Doctoral Degrees (Mechanical and Mechatronic Engineering)

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https://scholar.sun.ac.za/handle/10019.1/780

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Fracture 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.
Using 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.
Augmentation 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.
An 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.
Towards a protocol for evaluating unrestrained torso neck braces.
(Stellenbosch : Stellenbosch University, 2024-02) De Jongh, Cornelis Uys; Basson, AH; Knox, EH; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The relatively recent introduction of neck braces for the unrestrained, helmeted rider in extreme activities has necessitated an understanding of the underlying biomechanics resulting from headfirst impacts while wearing these devices. Currently, no established or commonly accepted pathway exists to independently evaluate, and subsequently approve, these devices. The aim of this dissertation is to propose key elements of a protocol for the evaluation of unrestrained torso neck braces resulting in a reliable determination of intervention efficacy. Specific objectives include identifying the relevant neck injury mechanisms from literature, recreating those mechanisms in testing and computational simulations, and identifying applicable neck measures, criteria, and injury risks to evaluate. A further objective is to critically evaluate the proposed methods, measures, and correlates through a case study, using a neck brace. The dissertation presents two tests and a computational model to evaluate neck brace efficacy. The first test, an inverted pendulum test, is proposed to evaluate compression flexion, tension flexion, and tension extension using an HIII ATD neck, and a motorcycle-specific ATD neck (MATD). The second test addresses the most important neck injury mechanism related to motorcycle accidents, compression flexion. This test distinguishes itself from the first test in that the degree of anterior head impact eccentricity is reduced and the baseline impact energy results in neck axial forces approaching injury assessment reference values (IARV). Using a current neck brace as a case study, the proposed tests bring to light important observations in evaluating a neck brace for each mechanism investigated. The ability of the tests to underscore the potential benefits and adverse effects of a neck brace is investigated through the evaluation of the appropriate upper and lower neck response measures. Lastly, a solid-body computational model is proposed to simulate neck response with and without a neck brace for a variety of head impact conditions. These simulations may be used to augment tests that use an HIII ATD neck, considering the challenges associated with using these instruments. The computational model can highlight aspects such as changing neck brace efficacy for varying impact configurations. The proposed method identifies a set of novel methods to visualize and interpret computed neck response data with and without a neck brace when large datasets are created. This work contributes towards the establishment of a novel protocol by which to gauge neck brace performance using applicable biomechanical considerations through testing and computational biomechanics. The chosen loading modalities, neck injury mechanisms, resulting neck response measures, injury criteria, and injury risks evaluated are relevant to the proposed analyses and create a basis for the establishment of a formal testing protocol. A protocol whereby unrestrained torso neck braces can effectively and critically be evaluated will allow product designers to be creative in their endeavors while conforming to a set of safety measures that effectively address important biomechanical considerations required for these device types. The combination of testing and real-world impact simulations enables the efficacy prediction of neck braces to converge with real-world effectiveness.

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