Structural joining of dissimilar materials has recently been recognized as one of the primary challenges limiting the wide acceptance of composite materials in mass-produced vehicles. Joints are considered the 'weak-links' in a structure as they experience complex stress distributions during load transfer and have direct implications on the safety of resulting structural components. This study aims at developing a computational materials design approach of integrating experiments and numerical simulations to better understand the behavior of bonded joints using novel 'reversible adhesives (RA).'Thermoplastic adhesives reinforced with conductive nanoparticles allow for selective heating of thermoplastics through coupling with electromagnetic (EM) radiations via non-contact methods. This allows for increasing the adhesive temperature above processing temperatures in a short duration which upon cooling forms a structural bond. Hence this process is attractive as it enables quick assembly, removal and re-assembly of joints without the need to heat the entire component. Hence, the term "reversible adhesive (RA)" was coined to indicate the ability of these adhesives to be dis-assembled, and re-assembled by selective heating.RA consisting of Acrylonitrile Butadiene Styrene (ABS) polymer reinforced with conductive nanoparticles, namely ferromagnetic nanoparticles (FMNP - Fe3O4), and short carbon fibers (SCF) were developed using melt-compounding. Detailed thermo-mechanical characterization was performed on both the polymer nanocomposites and resulting joints. Also, the effect of repeated EM exposure on degradation of the RA was explored. Surface preparation studies to enhance structural joint performance was also performed. Lastly, a computational materials-based approach, wherein integration of multi-scale simulations and experiments was developed to explore these novel materials beyond the experimental matrix.Results indicated that the percolation limit of FMNP to ensure melting and flow during EM exposure was 8 wt.%, and the flow time decreased with increase in FMNP content. ABS adhesive with 16 wt.% FMNP showed good balance of stiffness and strength relative to other concentrations. RA with 16 wt. % FMNP can be heated to its melting temperature of 240°C within 20 seconds under EM heating (200KHz, 1KW). The drawbacks of using Fe3O4 nanoparticles as reinforcements (aspect ratio 10303) to enhance mechanical properties was overcome through addition of high aspect ratio short carbon fibers. The results from thermal degradation study of RA indicated that longer exposure to induction heating reduces the overall mechanical properties. However, repeated heating of RA within the melting temperature only effects the ductility as it loses the toughening agent butadiene within ABS. Bonded joints without O2-plasma surface treatment led to interfacial failures whereas induction-bonded joints with both O2-plasma and substrate preheating had 15% higher peak loads relative to oven-bonded joints. Finally, a multi-scale computational approach was developed and implemented to explore the design space beyond the experimental matrix and to provide an insight into nano-scale behavior and the local phenomena that cannot be experimentally measured. This work also addresses the limitations and challenges associated with RA. Overall, the integrated experimental and numerical approach such as the one presented in this work creates a benchmark for RA development and can be extended to other thermoplastics, to fully exploit the benefits of these reversible polymers for a wide range of applications.
Read
