Master of Engineering (ME)

Master of Engineering (ME)

Master of Engineering (ME)

<p>Document is missing page 93.</p> / <p>This thesis describes a photographic investigation of saturated nucleate boiling. Five different fluids were boiled on an oxide coated glass surface at various levels of heat flux. The nature of the boiling surface was such that the boiling phenomenon could be photographed through the surface from below. Active nucleation sites were identified and counted by analysis of still photographs obtained using this technique. Active nucleation site density data obtained from the analysis was correlated with surface temperature using the Gaertner site activation theory which requires the use of two arbitrary constants No and o. It is demonstrated that for the particular surface used, the product irrespective of the fluid boiled so that the relationship. satisfactorily predicts the active nucleation site density.</p> / Master of Engineering (MEngr)

<p>The accuracy of work on N.C. machine tools is much more affected by weight, clamping and thermal deformations as well as by variations in accuracy throughout the entire working zone than that of machine tools using other modes of operation.</p> <p>It is therefore very important to precisely define the concept of accuracy and the methods of testing and evaluating it.</p> <p>In this thesis the shortcomings and even possible fallacies of the classic form of accuracy evaluation procedure are discussed, together with an appreciation of recent significant contributions to the field which utilise modern instrumentation and high frequency response transducers in sound and informative experimental procedures.</p> <p>A new system is developed where the problem of formulating a comprehensive and meaningful machine tool accuracy capability statement is practically solved by directly relating the accuracy of the machine tool to the accuracy of a workpiece machined on it. This is achieved by deriving a tolerance law applicable to the evaluation of machine tool translative deviations where the law relates to the practical system of tolerancing a part dimension as well as to the usual mode of use of an N.C. machine.</p> <p>An extensive of the basic tolerance law provides for a comprehensive statement of accuracy achievable throughout any defined two-or three-dimensional working zone of a machine tool. For every type of machine tool configuration there is a finite (minimum) number of translative deviation measurements that is required to be evaluated only along well defined lines located at extreme offsets of the tool with respects to each moving member of the machine.</p> <p>Additionally the procedure for including in the machine tool capability statement the adverse "further" effects of weight, clamping and thermal deformations is outlined.</p> <p>While it is comparatively straight-forward to formulate tests such that the effect of weight and clamping deformations may be defined, the problem of correctly specifying a thermal test cycle warranted a rather more extended study.</p> <p>It is shown how the correct identification and classification of the type of heat source affecting a machine tool's structural components leads to rather precise knowledge of the particularly affected translative deviation measurements. The design of the test cycle, the general laws governing the specification of the tests and the test procedure, are developed by reference to many practically obtained measurements complemented by computational studied of simplifies and simulated machine tool structures.</p> <p>Two computational procedures are developed for the calculation of steady-state and transient temperature fields together with the resulting thermal deformations: a lumped-mass/time-continuous technique the solution of which leads to an eigenvalue problem, and the finite-element approach to the problem. In each case a step by step solution technique is outlined, and a Fortran IV computer program listing is included in the Appendix for the finite-element analysis of two-dimensional structures using triangular elements.</p> <p>Finally, an investigation is made of new concepts of errors of an axis of rotation. The experimental technique results in the generation of a polar trace on an oscilloscope screen, the form of which relates directly to a Talymonogram which could be obtained from a machine specimen if there were no unwanted disturbances of the cutting process. In this way, an exact picture of the influences of spindle rotation errors on the workpiece accuracy is obtained by an idle run test at normal spindle speeds without machining. The design of a comprehensive electronic instrument is included in the Appendix together with details of associated instrumentation.</p> / Doctor of Philosophy (PhD)

