DSpace Program:
http://hdl.handle.net/123456789/125
2020-07-07T23:12:43ZThermal Conductivity of Porous Sintered Metal Powder
http://hdl.handle.net/123456789/1092
Title: Thermal Conductivity of Porous Sintered Metal Powder
Authors: Ahmed Hussain Al-Saiafi
Abstract: Thermal conductivity is an important property in determining the flow of heat through porous media. The complexity of the microstructure of the solid matrix in po-rous sintered metal powder makes it difficult to come with general formulas that accu-rately predict thermal conductivity. The effective thermal conductivity of porous me-dia is usually predicted using simplified models based on parallel, series, and a combi-nation of parallel and series thermal resistances. All these models end up with equa-tions where porosity is used as an average weighting factor in different ways. In some cases, additional fitting parameters are used to fit specific experimental data and to improve accuracy.
The primary objective of this study is to develop more accurate semi-empirical thermal conductivity models for porous sintered metal powder. Sintered porous cop-per experimental data that are available in the literature were used to determine the degree of accuracy of several thermal conductivity models. The classical parallel, clas-sical series, and a combination of parallel and series models were investigated, and the assumptions behind them were re-examined. Analytical analyses were carried out to adjust the classical parallel and classical series models with parameters to fit the exper-imental data. The results of the analytical analyses reveal the effective cross-sectional area of the solid-phase as a possible fitting parameter. Using experimental data, the effective cross-sectional area was then calculated, and curve fitted to calibrate the theoretical models and obtain accurate semi-empirical correlations.
The secondary objective of this study is to develop a two-dimensional conduction heat transfer numerical model to predict the thermal conductivity. The numerical model gives more insight into the complex heat flow through a porous medium; how-ever, an accurate description of the porous medium structure is required to obtain reli-able results. In this study, the pores were modeled using simple geometrical shapes. The sensitivity of the effective thermal conductivity of the porous medium to the shape, size, and distribution of the pores were investigated. The results of the numeri-cal model were validated using the available experimental data and compared to the semi-empirical correlations.2019-01-01T00:00:00ZGrain Boundary Engineering of Inconel 625 using Iterative Thermomechanical Processing
http://hdl.handle.net/123456789/1028
Title: Grain Boundary Engineering of Inconel 625 using Iterative Thermomechanical Processing
Authors: Ali Bader Alshemali
Abstract: Grain boundary engineering (GBE) is relatively a new method for microstructural modification of metallic materials via altering characteristics and distribution of grain boundaries. GBE involves the use of thermomechanical processing (TMP) to improve the properties of FCC metals of low stacking energy. TMP is typically applied via iterative cycles of cold working and subsequent annealing of the metal. In such a case, annealing results in massive formation of twins leading to an increase in the fraction of special grain boundaries, such as Σ3 boundaries, and thus breaking the network of random grain boundaries. Inconel alloys have been the subject of recent studies for the aim of microstructural enhancement via GBE. This type of superalloys is known to have high-temperature strength, excellent formability, and outstanding corrosion resistance. In particular, Inconel 625 is characterized by high content of molybdenum making the alloy resistant to oxidization and corrosion. Yet, grain boundaries are susceptible to intergranular attack, particularly when sensitization occurs during welding. There are few studies for Inconel 625 and GBE can offer great benefit to improve its microstructure and mechanical properties. Therefore, the current study has focused on manipulating the grain boundary structure of Inconel 625 via GBE. This has been achieved by applying a TMP scheme of four iterations. Each iteration consists of cold rolling Inconel samples by 25% thickness reduction and subsequent annealing at 1000 °C for 15 min. The effectiveness of TMP was evaluated by looking into the formation of special boundaries, with increasing number of iterations, and its effect on the properties of Inconel 625. To do that, microstructure examination of GBE samples has been conducted using SEM and EBSD.2019-01-01T00:00:00ZVibration Reduction by Command Shaping on Flexible Rotating Beam Under the Effect of Gravity
http://hdl.handle.net/123456789/856
Title: Vibration Reduction by Command Shaping on Flexible Rotating Beam Under the Effect of Gravity
Authors: Worood Al-Munayyes
Abstract: Multiple sources of vibration exist in the environment, and their presence usually lead to
undesired events. Input shaping is a technique developed for minimizing residual vibrations
in rest-to-rest maneuvers. In this work, a smooth sine based waveform command shaping
profile is proposed to reduce residual vibrations of a vertically rotating flexible beam in restto-
rest maneuvers under the effect of gravity. The system contains nonlinearities with infinite
modes of vibration. The equation of motion is determined and then discretized for simplicity.
Only the first mode is considered because it is the only effective mode in the system. The
system inputs include the maneuvering time, maximum velocity, and final rotational angle.
A comparison is performed between the uncontrolled system, double-step technique, smooth
command shaping technique with optimization, and the smooth command shaping technique
without optimization. The respective techniques were applied in different cases with varying
maximum velocities, flexible beam lengths, and maneuvering times. The smooth command
shaping technique with optimization provided the best result in terms of eliminating residual
vibrations in rest-to-rest maneuvers. Its performance is followed by that of the command
shaping technique without optimization, and then that of the double-step strategy. The
command shaping technique proved to be very effective when compared with the
uncontrolled system. The results showed that the effect of nonlinearities arises as the
maximum velocity and beam length increase or maneuvering time decreases. Additionally,
the results show that optimizing the shaper parameters considerably reduces the residual
vibrations.2018-01-01T00:00:00ZAnalysis of Multi-Sectioned Beam Vibrations
http://hdl.handle.net/123456789/755
Title: Analysis of Multi-Sectioned Beam Vibrations
Authors: Abdullah A. Al Ajeel
Abstract: Beams are widely used in the industrial sectors where they usually have an uneven shape, multi-sections, different densities, and varying widths and thicknesses. Many re-searchers tend to simplify the system by assuming even shapes or by using Finite Element Methods (FEM) to predict the dynamic behavior or to control such systems. It is well known that, these methods might find a solution but it is either inaccurate or mathematically chal-lenging. Finding reduced order equations greatly simplifies the prediction of the dynamic be-havior and the choice of proper control scheme.
In this work, a mathematical method to reduce the partial differential equation of mul-ti-sectioned beams to ordinary differential equation is proposed. The beam nonlinear equation of motion is derived by utilizing Hamilton’s principle. This equation is linearized and then reduced by adopting Galerkin’s Method. The linear natural frequenciesarecalculated analyti-cally by using Maple and validated numerically using FEM.The generated analytical solution isdesigned to solve complex models fora multi-thicknesses cantilever beam which is separat-ed into multiple segments. Each segment can have different thickness, density, or shape but with common width. The change in density has been utilized to determine the effect of piezo-electric actuator dimensions and locations on a beam’s natural frequencies. Finally In order to study the effect of beam segmentsonthe natural frequencies,multi-sectioned examples are cal-culated analytically using the proposed method and validated numerically using FEM.
The results show that the analytical and FEM natural frequencies are almost identical for both uniform and non-uniformed cantilever beams with a minor percentage error. The same results are found as per changing the segment density. In fact, the percentage error is found to be less than 10%. The results confirm that the proposed analytical technique is suc-cessfully predicted the linear natural frequencies and can be used to predict the dynamical behavior of multi-sectioned beams with different used materials2018-01-01T00:00:00Z