temperature. Furthermore, physical, thermal-mechanical as well as microscopy of the laminates were performed. It was found that in order to create strong hybrid metal-resin interface, the surface chemistry on the metal side plays a significant role. Robust hybrid interfaces should be created via formation of covalent bonds at the interface region. This could be achieved via sol-gel surface treatment method as demonstrated in this work. Other important aspects of consideration include not only thermal degradation of the surface treatment but also the compatibility of the sol-gel chemistry with the polymer resin. It was shown in this work that the fracture behavior of the hybrid interfaces depends not only on the properties of polymer resin, which dictate whether the crack propagates in a stable or unstable manner, or whether the hybrid interface exhibits brittle or ductile-like failure, but also the architecture of the reinforcement in the composite. Moreover, the loading configuration also has an important impact on the mode of failure at these hybrid interfaces. It was shown that the same hybrid interface that exhibits cohesive failure mode after the mode I fracture toughness test failed adhesively under mode II loading. In addition, surface roughness or architecture of metallic surface played an important role. Fracture toughness could be higher where rougher metallic surfaces are present at the hybrid interface. This is ascribed to the more energy released when micro-cracks formed during the fracture process due to stress concentration caused by the surface roughness. One of the most significant contributions of this work is that for the first time, robust high temperature hybrid interfaces between metal (Ti)/shape memory alloy (NiTi) and polyimide matrix composite were successfully fabricated and tested. Laser ablation and a customsynthesized sol-gel treatment were employed to prepare the Ti and NiTi foils surfaces for strong, direct adhesion with the polyimide AFR-PE-4 resin system. This high temperature interface could sustain temperature up to 250 oC. One interesting finding was revealed by SEM/EDS and nanoIR analysis of the hybrid interface cross-section where it was observed that Si from the solgel treatment solution clearly penetrated into the ablated Ti surface. This was attributed to the porosities on the Ti surface created during laser ablation. Evidence of covalent bonds formation between the sol-gel treatment solution and Ti foil was confirmed by nanoIR analysis. It was found that the degree of hydrolyzation of the custom- synthesized sol-gel treatment solution or the time associated with sol-gel hydrolysis was important. In this work, the best results were achieved with a 16-hour hydrolysis although further optimization could be possible. It was also demonstrated in this work that in-situ Rayleigh backscattering fiber optics measurements were obtained during DCB experiments. This new technique to measure distributed strains allows us to interpret the DCB experiment in a new way and enables the visualization of strain energy release upon crack propagation. In addition to the experimental investigation, finite element analysis was carried out to model the double cantilever beam experiment performed. Axial strain profiles along the specimen’s edge obtained from FEA were compared to those measured in the experiment using optical fibers. Thermal residual stresses due to thermal curing and their effects on the strain energy release rates upon crack propagation were analyzed. In addition, shape memory alloy phase transformation in the hybrid DCB specimens containing NiTi foil occurring during the test at room temperature was observed experimentally. This phase transformation was captured and illustrated by the FEA performed. The FEA results obtained in this work further support experimental observations. Furthermore, microscale finite element model to study the effect of micro-roughness pattern created by laser ablation of the metal surface on crack initiation and propagation behavior at the hybrid interface was carried out. The results obtained from this model supported the evidence of micro-crack formation on the fracture surfaces observed from the SEM analysis.