<p>One of the central unsolved problems of engineering design is prediction of the probability distribution of fatigue life of structural members and machine components. Fatigue data scatter is a manifestation of the probabilistic nature of fatigue failure. A relationship is known to exist between the distributions of the number of cycles-to-failure and the applied stress through the probabilistic stress-life (S-N) curve.</p> <p>A method was developed for predicting the scatter of fatigue life for structures and components subjected to constant, complex or narrow band random amplitude cyclic loading. The method provides a unique way of predicting the statistical properties of fatigue life using a Monte Carlo simulation technique. In this method the randomness of material properties as well as that of the applied loading are incorporated into a stochastic model in conjunction with appropriate fatigue failure criterion to determine the fatigue life distribution.</p> <p>A modified probabilistic S-N diagram is used to reflect the variability of the endurance limit and the fatigue strength coefficient. Any damage criterion can be used with the simulated S-N diagrams. When the linear damage law is used, the analysis yields a damage criterion analogous to Miner's rule but with a random cycle ratio. The proposed method was used to analytically predict the characteristics of the cycle ratio distribution. Good agreement with available experimental results is obtained. This result is of particular importance since no assumption regarding the fatigue failure mechanism is made. A phenomenological treatment of damage accumulation below the endurance limit is presented.</p> <p>The validity of the method is demonstrated for different load histories and materials. Fatigue life data for axial and bending test specimens of SAE 1008 steel sheets subjected to constant amplitude, block and narrow band random loading were generated experimentally. The distributions of the endurance limit, ultimate strength and fatigue strength coefficient were obtained. The endurance life and cyclic stress-strain curve were also determined experimentally. The suggested method has proved successful in predicting fatigue life distributions for each case.</p> <p>The effects of the randomness of loads, material properties, and the shape of load probability density function were investigated. This study indicates that the large scatter exhibited in fatigue test data is mostly explained by the scatter in the material properties. It was found that the randomness of the fatigue strength coefficient does not significantly affect the scatter of predicted life or its estimated mean value. On the other hand, the variance of the endurance limit has a pronounced effect on the variability of fatigue life. Its effect on the estimated mean is also considerable.</p> <p>It was shown that the scatter in fatigue life under random loading is less than that under sinusoidal constant amplitude loading. The mean value of fatigue life decreases slightly as a result of increasing the variance of the applied load. Changes in the shape of the load probability density function affect the scatter of life more than its mean value. The predicted fatigue life distribution under random loading depends upon the accuracy of the load probability density function.</p> <p>The method has the capability of simulating complex load histories of the kind that occur in service. It can also be extended to include possible correlation between peaks of the applied load.</p> <p>The suggested method is believed to be an efficient tool that can be used at the design stage; it is ideal for parametric studies and provides an attractive alternative to the costly and time consuming prototype fatigue tests. Minimum experimental data is needed for the analysis.</p> <p>It is concluded that implementation of this new technique will substantially improve the ability of the engineer to design reliable fatigue resistant components efficiently.</p> / Doctor of Philosophy (PhD)

<p>The purpose of this thesis is to discover the mechanism of excitation and methods of alleviation of self-excited vibrations in a swing check valve following rapid pump shut-down. The problem was first encountered when the valve manufacturer incorporated an adjustable spring-damper into the original design to prevent its violent slamming. Tests on the modified design showed that, rather than eliminate the slamming, the valve disc bounced several times on its seat at a well-defined frequency. With increased damping the number of oscillations as well as the amplitude increased while the frequency decreased. For sufficiently high damping a stable limit cycle oscillation is established. This limit cycle oscillation continued until the valve pivot shaft pins failed. These vibrations are clearly hydroelastic in nature, the oscillations being perpetuated through a transfer of energy from the fluid flow.</p> <p>A two-dimensional geometrically-similar model of the valve was constructed with perspex sides for flow visualization. A central portion along the base of the model was also laminated with perspex to allow the projection of a collimated sheet of light. Aluminium powder tracer preparation was injected into the flow and cine-photography of the flow during vibration carried out. In addition, dynamic measurements of upstream and downstream pressures, valve angular displacement and the load on the damper arm were synchronized with the films. The data collected in this way for a number of restraining spring rates and initial spring deflection angles allowed a detailed stability map of the valve's dynamic behaviour to be plotted. The essential characteristics of the instability observed in the model are the same as those found in the prototype valve tests although the model was not scaled dynamically. This was necessary in order to guarantee the structural integrity of the model over the long period of tests.</p> <p>The results of the research show that there is a sudden increase in the hydrodynamic closing load as the valve approaches its seat, primarily as a result of the changing discharge characteristics. Although upstream and downstream waterhammer waves are produced as the valve slams onto its seat, the valve responds only to the pressure difference across it. It remains closed until this pressure difference reduces to the point where it either cracks the valve open or allows the damper spring to pull it open. On the opening part of the vibration cycle the hydrodynamic closing load is substantially lower than the load at the same angle during closing. This hysteretic effects shows that there is a net energy input from the fluid during each cycle and the motion is perpetuated.</p> <p>Tests on the model further show that if the damping spring is stiff enough to eliminate the slamming, either the valve will never close or it will exhibit limit cycle oscillations. Clearly, neither alternative is acceptable. Based on the aforementioned results, it was realised that another possible means of alleviating the problem is to alter the discharge characteristics of the valve at small angles of closure by suitable changes in geometry. In the second part of the thesis, a number of such changes were made in the model and the experiments repeated. It was discovered that by making the rate of change of discharge a more gradual function of the valve closure angle, the dynamic instability in the model could be entirely eliminated.</p> / Doctor of Philosophy (PhD)

