Engineering

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Now showing 1 - 10 of 15
  • (2021) Zhuang, Siyuan
    Thesis
    Rolling contact fatigue in rolling element bearing (REB) is a common surface degradation and failure mechanism. Limited experimental studies were reported on fatigue degradation, in particular, in grease lubricated conditions. This project examines the evolution of the fatigue degradation process through collecting, imaging and characterisation of the damaged surface to further understand the tribological changes in the process. Ball bearings with a seeded, irregular-shaped dent on the outer race were tested in a grease lubricated condition and on a bearing test rig. Moulding technique was used to replicate the damaged surface during the fatigue test. Also, 4 different shapes of initial defects and round-shaped dents in 3 sizes were introduced onto the outer raceway of test bearings to study effects of the shapes and sizes of initial defects on the fatigue degradation process as they are considered significant factors in fatigue propagation. This study has confirmed that the fatigue damage propagated on both the trailing and leading edges of the initial defect. The quantitative analysis reported that the wear depth on the trailing edge was measured more than that on the leading edge by over 100 μm, indicating different stresses distribution on the two edges of the seeded defect. In addition, the leading-edge damage surface was observed ‘self-healing’ or smoothening phenomenon. The shape study suggested similarities with propagating behaviour among the tested shapes of the initial defects. The initial shapes of defects mainly affected the fatigue propagation at the early stage where the first spall was initiated and fatigue progression on the leading-edge side of damaged surface in terms of the length of defected surface and the features of the damaged surface. The size influence study demonstrated that increasing size of the defect enlarged the effect of impact by rolling elements, evidenced by generating rougher surface, and thus shortened fatigue life. The study provides insights in the fatigue process of rolling bearings through monitoring the evolution of the tribological features of the degraded surface. The obtained information can be useful in fatigue model validation and development of effective online techniques for monitoring and prediction of the fatigue progress of REBs.

  • (2022) Tian, Rongying
    Thesis
    This thesis investigates the effect of fuel aromatic content on soot distribution, particle morphology and internal structure inside the cylinder of an optically accessible engine. A set of custom-made jet fuels with 4%, 14% and 24% aromatic content are studied first with the 24% aromatics fuel (AR24) used for more detailed follow-up study of soot particle evolution along the flame development path. Time-resolved imaging of cool flame, OH* chemiluminescence signals and soot luminosity are performed to visualise the overall reaction development. Planar laser-induced fluorescence imaging of HCHO and incandescence imaging of soot are also performed to obtain detailed understanding of reactions and soot distributions. Soot is analysed at a particle level. Using the thermophoresis-based particle sampling method, soot aggregates are collected from multiple in-bowl locations. Up to four soot sampling probes are installed on the piston-bowl wall with 60° spacing angles for simultaneous sampling from the same firing cycle. These sampling locations represent a jet-wall impingement point (JW), an up-swirl point (US), a down-swirl point 1 (DS1) and a down-swirl point 2 (DS2) along the sooting flame path. The subsequent transmission electron microscope (TEM) imaging of the collected soot particles enables structural analysis of soot particles as well as sub-nano-scale carbon layers. The results showed that the aromatic content has little impact on reactions and flame development among the tested fuels. However, the soot formation starts to occur earlier, and its growth rate is much higher for a higher aromatic fuel. The carbon-layer fringe analysis shows more mature, graphitised structures with higher aromatics at both formation-dominant and oxidation-dominant stages. For a selected AR24 fuel, the carbon-layer fringe analysis indicates continued oxidation during the flame penetration along the piston-bowl wall. Regarding the particle structure evolution, it is characterised by high formation of small aggregates at JW point, simultaneous aggregation and oxidation at US and DS1 point with the latter more prone to aggregation, and significant oxidation at DS2 point.

