Numerical modelling of power law constants established through impression and micro-Uniaxial creep methods for service exposed A234WPB steel
- Authors: Tembo, Blessed
- Date: 2024-04
- Subjects: Number theory , Numerical analysis , Mechanical engineering
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/64852 , vital:73931
- Description: Continuous monitoring of creep life in materials operating at high temperatures and pressures is imperative to prevent catastrophic failures and ensure timely replacement of worn-out components in industrial plants. Small-scale creep testing methodologies offer a valuable means of assessing material creep life while preserving structural integrity. Motivated by the need for reliable methods in creep life assessment, this study aimed to investigate the creep properties of A234WPB material subjected to service conditions using Impression creep and micro-uniaxial creep testing techniques. The research questions focused on establishing power law constants through small-scale creep testing, validating these constants using numerical modelling, and assessing their practical implementation in predicting material creep life. Samples extracted from service-exposed A234WPB steel alloy underwent step-load impression creep tests and step-temperature micro-uniaxial creep tests to derive the power law creep equation. The determined stress exponent of 3.967 indicated that dislocation creep was the dominant creep-controlling mechanism at 520 °C. A numerical model, utilizing the established power law constants, demonstrated a strong correlation with experimental findings in steady-state creep rates. Furthermore, the conventional Monkman-Grant approach was employed to predict the remaining life of the service-exposed material using impression creep data. The predicted remaining life aligned with the scatter band of uniaxial rupture life on a Larson-Miller plot, highlighting the practical utility of impression creep and micro-uniaxial creep testing techniques in assessing creep life. This study contributes to the advancement of small-scale creep testing methods and underscores their potential for practical implementation in industrial settings, thereby enhancing the reliability and safety of high-temperature and high-pressure operations. , Thesis (MEng) -- Faculty of Engineering, the Built Environment, and Technology, School of Engineering, 2024
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- Date Issued: 2024-04
Optimisation of laser welding for thin-walled Ti6Al4V glider pressure hull
- Authors: Nel, Matthew Ryan
- Date: 2024-04
- Subjects: Laser welding , Welding , Mechanical engineering
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/64739 , vital:73882
- Description: Laser welding is a type of fusion welding process characterised by deep penetration, low heat input and high welding speed. This dissertation investigates the suitability of this process for the fabrication of an underwater glider buoyancy engine from thin Ti6Al4V alloy sheet. Areas of interest include the effect of process parameters on weld microstructure, static properties (microhardness, tensile and bend tests) and dynamic properties (fatigue tests). The effect of welding speed and laser defocusing were evaluated considering experimental matrices consisting of four different travel speeds and three defocus distances. These were narrowed down to three travel speeds and a single defocus distance, resulting in a final test matrix delivering three different heat inputs. Thereafter, the effect of heat input on static and dynamic properties was investigated. Vickers microhardness tests were carried out to predict weld response during tensile testing, with the expectation being that harder welds would offload more strain. This was confirmed using digital image correlation, which allowed for virtual measurement and visualisation of strain offloading. Bend tests were carried out on parent and welded samples to confirm whether Ti6Al4V could be formed to the correct geometry. Forming Ti6Al4V into a U-shape was the first consideration, since dynamic testing required samples of this geometry with longitudinal weld orientation. A two-stage forming methodology was developed from these experiments. Formed samples were subjected to fatigue tests in a custom designed fatigue platform for testing weld orientation as it would appear in the final component. Prediction of the welded buoyancy engine life was the goal behind these tests. Fracture surfaces were analysed to gain understanding of where crack initiation and final fracture occurred. Porosity served as the primary cause for crack initiation in failed samples. Pore distribution was heaviest in low-heat weldments and decreased with increasing heat input, while pore size increased with increasing heat input. This resulted in medium-heat weldments exhibiting superior performance to that of low- and high-heat ones. It was concluded that the laser welding process is able to produce weldments of sufficient integrity in thin Ti6Al4V sheet-formed components intended for use in glider buoyancy engines. , Thesis (MEng) -- Faculty of Engineering, the Built Environment and Technology, School of Engineering, 2024
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- Date Issued: 2024-04
Quantify the shift in critical strain energy density for a dynamic loaded S355J2 weld section by small punch testing
- Authors: Snyman, Ross
- Date: 2024-04
- Subjects: Materials -- Testing , Materials -- Microscopy , Mechanical engineering
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/64830 , vital:73927
- Description: The potential of using “critical strain energy density” as an indicator to detect material degradation on fatigue-loaded components, where the stress amplitude was gradually increased, was investigated by applying the small punch test methodology. Small punch discs were extracted from four fatigue specimens. Discs were extracted from two zones within the fatigue specimens: the heat-affected zone of a shield metal arc welded butt joint, as well as from parent metal plate area. Each fatigue specimen having been exposed to a different stress amplitude but equal number of fatigue cycles of 20 million. This allowed the development of a Small Punch Test results database, permitting the calculation of material properties by using a Finite Element Analysis inverse method. Critical strain energy values did not reveal any conclusive shift or correlation for discs extracted from the HAZ. This is mainly attributed to variance in the grain structure for HAZ samples. For discs extracted from parent metal, a decreasing trend in strain energy density value was noted in relation to an increase in the fatigue test stress amplitude. This observed change was a clear indication of the potential for using “Strain Energy Density”, as extracted from this test methodology, as a ranking tool for quantifying the extent of degradation of in-service components. The application of this study demonstrated the ability to monitor and predict material degradation for a given stress range over the expected life of a cyclically loaded component utilizing strain energy density results. This is useful, particularly in heavy industries where structures and large-scale components are used beyond their original design life. , Thesis (MEng) -- Faculty of Engineering, the Built Environment, and Technology, School of Engineering, 2024
