Doctoral Degrees (Mechanical and Mechatronic Engineering)
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- ItemFracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4V(Stellenbosch : Stellenbosch University, 2024-02) Macallister, N; Becker, Thorsten Hermann ; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: This dissertation presents a study on fracture mechanics-based fatigue life assessment for Additively Manufacturing (AM). The mature laser powder bed fusion (LPBF) process with the Ti-6Al-4V alloy in particular is selected for study, as it is well suited to the South African context with regard to economic climate, strong AM relationships and abundant mineral titanium reserves available. Furthermore, the Ti-6Al-4V alloy is a staple of aerospace, automotive and biomedical industries which are amongst the largest promoters for using AM technology, and for whom fatigue characterisation remains a prevalent topic as many end-use applications are intended for cyclic loading. Though significant research in fatigue behaviour exists, the conundrum of reliably certifying fatigue life in AM parts persists. This problem stems from the complex relation between AM print parameters, build orientation, surface roughness, inherent defects, residual stresses, meso- and microstructure; and establishing reportable fatigue strength baseline values required by industry. Moreover, as the AM environment promises saving in cost and time, full fatigue testing schemas are undesirable. As such, alternate damage-tolerant methods are becoming increasingly popular, where adopting fracture mechanics-based frameworks accompanied by limited or non-destructive testing could aid in certification. For this purpose, the dissertation first presents a novel version of the fatigue predictive NASGRO model where parameters are established that are unique to LPBF produced Ti-6Al-4V meso- and microstructures. In establishing these parameters for LPBF produced Ti-6Al-4V, the influence of process inherent microstructure, residual stress, and orientational dependant meso- structure is considered through examining near-threshold in combination with steady-state fatigue crack growth rates. The analysis shows that the descriptors of material constraint are sensitive to build orientation and microstructure. Furthermore, the effect of residual stresses is observed to not be severe. In this a clear effect of build orientation and meso- and microstructure is established for selecting NASGRO model parameters. Secondly, the proposed NASGRO formulation is translated into a comprehensive novel damage-tolerant fracture mechanics-based model to estimate fatigue life. Non-uniform defect populations, typical of AM material, in terms of size, shape and location are captured through X-ray tomography and surface profilometry and used as inputs modelled as equivalent crack lengths. The fatigue strength estimations are shown to be sensitive to fatigue crack growth rate threshold parameters and short crack growth mechanic descriptions. Furthermore, by introducing multiple crack initiations, the fatigue estimates are shown as distributions and are sensitive to defect number. Finally, sub-size specimen testing is investigated as a potentially elegant solution to accompany fatigue life assessments for threshold validation. Where results show inconsistent near-threshold fatigue behaviour linked to the microstructure. In this, considering unique meso- and microstructural features of LPBF produced Ti-6Al-4V, the domain and suitability in using sub-size specimens for fatigue crack growth rate threshold testing is discussed. Overall, this dissertation walks the path required in establishing reliable damage tolerant fatigue life estimation approaches for LPBF produced Ti-6Al-4V. Providing fundamental insights into interactions of fracture mechanic mechanisms and descriptions necessary for reliably modelling fatigue behaviour, therefore contributing to the developing frameworks and philosophies in AM to help in certification of fatigue performance of LPBF produced Ti-6Al-4V components.
- ItemUsing full field data to produce a single indentation test for fully characterising the mooney rivlin material model.(Stellenbosch : Stellenbosch University, 2024-02) Van Tonder, John Dean; Venter, Martin Philip; Gerhard Venter; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: In the field of material characterization, a well-known problem in literature has been identified. The problem involves the solutions obtained from inverse Finite Element analysis when characterizing hyperelastic material model parameters, which are often non-unique. A gap in the literature exists regarding the handling of the non-uniqueness issue. Providing a solution for this has meaningful implications for engineering applications. The nature of these non-unique solutions is that they fit the dataset of the load case they were characterized on with indistinguishable errors from the actual optimal set of model parameters. These solutions prove to be sub-optimal when applied to load cases for which they were not characterized, failing to predict accurate material behaviour. The research presented in this dissertation addresses this non-uniqueness problem for the Mooney Rivlin model by introducing a novel contribution. The contribution involves a newly discovered concept known as hyperplanes, which manifest as flat, plane-like regions in the low-error regions of the design space. This discovery enables the isolating of a single, optimal set of material coefficients. The hyperplanes serve as the foundation for a new inverse Finite Element characterization method formulated as a constrained optimization problem. The main contribution of this formulation is that allows the the user to specify which loading state of the material deformation path they wish to fit. This is achieved by specifying specific measurement points. Additionally, this formulation allows for tolerances to be applied on these measurement points adding an additional level of compliance to the material characterisation. The behaviour of these hyperplanes was investigated, initially through a simulated indentation test that involved full-field digital image correlation experiments. However, these simulations provided a controlled environment to explore the characteristics of hyperplanes under noise-free conditions, leading to the development of the constrained optimization method. The applicability of the hyperplane concept was then validated using physical test data and compared with material testing standards. The results of this comparison study indicated that using hyperplanes in the inverse characterization process produced a more comprehensive set of material parameters than the test standards. In conclusion, this dissertation asserts the indispensable role of hyperplanes in isolating the true optimal set of Mooney Rivlin model parameters, thus addressing the identified gap in the literature and delivering a valuable contribution to the field of material characterization.
