Lightweighting of automotives and ground vehicles has facilitated the use of a wide range of materials such as high strength aluminum, advanced high strength steel alloys, ceramics along with various composites based on the desired application. This inevitably leads to multi-material joints, where fusion welding process is not possible. Adhesive bonding offers an alternative to fusion welding for mixed or multi-material joints. In military ground vehicle applications, these types of multi material joints not only undergo quasistatic and fatigue loading but also high strain rate events such as a mine blast and ballistic penetration. Adhesives that exceed 10.0 MPa in shear strength and a displacement failure greater than 3.81 mm are classified as ‘Group-1 adhesives’ as they exhibit excellent stiffness-toughness balance which are needed for high strain rate applications. In this work a multidisciplinary approach (experimental and modeling) is used to elucidate the effect of the GnP on the processing (chemistry), structure (phase separation), and properties (quasi-static, and viscoelastic) relationship of PUa based nanocomposite. A model polyurea with hard segment weight fraction (HSWF) of 20, 30 and 40 percent was developed to explore the combined effect of HSWF and nano-additions of 0.5, 1.0 and 1.5 weight percent GnP on the quasi-static and viscoelastic properties. For model PUa formulation with higher HSWF the additions of GnP on quasi static tensile and viscoelastic properties were negligible but at lower HSWF some improvement was seen in the viscoelastic properties and simultaneously improvements in strength and ductility were seen. Despite the complexity of the phase separated microstructure of the PUa and the nanocomposite, time-temperature (TTS) super position was shown to be valid for both the neat PUa’s and the PUa-GnP nano composite. Although the TTS shifts didn’t fit Arrhenius nor the WLF models, they did fit a more recently developed two-state, two-(time) scale model. Furthermore, a micro mechanical model utilizing fractional calculus-based modeling showed excellent correlation between the experimentally obtained TTS curves and the mechanical modeling for both neat and composite PUa. The micromechanical model developed utilizes a few physical properties such as modulus and relaxation time to predict material viscoelastic behavior instead of the conventional Prony series which has a large number of parameters with no relation to material properties. The micro-mechanical model parameters were evaluated at various nano-loading and hard segment weight fraction which showed that the effect of GnP was significantly less pronounced than the effect of HSWF. In addition, Single Lap Joints were used for an initial exploration of multiple formulations from both a chemistry perspective (changing isocyanate type and diamine type) and a microstructural perspective (weight fraction of hard segments). Results indicate that Group-I adhesive with cohesive failures can be achieved with PUa, showcasing the potential of PU as an adhesive. Overall, this work supports the feasibility of utilizing PUa’s in adhesive applications. The detailed characterization of PUas with varying HSWF and GnP content shows that the HSWF had a far greater effect on the properties of PUa than the additions of GnP. GnP did not have adverse or detrimental effects on the performance of the PUa. Future work can explore the advantage of GnP in creating multifunctionality to PUa such enhancing thermal and electrical conductivity. At the same time, GnP showed significant improvement in PUas with low HSWF creating a wide range of potential applications for PUa based bonded joints. Future work should also explore the high strain rate behavior of the PUa bonded joints.
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