One of the most important branches of applied mechanics is the theory of plates - defined to be plane structural elements whose thickness is very small when compared to the two planar dimensions. There is an abundance of plate theories in the literature modeling classical elastic solids that fit this description. Recently, however, there has been a steady growth of interest in modeling materials with microstructures that exhibit length-scale dependent behavior, generally known as Cosserat elastic materials. Concurrently, there has also been an increased interest in the construction of reduced dimensional models of such materials owing to advantages like reduced computational effort and a simpler, yet elegant, resulting mathematical formulation. The objective of this work is the formulation and implementation of a theory of elastic plates with microstructure. The mathematical underpinning of the approach used is the Variational Asymptotic Method (VAM), a powerful tool used to construct asymptotically correct plate models. Unlike existing Cosserat plate models in the literature, the VAM allows for a plate formulation that is free of a priori assumptions regarding the kinematics. The result is a systematic derivation of the two-dimensional constitutive relations and a set of geometrically-exact, fully intrinsic equations gov- erning the motion of a plate. An important consequence is the extraction of the drilling degree of freedom and the associated stiffness. Finally, a Galerkin approach for the solution of the fully-intrinsic formulation will be developed for a Cosserat sur- face analysis which will also be compatible with more traditional plate solvers based on the classical theory of elasticity. Results and validation are presented from linear static and dynamic analyses, along with a discussion on some challenges and solution techniques for nonlinear problems.One of the most important branches of applied mechanics is the theory of plates - defined to be plane structural elements whose thickness is very small when compared to the two planar dimensions. There is an abundance of plate theories in the literature modeling classical elastic solids that fit this description. Recently, however, there has been a steady growth of interest in modeling materials with microstructures that exhibit length-scale dependent behavior, generally known as Cosserat elastic materials. Concurrently, there has also been an increased interest in the construction of reduced dimensional models of such materials owing to advantages like reduced computational effort and a simpler, yet elegant, resulting mathematical formulation. The objective of this work is the formulation and implementation of a theory of elastic plates with microstructure. The mathematical underpinning of the approach used is the Variational Asymptotic Method (VAM), a powerful tool used to construct asymptotically correct plate models. Unlike existing Cosserat plate models in the literature, the VAM allows for a plate formulation that is free of a priori assumptions regarding the kinematics. The result is a systematic derivation of the two-dimensional constitutive relations and a set of geometrically-exact, fully intrinsic equations gov- erning the motion of a plate. An important consequence is the extraction of the drilling degree of freedom and the associated stiffness. Finally, a Galerkin approach for the solution of the fully-intrinsic formulation will be developed for a Cosserat sur- face analysis which will also be compatible with more traditional plate solvers based on the classical theory of elasticity. Results and validation are presented from linear static and dynamic analyses, along with a discussion on some challenges and solution techniques for nonlinear problems.

L’allégement des structures aéronautiques conduit à associer par stratification les composites aux métaux : on parle alors de multi-matériaux. L’assemblage mécanique des empilages nécessite au préalable des opérations de perçage qui s’effectuent majoritairement sur Unité de Perçage Automatique (UPA). L’objectif des travaux présentés dans ce mémoire est d’optimiser les opérationsde perçage effectuées sur UPA dans des multi-matériaux CFRP-7175-TA6V. Trois axes de recherche ont ainsi été étudiés. Le premier concerne l’optimisation de l’outil. L’utilisation d’une méthode de conception adaptée a conduit vers plusieurs pistes d’améliorations de la géométrie d’un foret. Le deuxième axe traite de la modélisation du perçage vibratoire. Cette méthode consiste à ajouter un mouvement de vibration axiale, au mouvement de coupe. Le dernier axe développe la technique du perçage auto-adaptatif. Une nouvelle méthode est proposée pour identifier les différents matériaux constituants l’empilage. / The weight reduction of aero structures has led to use composite materials combined to metallicparts to form multilayer materials. Stacked materials are drilled in one-shot during the assemblyprocess. The objective of this work is to find optimised parameters to drill efficiently CFRP-7175-TA6Vmaterial stack using Automatic Drilling Units (ADU). Three research areas have been explored. Thefirst one concerns drill bit optimisation. A customized functional analysis had led to several toolimprovements. The second area focuses on vibration-assisted drilling. This method consists in addinga reciprocating axial displacement. Formerly under ribbon form, the chips become well broken withthe vibrations and their evacuation gets better. At last, the self-adaptive drilling technique is studied.A new methodology for real-time material identification is proposed.