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- Date Issued: 2024-04
Influence of process energy on stress corrosion susceptibility of a friction hydro pillar repaired steam turbine rotor disc blade locating hole
- Authors: Pentz, Willem Gerhard
- Date: 2020
- Subjects: Friction welding , Mechanical engineering
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/47106 , vital:39810
- Description: Currently the power generation industry is struggling to keep older coal power plants running efficiently. One of the major hurdles is to keep repair and service cost low. Over time stress corrosion cracking (SCC) occurs in the locating pinholes of tier type rotors which locate the turbine blades. This is where this research aims to assist with an alternative repair technique, Friction Hydro Pillar Processing (FHPP) welding, to have longer service intervals thus saving cost and time. The same material can be used for welding and a new aligned hole can be drilled. FHPP welding is a solid state friction welding process. Four different FHPP axial forces were selected to compare their respective performance in subsequent tensile testing, impact testing and SCC testing. All the tensile samples extracted from preheated welds and post weld heat treated welds fracture in the parent material, which indicates good weld efficiency. The impact crack route from the weld nugget towards the parent material was identified in the energy and force graph. Axial force which promote impact toughness can be selected with this curve. SCC occurs when a tensile stress is applied to a susceptible material when in a conducive environment for cracking. A new SCC W-shape was designed and performed well during initial testing. With the SCC W-shape two specimens can be extracted opposite each other and tested. Both the preheated weld samples and the post weld heat treatment (PWHT) weld samples had improved SCC performance over their respective parent material samples. A high axial force, low process energy, and high process energy rate (low process energy and low weld time) produced a weld with improved SCC resistance. FHPP (with PWHT) is a promising repair technique as it improved on the SCC resistance and impact toughness as well as having 100% bond efficiency. More research is still required to identify the SCC mechanism of the FHPP weld.
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- Date Issued: 2020
Characterising the stress-life response of mechanical formed AISI-1008 steel plate components
- Authors: Müller, Ruan
- Date: 2012
- Subjects: Materials -- Fatigue -- Testing , Metals -- Fatigue , Mechanical wear -- Measurement , Mechanical engineering
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: vital:9616 , http://hdl.handle.net/10948/d1008102 , Materials -- Fatigue -- Testing , Metals -- Fatigue , Mechanical wear -- Measurement , Mechanical engineering
- Description: The main purpose of this research project was to determine the fatigue-life behaviour of AISI 1008 sheet steel which has been mechanically formed to a radius of curvature of 120mm and then to correlate the fatigue-life behaviour to that of the parent or “as manufactured” material. During the forming process it was felt important to induce plastic strain through stretch-bending by clamping the sides of a plate sample’s (width) edges in the bending fixture before being bent by a single acting mechanical press. It was determined through actual testing that there was a decrease in fatigue-life when the mechanical formed data was compared to fatigue data of the parent material. Standard fatigue mathematical models were used to relate the actual fatigue data. Due to the material being cold formed to a radius of curvature of 120mm, residual stresses induced during the forming process played an essential role during the fatigue-life prediction calculations. The maximum relieved stress in the parent material was compressive in nature having a magnitude of 11percent of the “as manufactured” yield strength (265 MPa). For the mechanical formed material compressive residual stresses were measured on the outer surface while tensile stresses were measured on the inner surface. The difference between actual number of cycles to failure to that calculated using the standard mathematical models for the parent material, ranged between 48 percent and 18 percent and for the mechanical formed samples between 35 percent and 1percent, depending on the strain amplitude used. An important aspect of this study was to determine the criteria required for mathematical modelling of the parent material as testing occurred between the limit of proportionality and yield point. Although this aspect requires further investigation the mathematical results obtained during this study were considered to be acceptable.
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- Date Issued: 2012
Preconditioning measurement and control system for a combustion engine in a vehicle
- Authors: Homann, Gregor
- Date: 2011
- Subjects: Internal combustion engines , Mechanical engineering
- Language: English
- Type: Thesis , Masters , MA
- Identifier: vital:9649 , http://hdl.handle.net/10948/d1010998 , Internal combustion engines , Mechanical engineering
- Description: Modern vehicles have to ful ll new CO2 emission and additionally customer comfort requirements to stay competitive. A major impact to the fuel consumption of an internal combustion engine (ICE) has the starting period. An ICE equipped with a preconditioning system which heats up the ICE much faster than a common ICE. This procedure of preconditioning is called peak heating. The main benet of preconditioning of an ICE is less fuel consumption. Recently the only way to obtain a fast heating up of an ICE is the injection of a higher amount of fuel during the starting period. This heat up procedure can be changed if a heat reservoir is available to the ICE during the starting period. In this case the additional injection of fuel is redundant and therefore the consumption during the starting period can be reduced. The major advantages of this strategy are achieved in cold ambient conditions. During this project di erent preconditioning strategies and di erent points of interaction in the coolant circuit of an ICE have been investigated. The preconditioning concepts have been evaluated according to their heating up performance and their implementation into the engine compartment. The results obtained by this project highlight that a system layout which enables a preheating of the cylinder block by a heat reservoir located in a bypass-line to the heater core is the most e ective point of interaction. The best results have been achieved with a coolant ow of 10 l/min at a temperature of 90 C. Furthermore, this project points out that the implementation of a preconditioning system into the oil cooler will achieve similar results. This strategy of preconditioning the engine oil reduces the internal frictions of the ICE which leads to a decreasing consumption. This solution is much more energy e cient and technically easier to implement into a modern vehicle with its limited space. An additional side e ect of the preconditioning of the oil is a longer service life of the ICE.
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- Date Issued: 2011