In this dissertation, four distinct but in some ways related topics, mostly related to experimental and theoretical investigations of friction and adhesion of elastomeric materials, are presented. First, an experimental and theoretical study of the interaction between elastic beams and granular media under compressive loading is performed. Buckling loads of beams with different dimensions and boundary conditions within granular media of different depths and grain sizes are measured, and theoretically approximated using the Ritz energy approach, based on the concept of beam on an elastic foundation. Several nondimensional parameters and a scaling law are derived to characterize different interaction regimes between the beams and granular support. The findings from this work is believed to be helpful for improved understanding of interactions between elastic beams and surrounding elastic foundation with applications to piles, oil pipelines, and robotic needle insertion into soft tissues. Second, the role of axial compliance on the friction of extensible strips is investigated. Significant changes were observed in the static and kinetic friction of strips, when the effective axial compliance was changed. The underlying causes of the changes in the frictional response are explained and quantitatively predicted using an extended shear lag model. We believe that this study provides insights into the effect of axial compliance on the frictional response of materials, paving the way for design and optimization of systems where the static and kinetic friction forces play an important role. Third, the effect of normal force and rate on the kinetic friction of two different elastomers, namely acrylic and silicone-based elastomers is evaluated. A custom-built pendulum test setup was used to perform the friction test in dynamic conditions. Two substantially different responses with respect to the change in normal force were observed and the role of different contributions to the frictional response of viscoelastic materials, i.e. bulk hysteresis friction, adhesion friction, and cohesion friction, are discussed. Different scenarios such as modifying the surface by using graphite powder, reducing test velocity, and also performing drop tests to characterize the surface hysteresis of the elastomers, were considered to further explore the origin of frictional responses of the elastomers. This study could improve insights gained from Dynamic Mechanical Analysis (DMA) data when obtaining and interpreting the effect of normal force on kinetic COF of elastomers with potential applications to tires, shoes, etc. where friction plays an important role. Last, a generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling law, which depends on the rate of change of bond area with compliance, is in apparent discrepancy with the previously reported scaling relationship that depends on the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when failure mechanism changes or the compliance of the load train is increased. Furthermore, the shear lag model is implemented to derive a closed-form relation for the system compliance and the conditions where the two models deviate from each other are discussed and demonstrated. The results obtained from this approach could lead to a better understanding of the relationship between the bond strength and the geometry and mechanical properties of adhesive systems, with applications to different types of adhesive joints such as bio-inspired adhesive, biomedical adhesive tapes, and structural adhesive joints. / Ph. D. / In this dissertation, four distinct but in some ways related topics, mostly related to experimental and theoretical investigations of friction and adhesion of elastomeric materials, are presented. The theoretical models are based on classic solutions for load transfer between two members through shearing an adhesive layer or frictional interface on extensible support layers. First, an experimental and theoretical study of buckling of elastic columns embedded in granular media is performed. In many engineering applications, it is desirable to insert and manipulate an elastic column like needle or drill rod within complex media, such as soft tissues or granular beds like sand and gravel. In these procedures the column is subjected to axial loading and it tends to buckle and lose stability due to a high length to thickness ratio. Burrowing a flexible structure through fragile media requires understanding the coupled interactions between a geometrically non-linear structure and its reconfigurable surroundings. Several nondimensional parameters and a scaling law are derived to characterize different interaction regimes between the columns and granular support in order to better understand the stability of elastic structures confined in a granular bed. Second, a comprehensive study that combines theory and experiments to investigate frictional responses of a system, i.e. static and kinetic friction, with change in system stiffness is presented. Friction plays an important role in many technologies such as tires, brakes, rubber seals, conveyer belts, and footwear. Understanding the role of system stiffness on the frictional properties of materials, from both experimental and theoretical points of view, has important implications for such technologies. Significant changes were observed in the static and kinetic friction of strips when the effective axial stiffness was changed. The underlying causes of the changes in the frictional response are explained and quantitatively predicted by a theoretical model. Furthermore, a permanent increase in kinetic friction of sufficiently soft extensible strip was found, with potential application to improved friction performance of materials where the kinetic friction plays a major role. Third, the effect of normal force and rate on the kinetic friction of two different elastomers, namely acrylic and silicone-based elastomers, is evaluated. A custom-built pendulum test setup was used to perform the friction test in dynamic conditions. Two substantially different responses with respect to the change in normal force were observed and the role of different frictional mechanisms is discussed. This study could improve insights gained from mechanical testing data at different temperatures and speed when obtaining and interpreting the effect of normal force on kinetic COF of elastomers, with potential applications to tires, shoes, etc., where friction plays an important role. Last, a theoretical model, to predict the bond strength of adhesive systems, is developed. The proposed model, which depends on the rate of change of bond area with compliance, is in apparent discrepancy with the previously reported scaling relationship thatdepends on the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when failure mechanism changes or the compliance of the load train is increased. The conditions where the two models deviate from each other are discussed and demonstrated. The developed model could help to better understand the role of system compliance on the bond strength of adhesive systems such as bio-inspired adhesive, biomedical adhesive, and structural adhesive, where the system stiffness changes significantly depending on the applications.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/79818 |
| Date | 26 October 2017 |
| Creators | Rezaei Mojdehi, Ahmad |
| Contributors | Engineering Science and Mechanics, Dillard, David A., Holmes, Douglas P., Boreyko, Jonathan B., Williams, Christopher B., Long, Timothy E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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