Excessive vibration exerted on the human body can cause many harmful phenomena that can result in permanent bodily damage or permanent disability. Human vibration is classified into two main categories: Hand,arm vibration and whole,body vibration. Hand,arm vibration is vibration transmitted through a percussive tool handle via the hand,arm system to the rest of the body. The main diseases concerning hand,arm vibration are Vibration White Finger (VWF), neurological diseases in the hand and fingers and musculoskeletal diseases like carpal tunnel syndrome. These diseases, especially VWF and musculoskeletal disorders, are mainly associated with lower frequencies. VWF in particular is more likely to occur when an operator is subjected to vibrations with high magnitudes in the 25-40 Hz region. The operating frequencies of most rock drills vary between 30 and 50 Hz. Although there are many other contributing factors like grip force, hand temperature and subject variability, prevalence of VWF among rock drill operators is relatively high in the world. The situation in South Africa is not yet very clear, and further research must be done to evaluate the current status of VWF in South Africa. Vibration energy at higher frequencies can be attenuated with rubber grips or gloves, and these types of dampers are already available on the market. The problem concerning the operating frequency of the drill has not yet been successfully addressed, and thus remains a problem in the rock drill industry as it is at the moment. The main objective of this thesis was the development and testing of a concept that can potentially be implemented on a rock drill to attenuate the operating frequency of a rock drill. The concept must be able to account for minor changes in operating frequency on a specific drill. In addition to that, the drill operating frequency varies from one drill to another. The thesis also aims to lay the mathematical foundation to design an attenuating handle for a specific drill with a specific operating frequency. All the objectives must be obtained without noticeable sacrifices in drill control or performance. The thesis includes the concept generation, optimisation, design and manufacture of a rock drill vibration absorber. The absorber has been tested, and the results are presented. / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

No description available.

Master of Science

Lubrication is used during the drilling of aluminum to counter the formation of a built-up-edge (BUE), among other reasons. The elimination of the use of lubricants in drilling of aluminum is important because of the associated high costs of cleaning and disassembly involved in lubrication. The optimal conditions sought in this work include the elimination of the use of lubricants along with the possible attainment of a high material removal rate, which could help in reduction of cost and increase productivity at the same time. BUE has been found to be almost always present in the process of metal cutting at low to moderate speeds. It has been found that a necessary condition for the formation of a BUE is the presence of a negative stress gradient away and normal to the tool rake face. The quantitative equivalence of the effects of temperature and strain rate on flow stress described by the Zener-Hollomon parameter (Z) [5]. The relationship between the Zener-Hollomon parameter (Z) and chip flow stress implies that a negative Z gradient could be considered equivalent to a negative stress gradient. A series of computer simulations with varied cutting conditions were analyzed to determine the combination of machining variables which yielded a low predicted BUE preferably with a high material removal rate. The results presented here include cases which have a low predicted BUE as well as a high material removal rate.

Among semi-destructive methods of measuring residual stresses in elastic materials, the blind hole-drilling strain-gage method is one of the best because it is simple, economical and accurate. It is based on the measurement of strains disturbed by machining a small diameter shallow hole in the test piece. The strains measured in three known directions permit the determination of the direction and magnitude of principal stresses and subsequently of any stress in any direction. This thesis presents the investigation of residual stresses in the longitudinal direction of a welded fabricated steel tube of 22 inch diameter, relating to a series of holes drilled in one half of a circular section of the tube. An initial assumption, substantiated later, was the existence of a uniform field of residual stresses through the thickness of the tube. Several methods for determining calibration coefficients are documented. The values of longitudinal stresses once computed are presented in a smooth curve. A straight line approximation is reconnnended for use in further studies of the effects of residual stresses on failure loads.

