The use of energy absorption materials and structures for protection in collision, explosion, and impact attacks has long been recognized as one of the most effective approaches to reduce and prevent personnel injuries and infrastructure damages. These systems have been widely used in many industrial, medical, and military applications. Recently, an advanced energy absorption material, liquid nanofoam (LN), has been developed with high energy absorption capacity as well as high energy mitigation rate. The LN system, composed of a liquid phase and a hydrophobic nanoporous media, employs the pressurized liquid flow in nano-channels as its energy absorption mechanism. However, previous studies of the LN mainly focused on the quasi-static behaviors. Only limited effort had been made to understand the working mechanism of the LN under dynamic impacts which are the practical loading condition in scenarios such as auto collisions, blunt impacts and blasts. This dissertation presents the first systematic experimental study on the dynamic behavior of the LN system and reveals the deformation mechanism of LN under high strain rates. These scientific findings open up new applications of the LN functionalized materials and structures.The intermediate and high strain rate responses of LN systems have been characterized by a lab-customized drop tower apparatus. The competition between liquid infiltration and porous structure deformation at high strain rates has been elucidated at nanoscale. Results show that liquid infiltration into nanopores is independent of the axial buckling stress of the nanopore, and thus is the dominating deformation mechanism of the LN. More importantly, the activation of liquid infiltration as well as liquid flow in nanopores are much faster than the nanoscale porous structure deformation. This much-enhanced liquid flow speed in nano-environment is experimentally quantified for the first time. It has been demonstrated that the liquid infiltration speed is adaptive to the impact energy level, which provides mechanistic explanation for the high energy absorption efficiency of LN at high strain rates. Results also suggest that LN in the liquid marble form performs better than the liquid form upon high strain rate impact due to the macroscopically homogenous structure in the liquid marble form.Based on the fundamental understanding of the deformation mechanism and the adaptive nanoscale liquid flow, LN has been integrated into other materials and structures to generate multifunctional materials and structures, e.g. LN-filled tube (LNFT), hybrid hydrogel, and advanced seat belt retractor system. In LNFT, LN is utilized as a novel filling material in thin-walled tube. The resulted LNFTs possess enhanced average post-buckling strength and energy absorption capacity due to the "perfect bonding" between the LN and the tube wall. Also, based on the adaptive nanoscale liquid flow, the LNFT is more efficient for energy mitigation at elevated strain rates. In LN-based hybrid hydrogels, LN is formulated and encapsulated in hydrogel by integrating nanoporous particles into the 3D polymer network. Liquid infiltration mechanism, combined with the chemical and physical cross-linking effects, leads to the improvement of both strength and toughness of the hybrid hydrogel, which is not seen in current hydrogels. In LN-based seat belt retractor system, LN is employed as the load-bearing component, which allows additional payout tunability, adaptability, and reusability in the system.The knowledge gained in this study will facilitate the design of next generation of advanced LN-functionalized materials and structures for extreme working conditions.
Read
