2014 Theses Doctoral

Dynamic energy dissipation using nanostructures: mechanisms and applications

Xu, Jun

The emerging subject of mechanical impact protection at nanoscale where solids, fluids interact closely, has raised many multi-disciplinary and challenging questions which cannot be answered by the analogy from their macroscopic counterparts. A series of counterintuitive phenomena have been observed without further profound theoretical explanations. It is pretty straightforward to take advantage of these novel properties for fascinating applications which cannot be achieved by traditional materials and structures. Among various exciting areas, one of the most promising and ever expanding topics is the impact protection at nanoscale.
Primarily inspired by the excellent mechanical properties of carbon nanotube (CNT), a water-filled CNT system for impact protection is designed. The nanoconfined water molecules enhance the stiffness of the tube and also help to stabilize the tube such that the buckling force and the post-buckling plateau is higher than empty CNTs, indicating a much higher energy dissipation performance. Also, the energy dissipation performance is dependent on the aspect ratio which is similar in its bulk counterpart. Additional support may come from adjacent tubes in CNT bundle and forests and the actual energy dissipation per unit volume/mass is improved.
Unlike the tube or beam shape, once the single layer graphene is rolled into a spherical shell, it becomes buckyball whose mechanical properties are seldom studied. Thus, firstly, the quasi-static and dynamic behavior of buckyball is investigated based on MD simulation. Buckyball may be categorized according to the mechanical behavior. The force-displacement curve obey the continuum shell model prediction perfectly except for the slight difference in coefficient change due to the size effect. Interestingly, it is discovered that larger buckyball cannot recover to its original shape after unloading while the smaller buckyball can. Stronger van der Waals interaction between the buckled layer and the bottom layer may be the responsible reason for the non-recovery phenomenon. The buckled shape should have higher strain energy such that more mechanical energy is dissipated.
Motivated by the novel behavior of buckyball, the various stacking forms of buckyball are investigated. By analogy for the 1D granular energy dissipation system, a 1D long chain buckyball system is designed and studied. With relatively small elastic deformations of C60 buckyballs during impact, a modified Hertz contact model is proposed, with critical parameters calibrated via MD simulations for given impact loading conditions. The major energy dissipation mechanism for the buckyball chain is the wave reflection among the deformation layers, covalent potential energy, van der Waals interactions as well as the atomistic kinetic energy. For nonrecovery buckyballs such as C720, energy mitigation effect is more obvious. Over 99% and 90% of impact energy for C720 and C60 chain systems could be mitigated under particular impact conditions. Moreover, protective system based on short 1D vertical and horizontal alignments and various pseudo 3D stacking forms with different packing densities are also studied. It is found that stacking form with higher occupation density yields higher energy absorption and proves the available buckyball in bulk is ready for impact protection.
In addition, to quantify the material behavior, prove the recently suggested new theories and discover new phenomenon, desktop experiments serves as an important part of this thesis research. The governing factors, i.e. pre-treatment temperatures, solid-to-liquid mass ratio, particle size and electrolyte concentration are parametrically studied. Experiments show that the optimum processing pretreatment temperature is about 1000 degree; with higher zeolite mass ratio. Also, the dynamic behavior of zeolite beta/water system is investigated and the system has a better dynamic performance than quasi-static since the infiltration pressure increases due to the water molecule inertia effect.
To conclude, impact dynamics and energy dissipation/mitigation at nanoscale is a lasting topic in mechanics related research area whose knowledge is highly desire in engineering application with the advancement of material science, physics and chemistry.