  • (2021) He, Peidong
    Thesis
    Aluminium alloys have a broad spectrum of applications in many key industries due to their high specific strength, excellent processability and low cost. Materials for structural applications usually require complex geometry and high strength which is considered a tricky problem for conventional manufacturing. However, metal-based additive manufacturing, or three-dimensional (3D) printing, frees products from geometric constraints proving potential benefits for structural applications. Therefore, it is of interest to develop high strength additive manufacturing fabricated aluminium alloys or aluminium metal matrix composites. The primary goal of this thesis is to improve mechanical properties of additive manufactured aluminium alloys and composites by exploring the correlation between alloying elements, reinforcement particles, processing, microstructure and properties. The first aim of this study was to investigate the effect of adding secondary particles to an aluminium metal matrix during laser powder bed fusion (LPBF). Specifically, TiCN reinforced AlSi10Mg composites were produced in this study and the microstructure and phase evolution of the produced materials were explored using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The mechanical properties were characterised via elevated tensile tests from room temperature up to 200 ℃. Optimised printing parameters to produce fully dense and crack-free TiCN/AlSi10Mg samples were selected. Additionally, LPBF AlSi10Mg samples were also fabricated with the same parameters as a reference. The results showed that LPBF TiCN/AlSi10Mg exhibited a bi-modal Al-Si eutectic structure consisting of equiaxed coarse and fine grains and possessed enhanced elevated temperature tensile strength compared with LPBF AlSi10Mg. The novel bi-modal structure formation mechanisms were attributed to the modified temperature gradients and heterogeneous nucleation due to the addition of TiCN particles. And the enhanced elevated temperature strength was related to the refined Al-Si eutectic bi-modal structure and the thermal stability of the TiCN particles. Besides, it was found that micrometre-sized TiCN particles are possible to be fractured during the LPBF process due to thermal shock. The second aim of this study was to explore the effect of introducing the alloying element Sc to an Al-Mg alloy (Al-5024) fabricated by LPBF. A thermal modal was developed using COMSOL Multiphysics Modelling Software for simulating the solidification behaviour within a single melt pool. Heat treatments were examined to improve mechanical properties by generating a well-dispersed strengthening phase of secondary Al3Sc in the Al-5024 alloy and the microstructure and precipitate evolution were characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Mechanical properties were characterised via tensile and uniaxial fatigue tests. The LPBF Al-5024 also showed a bi-modal structure but consisted of both columnar and fine equiaxed grains. Simulation of thermal field and the calculation of solidification front velocity indicated that the speed of solidification interface less than 110 mm/s would benefit the generation of Al3Sc precipitates to further facilitate the near full fine grain microstructure. Also, a fast solidification velocity close to the top of the melt pool caused the effect of solute trapping, limiting the precipitation of primary Al3Sc. A trade-off trend between yield strength and ductility as a function of the extended heat treatment duration, and a linear correlation between fatigue life and yield strength were observed, which were related to the properties of secondary Al3Sc precipitates. The maximum tensile strength of 450 MPa and corresponding 107 cycles fatigue strength of 105 MPa were realised by the hot isostatic pressing treatment of 325 ℃ for 4 h with 100 MPa pressure. Last, the corresponding dynamic strain aging behaviours of each state obtained by different treatments were studied. The dynamic strain aging behaviour is related to Mg solute atom clusters, the underlying reasons for the unstable plastic flow in this study were attributed to (i) the ‘Mg wall’ resulted from repetitive melting and fast cooling in LPBF, and (ii) the growth of intragranular (Al3Sc) and intergranular (Fe-, Mn-rich) precipitates in subsequent heat treatment that led to the increasing amount of misfit dislocation promoting Mg atoms clusters.

