The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

The observed multiscale phenomenon of rough surfaces, i.e. the smaller mountains mount on the bigger ones successively, renders the hierarchical structures which are described by the fractal geometry. In this situation, when two rough surfaces are loaded together with a higher load, the smaller asperities will undergo plastic flow and immerge into the bigger asperities below them. In other words, the higher load needs to be supported by the bigger asperities. However, when the GW model was proposed in 1966, its analytical method considered that the length-scale of asperities is fixed, which is independent of load (or surface separation). In such condition, the analytical results for a specific asperity length-scale can only suit the situation of a certain narrow range of load. In this research, a new model, called the multiscale GW model, has been developed, which takes into account the relationship between the load and the asperity length-scale. At first, based on the Nayak¡¦s model the multiscale asperity properties with different surface parameters have been derived, and based on the material yielding theory a criterion for determining the optimal asperity length-scale, which functions as supporting the load, is developed. Then both of the above are integrated into the GW model to build the multiscale GW model. The new model is compared with traditional one qualitatively and quantitatively and show their essential differences. The effects of surface parameters and material parameters are discussed in this model. Finally a comparison with the experiment is made, and reveal the good coincidence.

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This thesis describes a series of fretting fatigue experiments carried out under closely controlled conditions of partial slip. These experiments confirm the existence of a size effect whereby the fretting fatigue life of an aluminium alloy is shown to vary with contact size. The configuration chosen, of cylindrical fretting pads contacting a plane specimen is amenable to classical stress analysis and the surface tractions between the contacting bodies are derived. The effects of tension in the specimen, finite specimen thickness, differing elastic constants, and surface roughness are all investigated and incorporated into the analysis where appropriate. A technique is then developed to calculate stress intensity factors for plane cracks growing under the contact load at an arbitrary angle to the free surface. The analysis is then applied to the experimental results and three possible explanations for the size effect are proposed, based on statistical effects, crack arrest, and crack initiation. These are examined in the light of the experimental evidence and it is proposed that the variation of fatigue life with contact size is due to an increase in the amount of fretting damage above a threshold level for crack initiation. A composite parameter is chosen to characterise the severity of fretting conditions and this is shown to describe the experimental results accurately. Finally, the use of this parameter in design calculations is discussed.

The remarkable attachment system of geckos has inspired the development of dry microfiber adhesives through the last two decades. Some of the notable characteristics of gecko-inspired fibrillar adhesives include: strong, directional, and controllable adhesion to smooth and rough surfaces in air, vacuum, and under water; ability to maintain strong adhesion during repeated use; anti-fouling and self-cleaning after contamination. Given these outstanding qualities, fibrillar adhesives promise an extensive range of use in industrial, robotic, manufacturing, medical, and consumer products. Significant advancements have been made in the design of geckoinspired microfiber adhesives with the characteristic properties listed above, with the exception of the anti-fouling and self-cleaning features. The self-cleaning mechanism of the gecko’s adhesion system plays an important role to its ability to remain sticky in various environments. Similarly, enabling self-cleaning capability for synthetic microfiber adhesives will lead to robust performance in various areas of application. Presently, the practical use of fibrillar adhesives is restricted mainly to clean environments, where they are free from contaminants. The goal of this thesis is to conduct a detailed study of the mechanisms and mechanics of contact-based self-cleaning of gecko-inspired microfiber adhesives. This work focuses on contact self-cleaning mechanisms, as a more practical approach to cleaning. Previous studies on the cleaning of microfiber adhesives have mostly focused on mechanisms that involve complete removal of the contaminants from the adhesive. In this thesis, a second cleaning process is proposed whereby particles are removed from the tip of the microfibers and embedded between adjacent microfibers or in grooves patterned onto the adhesive, where they are no longer detrimental to the performance of the adhesive. In this work, a model of adhesion for microfiber adhesives that take the deformation of the backing layer under individual microfiber is developed. The dependence of adhesion of microfiber adhesives on the rate of unloading is also modeled and verified using experiments. The models of adhesion presented are later used to study the mechanics of contact self-cleaning of microfiber adhesives. Three major categories of self-cleaning are identified as wet self-cleaning, dynamic self-cleaning, and contact self-cleaning. A total of seven self-cleaning mechanisms that are associated with these categories are also presented and discussed. Results from the self-cleaning model and experiments show that shear loading plays an important role in self-cleaning. The underlying mechanism of contact self-cleaning due to normal and shear loading for spherical contaminants is found to be the particle rolling between the adhesive and a contacted substrate. Results from the model and experiments also show that small microfiber tips (much less than the size of the contaminants) are favorable for self-cleaning. On the other hand, large microfiber tips (much larger than the size of the contaminants) are favorable for anti-fouling of the microfiber adhesive. Results from this work suggests that the sub-micrometer size of the gecko’s adhesive fibers and the lamellae under the gecko toes contribute to its outstanding self-cleaning performance. The results presented in this thesis can be implemented in the design of microfiber adhesives with robust adhesion, self-cleaning and anti-fouling characteristic, for use in numerous applications and in various environments.

Rail corrugation is a periodic wear pattern that develops on the wheel and rail contacting surfaces in railway systems. It is a commonly observed phenomenon worldwide and is a serious problem because it causes degradation of the track and its components and is a significant source of noise. Currently the only reliable method of ameliorating the negative effects of rail corrugation is to periodically regrind the rail surface to a smooth profile, at great expense to the railway operator. It is therefore of interest to investigate other possible control strategies to reduce corrugation growth through an understanding of the mechanism of corrugation formation. This thesis presents an investigation into the effect of speed variation on the corrugation formation mechanism. The research presented is intended to highlight the significant role that speed variation has on corrugation formation via a disruption of the feedback mechanism which leads to corrugation growth over successive train passages. This discovery motivates the investigation the feasibility of altered speed variation as a novel corrugation control method, due to the large effect that the variance of train speed has on corrugation growth rates. The effect of variable pass speed on corrugation formation has been investigated in this thesis through the use of efficient models of corrugation formation in straight track and cornering conditions. These models are simple enough to readily perform corrugation control studies without neglecting any relevant physics, obscuring the corrugation formation mechanism with overly detailed modelling or imposing a significant computational burden on performing control studies. These novel models have been outlined and their predictions elucidated in detail. A theoretical investigation into the effect of speed variation in the presence of a resonance free mechanism for corrugation growth via a contact filter has been performed and shown to only be important when the dynamic wavelength of formation approaches the size of the contact patch. The results of these corrugation models have been validated via test rig and field experiments. An investigation of the effectiveness of speed variation as a corrugation growth control measure has also been investigated via test rig experiments. The results of this thesis have formed the basis of an industry supported field trail of this technique for corrugation mitigation that is currently in progress.

Thesis (M.S.)--University of Alabama at Birmingham, 2008. / Description based on contents viewed Oct. 8, 2008; title from PDF t.p. Includes bibliographical references (p. 77-82).