Exploring the dynamics of viscoelastic adhesion in rough line contacts

Modeling the adhesion of viscoelastic rough surfaces is a recent challenge in contact mechanics. Existing models have primarily focused on simple systems with smooth topography or single roughness scale due to the co-action of roughness and viscoelasticity leading to elastic instabilities and rate-dependent behavior, resulting in complex adhesion dynamics. In this study, we propose a numerical model based on a fnite element methodology to investigate the adhesion between a randomly rough profle and a viscoelastic half-plane. Approach-retraction simulations are performed under controlled displacement conditions of the rough indenter. The results demonstrate that viscous efects dampen the roughness-induced instabilities in both the approach and retraction phases. Interestingly, even when viscous efects are negligible, the pull-of stress, i.e., the maximum tensile stress required to detach the surfaces, is found to depend on the stifness modulus and maximum load reached during the approach. Furthermore, when unloading is performed from a relaxed state of the viscoelastic half-plane, both adhesion hysteresis and pull-of stress are monotonic increasing functions of the speed. Conversely, when retraction begins from an unrelaxed state of the material, the maximum pull-of stress and hysteretic loss are obtained at intermediate velocities

Design and fabrication of ceramic beads and laminated composites for the study of stress wave mitigation

Research on theoretical models to study stress wave management has created new opportunities to design and fabricate beads and laminated composites in order to improve the material performance of engineering devices such as armors, casing for sensitive equipment, and heavy machinery. This thesis provides in detail the steps to formulate, test, and characterize the beads developed by two different approaches (sol- gel and vibration method) as well as laminated composites. Through the sol-gel method, the forming techniques and parameters for producing alumina beads using sodium alginate were developed. This simple, inexpensive, and environmentally friendly approach to producing alumina beads using bead-forming equipment occurs when a flat-tipped needle produces droplets that cross-link, forming green bodies upon contact with a CaCl2 solution. An exchange of ions takes place, where sodium alginates substitute their Na+ for Ca2+ ions to form semi-rigid bodies. Spherical ceramic beads using 50 wt% alumina suspension with 0.04 wt% polyacrylate dispersant are produced when: the viscosity of the slurry is below 0.3 Pa•s, the surface tension of the gelling solution is below 50 mN/m, and the distance of the nozzle tip to the reacting solution is approximately 3 cm. The sol-gel method approach for producing alumina beads using alginates will allow its use for any type of ceramic material, changing its chemical composition and controlling the microstructure and shape of the beads. Moreover, a second technique to make beads larger than 5 mm in diameter was established. Through the vibration approach, several types of alumina beads such as oblate, prolate, and tri-axial were produced with variable size and levels of porosity. In this approach, a formulation of 82.7 wt% alumina powder dispersed in 17.3 wt% water using 0.8 wt% ammonium salt dispersant with 0.2 wt% polyvinyl alcohol (PVA) binder was used. After the drying of the alumina slurry, the mixture becomes a paste and it is fairly solid when left at rest, but will begin to flow under applied stress. With the correct moisture content (~9 wt% water content), the alumina paste can then be placed in a vibrating table within enclosures to form beads. Through an understanding of the formation of the alumina beads, it was found that the alumina paste is a viscoelastic solid with limited strain recovery. Based on the thermal treatment process, the optimum conditions were 700 oC/1hr for calcination and 1650 oC/4hrs for sintering, where the density was 97% of the theoretical value and the compressive load of the alumina beads were 3954 + 93 N. Furthermore, the refinement step provided more insight in to mechanical performance of the alumina beads. The three stages under longer milling times revealed that during the first 50 hours at 220 rpm, the bead diameter was reduced by 0.7 %; during the second stage, the diameter remained constant; and during the third stage, the alumina bead either fractured or became misshapen. Lastly, this thesis presents the Split Hokinson Pressure Bar (SHPB) results using Brazilian disk geometry to understand the stress wave propagation in a laminated alumina/epoxy system. The impact orientation of the layered alumina/epoxy system was gradually changed every 45o from 0o to 90o in order to understand the evolution of the fracture. In addition, laminated alumina and pure epoxy samples were the control experiments to compare their dynamic responses and fracture behavior. Through careful evaluation of the tested laminate samples, it was concluded that as the angular orientation of the laminate alumina/epoxy disks increased within the tested angle of orientation from 0o to 90o, transmitted force of the laminates decreased and their failure mode changed from major delamination to minor cracks. In the case of the control experiments, the epoxy sample did not fracture because of the nature of the polymer. However, the laminated alumina sample had a minor indentation in the impacted area