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.
- ItemTowards 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.
- ItemA human cyber-physical system to study the motion sickness of seafarers.(Stellenbosch : Stellenbosch University, 2023-11) Taylor, Nicole Catherine; Kruger, Karel; Bekker, Anriette; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Maritime 4.0 offers technical opportunities to digitally enable ships. Systems are equipped with virtual counterparts, forming cyber-physical systems, to manage operations. However, the human element remains constrained to the physical layer. This dissertation proposes the Mariner 4.0 concept that encourages equipping seafarers with virtual counterparts. Mariner 4.0 is a contribution formulated to serve as a branch of digitalisation that addresses opportunities and challenges associated with human factors in Maritime 4.0, such as a lack in accessibility of human-centric data during operation. Human cyber-physical systems offer a promising means for human-system integration and human digital representation amidst technically-centred developments. This dissertation contributes an architecture for a human cyber-physical system that defines core elements – a physical layer, where seafarers are present, a cyber layer, which contains virtual counterparts for seafarers, and communication between the layers. A human cyber-physical system for seafarers is implemented and deployed, which is the first objective of this dissertation. The trialled human cyber-physical system is a contribution that facilitated human-centric data acquisition and processing for seafarers on a ship over the course of a 20-day long voyage. The motion sickness of seafarers is monitored subjectively and objectively in near real time, a novel feat in shipping, in a case study with 63 participants on board. Seafarers recorded their subjective experiences of motion sickness through two methods. The first, traditional method used paper-based questionnaires, while the second, novel method used the human cyber-physical system. The second objective of this dissertation is to use the human cyber-physical system as a mechanism for advancing the study of motion sickness in shipping. The human cyber-physical system enables personalised data analysis as well as conventional aggregation of results. As such, novel understanding of motion sickness and the study thereof in shipping is uncovered. Measures of motion exposure are determined objectively according to recommended procedures in ISO 2631-1 (1997) and are integrated with the percentage of motion sick individuals on board, forming motion sickness criteria. The human cyber-physical system provides a platform for inspecting the evolution of criteria development during ship operation. The results contribute diagnostic thresholds that could be used beyond the operational stage to assess the levels of sickness that individuals or a cohort may present if exposed to certain measures of ship motion. The motion sickness criteria accommodates factors, such as exposure duration, for tailoring diagnostic thresholds to voyage missions – potentially applicable for voyage planning and ship design. The human cyber-physical system provides a customised platform for addressing challenges and opportunities associated with human factors in Maritime 4.0. Moreover, the human cyber-physical system extended the knowledge basis of motion sickness on ships innovatively. The human cyber-physical system served as a human-centric platform for seafarers that empirically revealed the importance of performing data acquisition and analysis at an individual level in addition to an aggregate level.
- ItemFlow and heat transfer in packed beds of rock.(Stellenbosch : Stellenbosch University, 2023-11) Hassan, EBE; Hoffmann, JE; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Utilizing thermal energy storage systems in a solarized Brayton cycle increases the capacity factor of the power plant by using waste heat. The waste heat can be used to power a Rankine cycle to produce additional energy after sunset or during periods of high demand. Packed beds of crushed rock have been proposed as a promising storage solution since it is readily available, inexpensive, and able to withstand high temperatures. Dolerite is one of the rock types selected as the optimal choice for storing high temperatures. Nevertheless, predicting the pressure drop over a packed bed is a crucial parameter for estimating the pumping power of the system. There are various parameters that influence the pressure drop through the bed, such as the shapes and sizes of the crushed rocks, which affect the packing density and particle orientation. In this study, the investigation of the pressure drop through the packed bed of crushed rock depending on the flow direction was conducted. The crushed rock was represented by an ellipsoidal shape with the same volume and aspect ratio as the average of randomly collected crushed rock samples. This is because the aspect ratio is considered one of the particle characteristics that enables one to capture the flow directional effect. Simulation models were developed to assist in deriving a pressure drop correlation as well as the effect of particle orientation on pressure drop. For simulation, a discrete element model (DEM) was used to generate the particles and computational fluid dynamics (CFD) to simulate the flow at pore scale over the particles. Firstly, a packed bed of ellipsoidal particles was developed using a DEM-CFD method. Afterward, an experimental model was developed to validate the DEM-CFD model. Following the validation, the DEM-CFD model was compared with a crushed rock packed bed to examine how well the ellipsoid particles represent the crushed rocks. The findings reveal that the model successfully captured the flow direction effect across the crushed rock bed. However, it underestimated the pressure drop through the crushed rock by 5 % in horizontal flow and overestimated it by 20 % in vertical flow. The wall has a direct effect on the particles' alignment, where at the bottom of the container the particles are aligned with their flat faces. The wall effect is high for the particles close to it; however, it does not extend deep into the bed. Additionally, it depends on the bed-to-particle diameter ratio. For a large rock bed where it is a free packing bed, the wall has an insignificant effect on the particles' alignment. Therefore, the wall effect was then eliminated from the DEM-CFD model, and a correlation was proposed to predict the pressure drop through a crushed rock packed bed using the porous media approach. The proposed correlation was used to predict the pressure drop across a packed bed of 10 MWth. The findings were compared with the isotropic Ergun model. After investigation of the outcomes, it was found that the proposed correlation captured the directional effect. Also, it was predicted that the pressure drop across a porous bed would be about 22 % lower than that predicted by the Ergun equation in the vertical direction.