  • (2020) Gao, Shanshi
    Thesis
    Mechanical metamaterials have attracted considerable attention due to their programmable internal structure and extraordinary mechanical properties. With their specially designed internal microstructure, mechanical metamaterials can significantly improve some specific properties of a material or even bring incredible mechanical properties, such as negative Poisson's ratio and modulus, which cannot be achieved by conventional natural materials. However, most mechanical metamaterials are still in their prototype stage and have no direct applications. Processing methods are critical reasons restricting the development of mechanical metamaterials. Such micro-structured metamaterials were previously challenging to manufacture using traditional processing techniques. However, the recent emergence of advanced 3D-printing techniques has removed these barriers. This thesis aims to develop an easy-to-use mechanical metamaterial with tailorable large negative Poisson′s ratios. This metamaterial was microstructural, had cylindrical-shell (CS) based units, and was manufactured by 3D-printing. This Poisson's ratio related metamaterial was designed via Fusion 360 computer-aided design software. Its Poisson's ratio, internal stress distribution, and bending deformation were simulated using the finite element method (FEM) software. The following physical experiments were conducted to verify the simulation results. Numerically, the present metamaterial was found to capable of achieving large negative Poisson′s ratios up to −1.618 under uniaxial tension and −1.657 under uniaxial compression, and the results of the following verification tests agreed with the simulation findings. Moreover, stress concentration in this new metamaterial was much smaller than that in most existing re-entrance metamaterials. In the bending simulation, the transverse displacement at the simply-supported end of this metamaterial was only 28.25% as it of natural material, illustrating the strong geometry profile retention ability of this metamaterial under bending conditions. The proposed mechanical metamaterial was utilized to develop a wearable piezoelectric energy harvester (PEH) that converts electricity from mechanical energy associated with walking by using attached polyvinylidene fluoride (PVDF) membranes. This wearable PEH was structured with a 3D CS-based metamaterial and manufactured by multi-jet 3D printing technique. FEM analysis was conducted to simulate the dynamic compression on insoles corresponding to actual walking or running. The simulation findings matched well with the physical experimental results. According to the experimental results, a 4.15 V output voltage was achieved with a triple-layer PVDF structure. One insole with a triple-layer PVDF structure array also provided an 8.6 mW output power while running.

  • (2021) Yang, Jinxin
    Thesis
    Measurements of flow and turbulence within the flames are important to enhance fundamental knowledge of air-fuel mixing required to achieve high efficiency and low emissions in diesel engines. However, interferences from broadband soot luminosity signals pose a significant diagnostic challenge. To overcome this hurdle, the present study implements a new diagnostic method based on tracking of flame pattern changes detected in high-speed soot luminosity movies, namely, flame image velocimetry (FIV). In a small-bore optical diesel engine, time-resolved particle image velocimetry (PIV) is firstly performed to understand the in-cylinder flow field and turbulence in a motored engine condition, which shows an expected swirl structure and well distributed turbulence. With the direct fuel injection and flame development, the FIV is applied to show flow field evolution during the combustion event and its influence on in-flame turbulence distribution. A total of 100 engine cycles are recorded and processed for the FIV to address the inherent cyclic variations. The ensembleaveraged flow fields and turbulence intensity distribution extracted from individual cycles via the spatial filtering method are discussed with variations in injection pressure, inter-jet spacing, jet-swirl interaction and after injection condition. The FIV-derived flow fields show that increased injection pressure causes increased flow magnitude and turbulence intensity at the expense of cyclic variations. When the inter-jet spacing angle is narrower, an interesting flow suppression effect is found within the jet-jet interaction region due to flow collision. Regarding the jet-swirl interaction, the counter-flow condition on the up-swirl side leads to higher turbulence intensity suggesting better mixing. With the after injection, increased flow magnitude and turbulence intensity is found to be more effective when the injection timing is set close to the main injection. New knowledge developed from these FIV measurements provides guidance for engine developers to further optimise fuel injection conditions and injector geometry.

  • (2021) Ma, Chung-Hao
    Thesis
    Forward-Backward facing steps (FBS) are often seen in engineering applications, such as the uneven plate joints on the surfaces of vehicles or surface irregularities on aircraft. The presence of the step enhances the turbulence levels in the flowfield resulting in large wall pressure fluctuations, which are considered a source of noise, vibration, and structural fatigue. In this thesis, a combined experimental and numerical study on the aeroacoustics of low Mach number, turbulent flow over FBS with different aspect ratios is presented. The wall pressure measurement results indicate that the largest pressure fluctuations occur slightly downstream of the step leading edge and the downstream flow field is influenced by this upper corner disturbance. The downstream flow field modification is more severe for the small aspect ratio step when its length is shorter than the reattachment length. Large-Eddy Simulation (LES) was also performed to visualize the flow field around the FBS, which was validated by the wall pressure measurement results. The turbulent kinetic energy levels were found to be higher near the step leading edge, indicating that the flow was modified by this sharp corner. Meanwhile, spectral proper orthogonal decomposition (SPOD) was applied and identify the low-rank behavior in the flow field. The velocity-pressure cross-correlation showed that the wall pressure fluctuations were correlated to the wall-normal fluctuating velocity that was associated with the coherent structures. Also, the instantaneous flow field showed that when the step length was smaller than the shear layer reattachment length, the large scale vortex formed at the leading edge would shed downstream directly. This was believed due to the lack of the shear layer flapping motion, which was found to determine the trajectory of the shedding vortex. Without a reattachment point, resulting in a lower level wall pressure fluctuation on the top surface. Aeroacoustic beamforming results display the noise source location around the step leading edge and on the top surface of the step. Coincidentally, this is close to where the highest root-mean-square (r.m.s.) fluctuating wall pressure was recorded. The acoustic data combined with the wall pressure data suggest that for FBS noise generation, the leading edge plays a more crucial role compared with the trailing edge. Meanwhile, the integrated far-field acoustic spectra show that the levels of FBS noise were lower when the step length was smaller than the shear layer reattachment length. A new scaling law for FBS sound spectra was proposed to address the influence of the step AR.