- ItemA model for accurate error propagation in a convergent stereovision system.(Stellenbosch : Stellenbosch University, 2024-02) Ezebili, Ifeanyi Francis; Schreve, K.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Stereovision is a camera-based imaging technique that facilitates the reconstruction of the 3-space coordinates and depth of a scene point using the images acquired from two cameras. Generally, stereovision finds application in autonomous vehicle navigation, mobile robots, parts inspection for quality assurance, and tracking and identification of objects. Like the output of a typical metrological system, the 3D coordinates measured with a stereovision system have associated measurement uncertainties. Such uncertainties practically emerge from the errors which are concomitant with each of the steps involved in stereo camera measurement. In this dissertation an analytic epipole-featured model is developed and proposed for structure computation and 3-space depth measurement in convergent stereo camera imaging. The proposed reconstruction model is predicated on the image sensor parameters of both cameras, left and right, together with two extrinsic parameters, namely the baseline distance and the stereo projection angle of the scene point. The intrinsic parameters are normalized with respect to the focal lengths of the cameras. The proposed model is characterized by less computational complexity and short execution time and can be employed in active vision-based metrology in which the imaging stereo cameras are rotated about their vertical axes relative to each other. The terms virtual depth and depth factor or depth coefficient are subsequently introduced and described. Both quantities together define the depth of a world point relative to the coordinate frame of the reference camera. From the developed reconstruction model, an equivalence relation between coplanar parallel and convergent stereo camera imaging systems is established. The relation states that for double-view geometry in computer vision, disparity in a row-aligned, coplanar-parallel stereo camera configuration is equivalent to the baseline-to-depth-factor ratio in a convergent stereo camera configuration. This baseline-to-depth-factor ratio in convergent stereo camera imaging is termed convergent stereo disparity and can be identified and equated with the image rectification process in a practical conventional coplanar-parallel stereo camera setup. Incorporating the epipoles in the developed reconstruction model facilitates the establishment of the stereo equivalence relation and the definition of convergent stereo disparity in stereovision. Furthermore, generalized mathematical analyses are done to model and study the variation of depth sensitivity and relative depth uncertainty with respect to convergent stereovision system parameters for 3-space points using the developed reconstruction model. It is observed that different values of left and right focal lengths are required to achieve high sensitivity coefficient, a condition that is not conformable with the conventional practice of having the same left and right focal lengths in stereo camera imaging. Regarding the variation of the stereo projection and stereo convergence angles, there are trade-offs between depth sensitivity coefficient and relative depth uncertainty. It is found that a stereo convergence angle of 90° yields the best relative depth uncertainty value at which the focal length-normalized epipole-to-principal point distances on both image planes are reciprocals. The analytic derivations and graphical characteristics of the depth sensitivity coefficient would give a stereovision system designer some information regarding the margin by which the estimated depth of a scene point changes for a drift in the value of any stereo camera parameter, and also some idea in respect of the potential trade-offs involved in the choice of certain parameters. The performance of the developed reconstruction model is studied, and its accuracy tested by comparing the 3-space coordinates it predicts to those obtained by Gold Standard triangulation algorithm and to the ground truth results. In terms of execution speed the proposed reconstruction model exhibited a computation time of 0.6 ms compared to 6.2 ms and 9.9 ms recorded for the Direct Linear Transformation (DLT) and Gold Standard triangulation algorithms respectively. The measurement errors determined by theoretical methods based on the law of error propagation incorporating the analytic reconstruction model (with and without full input covariance matrices) are compared with those obtained by the experimental approach. Strong correlations are found to exist between the two sets of values obtained, indicating the validity of the error model. The study of measurement error using the reconstruction model shows that accounting for the covariances of all the stereo camera parameters in vision-based metrology predicts smaller errors compared to when the covariances of the parameters are ignored. It is also found that it makes no significant difference if full or diagonal input covariance matrices are used in the theoretical computation of error compared to the experimental approach to determining the error. The error model derived in this work and predicated on the developed epipole-dependent reconstruction model would be useful in the design of high-precision stereovision systems.