A thesis submitted to the Faculty of Engineering and the Built Environment in fulfilment of the requirement for the Degree of Masters of Science, University of the Witwatersrand, Johannesburg, 2019 / Drilling Mud holds an important role in the drilling process in such a way that it is a determinant key to the success of the operation as well as the money spent throughout the process. Indeed the success and the cost of the operation can be severely impacted by some challenges experienced while drilling such as temperature and pressure conditions which leads to fluid loss, fluid deterioration...As a result there is a need to formulate a fluid with desirable rheological properties to withstand such undesirable parameters. Therefore this work was aimed to improve emulsion drilling fluids (EDFs) based nanoparticles with enhanced properties. Many investigations were performed to find a proper emulsion stability as well as a good drilling fluid performance. The stability of the prepared emulsion drilling fluids was done using surfactant with different concentrations for several days. After several days of preparation, the EDFs containing DTAB as surfactant have showed a better emulsion stabilizer compared to the Triton X-100 ones. In addition an investigation combining both NPs and surfactants confirmed the used of NPs to improve DF and revealed the effective use of ZnO NPs for drilling fluids application and preferentially with DTAB as surfactant. Following that result, the 2nd part of the work was based on the synthesis and characterization of CNCs as NPs to formulate EDF with DTAB as surfactant. The CNCs NPS were successfully obtained via the method of oxidation of microfibrillated cellulose through TEMPO-mediate and after characterization using TEM, spherical NPs with small size varying from 10-50nm were observed. The FANN® Model 35 viscometer served to display the behavior of the shear stress and viscosity of the prepared fluids against variable shear rate at variable NPs and temperature concentration. The rheological and filtration properties were increase with increase in CNCs content from 0.8 to 1.2% of fluid in room temperature and with an increase in temperature. / PH2021

No description available.

This thesis was submitted to the School of Graduate Studies in partial fulfillment of the requirements for the Degree Master of Applied Science. / <p>Accuracy and surface finishes play an important role in modem industry. However, the deformability of ductile materials induces challenges in achieving high accuracy and surface finish. Undesired projections of material produced during metal cutting, known as burrs, significantly reduces the accuracy of the parts and affect both the assembly process and product quality. Around 30% of total production costs are used for deburring processes. This thesis presents one modem and promising method in reducing burr size through the use of ultrasonic assistance. With ultrasonic assistance, high frequency vibrations are added in the feed direction during cutting. In particular, ultrasonic assisted drilling of 1100-0 aluminum using high speed steel standard twist drills was investigated.</p> <p>Two simulation studies were conducted. Firstly, a finite element model of orthogonal cutting with and without ultrasonic assistance was developed. The results predicted that ultrasonic assistance should produce smaller burrs for the simulated operating conditions. Secondly, a drilling exit burr model was created based on a circular plate deflection model. Unfortunately this simulation failed to predict the experimentally observed burr sizes and thus the model requires further development.</p><p>To provide the ultrasonic vibration a preloaded workpiece holder and a drive circuit were designed and implemented for use with a commercial piezoelectric actuator. This equipment was cost effective (costing about 400 CAN$) and functional.</p><p>The effects of ultrasonic assistance under different vibration and cutting conditions were investigated experimentally. The experimental results demonstrated that, for certain combinations of vibration frequency and amplitude, burr size reduction can be achieved. Under these conditions the ultrasonic impact actions become significant, causing chip segmentation and smaller burrs. The results also show that ultrasonic assisted drilling allows a higher spindle speed and feed to be used without increasing burr size. However, ultrasonic assistance has a negative impact on tool life. Compared with conventional drilling, chipping of the chisel edge and greater wear of the cutting lips was observed after drilling 10 holes. This situation may be improved in the future if drills with suitable coating are used in place of the standard high speed steel drills.</p> / Thesis / Master of Engineering (ME)