  • (2021) De Cachinho Cordeiro, Ivan Miguel
    Thesis
    In the past decades, water droplet-based suppression systems (i.e. fire sprinklers, water mist) have been extensively utilized as building fire suppression systems. Nevertheless, current suppression systems operate under considerable design limitations due to high-rise structures and rapid increase in building complexity, such as pressure supply and water storages. Challenges can also be foreseen in suppressing water-reactive chemicals (i.e. alkali metals, hydrides) as the violent explosive reaction will be triggered. Therefore, it is vital to investigate potential suppression agents to cope with the increasing building fire risk associated with complex building materials and hazardous combustibles. A large eddy simulation (LES) model incorporated with novel user-defined functions (UDFs) to consider particle expansion and charring was proposed in this thesis. This model has been utilised to numerically study an experimental case to investigate the fire suppression behaviour of expandable graphite (EG) in building structures. This approach has provided an in-depth characterization of EG's thermophysical properties. This includes the expansion, barrier effect from char formation and the decomposition of EG. It was discovered that EG is more effective in fire suppression compared to natural graphite. Among the diameter range of EG (400 µm - 1000 µm), the smaller diameter of EGs tend to be efficient in suppressing metallic fire. The WALE SGS model has provided the most accurate temperature prediction among other SGS models, with average relative errors of 7.71% and 8.93%. The novel multiphase model was comprehensively validated by material testing and other experiments, and proven to be an effective tool to investigate particle-infused suppression in a structural fire. Moreover, the thermophysical properties (i.e. pyrolysis kinetics) of graphite have also been characterized through Molecular Dynamics (MD) simulations. The extracted Arrhenius kinetics parameters (i.e. activation energy) were compared to the experimental results, with an averaged relative discrepancy between 1.71 % - 5.38 %. The species breakdown analysis from the ReaxFF simulation will also further explore the feasibility of MD as a practical approach to analyse graphite's pyrolysis and chemical reaction mechanisms. The MD simulations have improved the understanding of particle pyrolysis and volatile emission during the oxidation of graphite and provided insights into the future of multiscale simulations.

  • (2021) Chen, Wenliang
    Thesis
    Among the shape memory alloys, nickel-titanium (NiTi) alloy finds applications in automotive, aerospace, biomedical and robotics. Recently, laser power bed fusion (LPBF) additive manufacturing, which fabricates metallic components by selectively melting layers of material, offers great potential for the design and fabrication of NiTi samples with complex geometries. Firstly, the aim of this thesis was to explore the role of laser power in controlling element evaporation and oxygen pick-up, and the subsequent microstructure and properties of LPBF fabricated NiTi. Two sets of optimized LPBF parameters with similar energy density but different laser powers (HP: high laser power with high scanning speed, LP: low laser power with low scanning speed) were used to produce nearly fully dense and crack-free NiTi samples. The results showed that the HP and LP NiTi samples had different microstructure, phase transformation and mechanical properties and even in the same HP or LP NiTi sample the microstructure, phase transformation and mechanical properties also vary in different regions along the building direction. Based on the different transformation temperatures, better pseudoelastic effects and further medical application direction, HP parameters were chosen to manufacture porous NiTi with three different structures. Results showed that all of the LPBF fabricated NiTi structures exhibited good geometric agreement with the designed structures. The struts of the lattice structures exhibited a rough surface and their sizes were slightly higher than the designed values. All the LPBF NiTi structures exhibited similar nominal compressive elastic modulus (5-7 GPa) to human bones. By altering the density distribution of the novel gyroid cellular structures, different deformation and fatigue properties were achieved. GCSs with graded density perpendicular to the loading direction (Y-GCSs) exhibited a similar shear compression failure behavior and slightly better fatigue behavior (i.e., longer fatigue life under high cycles) compared to the uniform structure. In contrast, GCSs with graded density parallel to the loading direction (Z-GCSs) exhibited a different layer-by-layer deformation and collapse behavior under compression while the fatigue life was worse than for the uniform structure.