- ItemAugmentation of the actuator-disk method for low-pressure axial flow fan simulation.(Stellenbosch : Stellenbosch University, 2024-02) Venter, AJ; Owen, Michael ; Muiyser, Jacques; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Actuator-disk rotor models are an invaluable simulation tool for cost-effective turbomachinery simulation. Actuator-disk models implicitly represent turbomachine rotors as momentum sources where the source term magnitude is determined from classical two-dimensional blade-element theory (BET) force calculations. Actuator-disk models accordingly require appropriate lift and drag coefficients as input to complete the force calculations. Conventional actuator-disk models utilize standard two-dimensional airfoil coefficient data, but this limits the accuracy of the models to only a small operating window where the bulk of the flow over the rotor itself is principally two-dimensional. This, consequently, limits the application of traditional actuator-disk models in industrial system analyses where complex flow environments prevail. This study considers the particular example of low-pressure axial flow fans, widely applied in thermoelectric air-cooled condenser (ACC) systems. ACCs are a key water-conservative cooling solution to the thermoelectric power industry, yet their operation is beset by inefficiencies and corresponding high operating costs. Given the scale of ACC systems, numerical investigations are forced to rely on simplified implicit fan models like actuator-disk rotor models, which provide limited approximations of actual ACC fan performance over a wide range of flow conditions. Expanding the usable window of actuator-disk axial fan models is therefore vital to providing an enhanced capacity to robustly analyse and ultimately improve ACC systems (and other industrial cooling fan systems alike). To realize this enhanced analysis capability, a new means of appropriately defining the actuator-disk model input coefficient data is required. The input coefficient data needs to appropriately reflect actual fan blade behaviour in a three-dimensional rotating context. Physical fan blade behaviour, however, has not been comprehensively investigated, and the multi-dimensional effects of rotation and blade solidity remain somewhat obscure. This study therefore sets out to define generalizable axial fan behaviour and to use the newly acquired insight to fabricate new coefficient formulations. This study constitutes a numerical analysis in which two low-pressure axial flow fans are both explicitly (full, solid rotating fan geometry) and implicitly simulated. Novel insights into generalizable aerodynamic behaviour of axial flow fans at off design operating conditions are presented and key details on the underlying phenomena are uncovered. Furthermore, this study rigorously explores the feasible potential of the actuator-disk method for axial flow fan simulation and ultimately proposes its revised coefficient formulation. The augmented actuator-disk method (AADM) is shown to more accurately simulate axial fan performance compared to existing model variants, and to resolve flow fields that are more representative of the physical case – an important feature for ACC and other industrial heat exchanger system analyses. Over a wide range of axisymmetric operating conditions (and across both considered fan types), the AADM is shown to approximate reference static pressure rise results with a maximum error of 10%, shaft power results within 8% and blade force magnitudes within 10%, thus offering a marked improvement in comprehensive accuracy relative to existing models.
- ItemAn outlook on the energy mix at South African beverage manufacturers and opportunities for greater adoption of renewable energy solutions.(Stellenbosch : Stellenbosch University, 2024-02) Rozon, François Joseph André; McGregor, C; Owen, Michael ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: For the world to decarbonise, industry, which accounts for half of total energy demand, must embrace renewable and sustainable energy solutions. This, not only to meet electricity requirements but, more importantly, to substitute fossil fuels used to generate process heat. This research aims to assess the technoeconomic and carbon emission reduction benefits of renewable and sustainable energy solutions for the South African beverage sector, which represents an estimated 3020 GWh per annum in energy demand, and to provide a framework for the broader adoption of these technologies. This dissertation consists of three manuscripts collectively achieving the study’s ambition. In Chapter 2, the historical generation capacity and learning rates of leading solar and wind electricity generation technologies are used to derive a data-driven methodology to forecast costs. The approach to long-term forecasting is then applied to more novel technologies, such as energy storage systems, high temperature heat pumps and solar thermal energy solutions, which lack meaningful cost projections. The transparent approach to forecasting technology cost evolution supports decision-making regarding future investments in renewable energy solutions. Chapter 3 includes a proposed three-stage framework for the progressive reduction of fossil fuel usage by industry. Stage 1 advocates ongoing energy efficiency as the primary pillar. Under Stage 3, the framework introduces a broader role for industry as a catalyst to create municipal heating and cooling networks to lower overall community energy consumption. However, the paper focuses on Stage 2 and the large-scale adoption of renewable and sustainable energy solutions. This is particularly relevant to the South African beverage sector, which operates many coal-fired steam boilers and is primarily owned by international shareholders who have made bold carbon emission reduction commitments. A detailed evaluation of energy usage in the South African beverage sector is used to quantify the relative financial benefits and potential carbon emission reductions. For South African beverage producers, the paper concludes that investments in photovoltaic and battery energy storage systems will continue to take priority, given the 17–21 % Year-1 return on capital. However, given spatial and capital constraints, the judicious planning of solar thermal energy system requirements is advocated, to address the in-situ nature of process heat generation. Finally, a detailed techno-economic analysis of solar thermal parabolic trough collectors is presented in Chapter 4. In 2020 real US Dollar value, a levelised cost of heat of US$38–70/MWhth, was calculated. This remains higher than the cost of coal at US$20–40/MWhth at contract prices of US$100–200 per tonne. However, with the turmoil in energy markets and large-scale installations being commissioned in Europe in 2023, the article argues that a tipping point has been reached and that coordinated efforts between industry and service providers can lead to large-scale systems to be commissioned. This research demonstrates that the South African beverage industry can reduce carbon emissions through continued energy efficiency initiatives and value creating investments in renewable electricity generation and storage. Solar thermal energy technologies will need policy and fiscal support to compete against alternative investments and to displace coal and gas.