<p>The maching of hardened steels is becoming a viable technology. At the inception of the present research, this technology for milling processes was in its infancy. Advancements in cutting tool materials such as poly-crystalline cubic boron nitride (PCBN) have enhanced the ability to machine these difficult to cut alloys. The machining of hardened tool steels have been explored in the recent literature to a great extent because of the possible gains and benefits this technology has to offer. Near net shape manufacturing can become a viable technology and save the die and mold industry considerable investment by reducing the lead time for production of dies and molds.</p> <p>In this thesis, an experimental investigation is presented regarding the optimal process parameters that make high speed hard machining a viable technology. The experimental investigation shows that hard machining of AISI H13 (55 HRc) is possible, and is an extremely effective technology if the proper conditions are used. Because of the nature of hard machining, the fundamental aspects of chip formation, tool wear and life is explored through a detailed investigation. It was confirmed that the ball milling of hard materials produce segmented saw toothed chips even at low chip loads for specific cutting conditions. It was noticed that the chip morphology was very repeatable and consistent and therefore formed a preliminary basis for the modelling strategy. Tests were also performed on hardened AISI D2 tool steel (62 HRc), which showed that this material in its hardened state challenges the ability to machine this material in the fully hardened state. The primary tool failure modes are outlined and a detailed analysis of the chip formation mechanisms is reviewed.</p> <p>Owing to the difficulty associated when machining hardened AISI D2 tool steel, the development of an analytic force model was attempted. The modelling methodology required a correlation of the flow stress (the mechanical response of the material) with the cutting conditions in the form of kinematic parameters derived from chip morphology. The hardened material was characterised using high strain rate ballistic impact tests (using the compressive split Hopkinson pressure bar) in a punching shear configuration. This configuration was chosen as it represented the shearing process in metal cutting more accurately in terms of strain and strain rate. The tests were modified and performed on the fully hardened tool steel. The ultimate result was a precise representation of the flow stress of the hardened material in shear as a function of the strain, strain rate and temperature where the fitted correlations represented the experimental data with an accuracy of approximately 10%. The temperatures in this part of the investigation exceeded 600 °C with strains in excess of unity and strain rates approaching 50000 s^-1.</p> <p>Having a correlation of the flow stress of the fully hardened material, the force model was derived using the chip morphology and chip formation kinematics to represent the governing strain and strain rate conditions during machining. The resulting formulations allowed for a time domain orthogonal machining simulation represented by specific inputs such as cutting speed and feed rate. The orthogonal formulation was verified against experimental data and showed good correlation with observation.</p> <p>The orthogonal formulation was extended to the ball milling process (an oblique cutting configuration) to test the validity of the force model. Good correlation was realized between the experimental and predicted results. The ball milling process challenged the validity of the force model by applying the modelling strategy to small chip loads and low cutting speeds. The predicted results also rationalized the tool failure mode that was observed in the ball milling investigation.</p> / Doctor of Philosophy (PhD)

<p>Detailed flow field measurements were performed to investigate the development of the three-dimensional wall jet, the effect of initial conditions on the development of wall jets and the evolution of the large-scale vortex structures that causes the large lateral growth rate in the flow. Single-point measurements in the wall jet exiting a contoured nozzle indicated that there were two regions in the intermediate field 10 ≤ x/D ≤ 40. The flow underwent a significant change in the region 10 ≤ x/D ≤ 20 as it adjusted to the wall. The changes of the profiles of the moments and the reorientation of the regions of the mean streamwise vorticity slowed down in the region beyond x/D=20 and were not apparent by x/D=40. The comparison of measurements of wall jets exiting the contoured nozzle and the fully developed long pipe indicated that changes in intial conditions do affect the jet half-widths and the decay of the maximum streamwise velocity through near and intermediate fields. The differences in growth rates of wall jets, profiles of moments and contours of the mean streamwise vorticity in the two jets were reduced in the region after x/D=10 and were not apparent in the region beyond x/D=20 - 30. Measurements of two-point, two-time correlation of the streamwise fluctuating velocity indicated that the large-scale vortex structures that consist two pair of horseshoe vortices in the flow continued to develop throughout the intermediate field. In particular, the inner vortex structures were induced towards the wall by the outer structures and the legs of the outer structures inclines relative to the streamwise direction as the flow evolved downstream. Measurements also indicated that the flow below the outer region of the streamwise vorticity was laterally convected faster than the outer vortex structures.</p> / Doctor of Philosophy (PhD)

<p>The dynamic response of the railway track modelled as a continuously supported beam on a Kelvin type foundation and subjected to an axial force and time dependent moving loads is studied. The transient and steady state solutions are found for the general case including all linear effects. This study shows the effects of axial force and damping on the dynamic response. The results also show that the effect of the velocity of the moving load on the dynamic response is small, and hence it is not necessarily to consider the wave type expression to study the effect of track elasticity on the dynamics of railway vehicles.</p> <p>An analysis for the dynamics of a railway vehicle including the effect of vertical track elasticity is presented, with particular emphasis on the lateral stability and the response to vertical track irregularities. The model used in the analysis is that of a six-axle locomotive of the type commonly used in North America. Wheel tread profile parameters, gravity stiffness effects and creep forces are included in the mathematical model.</p> <p>The results show that an increase in vertical track elasticity results in a small increase in the critical speed at which hunting instability occurs. The increase in track elasticity results in appreciable increase in the amplitude of the response to track irregularities especially at high frequencies.</p> <p>A method for the minimization of the vibrations transmitted due to track irregularities using the minimax principle and mathematical programming techniques is suggested. The method is demonstrated by considering the minimization of the lateral cab acceleration within the frequency range of interest.</p> <p>The analyses and digital computer simulations are viewed as analytical tools for studying the effect of changing the vehicle and/or track parameters on the dynamic response of both the vehicle and the track. The methods developed are general and can be used in the design stage to adjust geometry and/or suspension characteristics for any proposed design of a railway vehicle.</p> / Doctor of Philosophy (PhD)