  • (2021) Liu, Qian
    Thesis
    In this study, a machine-learning approach based on Gaussian process regression was developed to identify the optimized processing window for laser powder bed fusion (LPBF). Using this method, we found a new and much larger optimized LPBF processing window than was known before for manufacturing fully dense AlSi10Mg samples(i.e., relative density ≥ 99%). The newly determined optimized processing parameters (e.g., laser power and scan speed) made it possible to achieve previously unattainable combinations of high strength and ductility. The results showed that although the AlSi10Mg specimens exhibited similar Al-Si eutectic microstructures (e.g., cell structures in fine and coarse grains), they displayed large difference in their mechanical properties including hardness (118 - 137 HV 10), ultimate tensile strength (297 - 389 MPa), elongation to failure (6.3 - 10.3%), and fracture toughness (9.9 - 12.7 kJ/m2). The underlying reason was attributed to the subtle microstructural differences that were further revealed using two newly defined morphology indices (i.e., dimensional-scale index Id and shape index Is) based on several key microstructural features obtained from scanning electron microscopy images. It was found that in addition to grain structure, the sub-grain cell size and cell boundary morphology of the LPBF fabricated AlSi10Mg also strongly affected the mechanical properties of the material. The method established in this study can be readily applied to the LPBF process optimization and mechanical properties manipulation of other widely used metals and alloys or newly designed materials.

  • (2020) Xing, Sensen
    Thesis
    Gasoline compression ignition (GCI) engines have significant potential to improve fuel economy and reduce emissions harmful to health and the environment. The under-pinning knowledge of key phenomena of fuel-air mixture formation, ignition, combustion, and pollutant formation, however, is lacking at present. In this work, an experimental study was performed to assess the combustion characteristics of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. The fuel was injected into a quiescent-steady environment inside an optically-accessible constant-volume combustion chamber with 22.8 kg/m3 ambient gas density and 21 vol.% O2 concentration. Optical techniques including OH-chemiluminescence and shadowgraph imaging were performed to compare the combustion characteristics. Measurements were also performed for n-heptane (a diesel surrogate) for reference purposes. From the measurement results, the lift-off lengths (LOLs), ignition delays (IDs) and their corresponding uncertainties for the fuels are observed to increase with decreasing ambient temperature conditions. The LOLs, IDs and their uncertainties for the iso-octane flames are also consistently higher than that of n-heptane, across the tested temperature range. The results reveal that the highest variability detected for the flame stabilisation distance of the iso-octane flame at the lowest tested ambient temperature condition is attributable to the long transient stabilisation phase that the flame exhibits after ignition. Additional tests performed using a split-injection strategy with iso-octane as fuel demonstrate their potentials to reduce the transient stabilisation phase of the test flames, when compared with a single-injection test case with iso-octane as fuel. In the second part of this work investigated the ignition and combustion interaction processes between two consecutive jets of iso-octane and n-heptane. High-speed schlieren imaging, pressure trace measurements, combustion luminosity detection and closed-homogeneous reactor (CHR) simulations revealed that under the test conditions, relative to the long single-injection reference cases, changes in the local temperature and reactive intermediates, as induced by the pre-high temperature ignitions of the pilot injections, can considerably affect the pre-high temperature ignition reactions of the main injections that followed. The results show that the interaction effects are dependent on the ignition and combustion characteristics of the fuels involved, as well as the temporal separation between the consecutive injections.