http://localhost/xmlui/handle/123456789/1278 2026-05-08T12:31:38Z http://localhost/xmlui/handle/123456789/6944 Bioplastic Development, Characterisation, and Optimization of Fused Filament Fabrication Parameters Andanje, Maurine Naliaka Additive manufacturing, commonly known as 3D printing, is a rapidly expanding technology that has the potential to support a circular and sustainable economy. This technology supports a wide variety of raw materials and offers design flexibility, expanding its use in prototype and custom part production. One of the most common additive manufacturing technologies is Fused Filament Fabrication (FFF), which utilizes thermoplastic polymers as the raw material. Thermoplastic polymers commonly used in FFF include polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyamide (PA), polycarbonate (PC), and Nylon 12. Despite its popularity, high-density polyethylene has not been thoroughly studied in fused filament fabrication due to problems with warping and significant thermal shrinkage of printed parts after solidifying. It has been suggested that adding organic fillers will lessen these difficulties. The use of organic fillers in polymers results in biocomposites that have improved thermal properties and potential for biodegradation. However, printability, low-layer agglomeration, and reduced mechanical properties are some of the challenges that have to be overcome during FFF. Determining the best combination of printing parameters can significantly improve the printability of these biocomposites. In this study, rice husk waste was used as an organic filler in recycled high-density polyethylene to develop a biofilament for FFF. High-density polyethylene was chosen as the polymer since, though it is highly recyclable, it has not qualified as a potential raw material in FFF. This is due to challenges such as high thermal shrinkage that causes it to warp during printing. Organic fillers in polymers have been recommended as a means of reducing warpage of HDPE and enhancing printing directionality. Through the design of an experiment, various filler-to-polymer combinations were tested with the addition of a compatibilizer to enhance the filler’s miscibility in the polymer matrix. Using the ball mill, the rice husks were ground into powder with particles smaller than 75 μm. The biofilament's highest composition included 35 wt.% rice husk filler, 35 wt.% recycled high-density polyethylene, and 30 wt.% compatibilizer, indicating an improvement in rice husk filler content compared to earlier research. Digimat 2024.1 was used as the platform for material modeling and printing simulation to identify printing issues, such as warpage and residual stresses. Through a coupled simulation, a finite element model was analyzed to predict part performance. The model was validated experimentally using the standard tensile test specimen. The Taguchi Grey Relational Analysis (TGRA) was used to optimize the printing process due to its efficiency and robustness for multi-response experiments. Printability was successful up to the biofilament whose composition comprised 30 wt.% rice husk filler, 40 wt.% recycled high-density polyethylene, and 30 wt.% compatibilizer. This biofilament's mechanical properties included a tensile strength of 8.53 MPa with a standard deviation of 1.32 MPa, a tensile modulus of 128.56 MPa with a standard deviation of 13 MPa, and a maximum tensile strain of 6.6% with a standard deviation of 0.03%. Experimental validation of warpage yielded a maximum error margin of 5.43%, while validation of residual stresses resulted in a maximum error margin of 5.56%. The incorporation of rice husk filler, a natural reinforcement, into recycled high-density polyethylene improved the crystallinity of the biofilaments, which helped reduce shrinkage and warpage in printed parts. Biodegradability was also enhanced up to 10 % in a period of 24 weeks. The outcome of this study will provide valuable information for the manufacture of functional parts, such as biomedical devices, including microfluidic substrates, from biocomposite materials using FFF. Doctor of Philosophy in Mechanical Engineering 2026-05-05T00:00:00Z http://localhost/xmlui/handle/123456789/6943 Evaluation of Ball Mill Performance and Energy Consumption through the Discrete Element Method Kyalo, Mathew Ndeto Ball mills are a frequently used technology for comminution in the chemical and mineral processing industries, yet their characteristically low energy efficiency presents a persistent operational challenge; even marginal improvements can yield substantial economic and environmental benefits. This research investigates the relationship between mill geometry, operational parameters, and grinding efficiency by employing an integrated computational and experimental framework. The study compares the performance of two distinct mill designs - polygonal and cylindrical geometries—under varied configurations (with and without lifters). The Discrete Element Method (DEM) was used to simulate particle dynamics, charge motion, and wear, while a Population Balance Model (PBM) was applied to determine ore-specific breakage parameters and predict product size distributions. Simulations were validated against experimental data obtained from a custom-designed milling setup capable of precise speed control and in-line torque monitoring. Key findings from the DEM analysis demonstrate that geometry fundamentally alters charge behavior. At 75% of critical speed, a polygonal mill without lifters increased effective interparticle interactions by 10% and minimized particle centrifuging compared to a lifter-less cylindrical mill. The introduction of lifters further optimized performance: in the polygonal design, lifter addition enhanced collision frequency, resulting in a significant 26% reduction in power draw and a reduced Archard wear rate of 1.18×〖10〗^(-3) m. The cylindrical mill equipped with lifters achieved high collision intensity similar to the polygonal design but exhibited superior operational stability and about 30% lower wear rate. The developed DEM model demonstrated strong predictive capability, achieving a high Pearson correlation coefficient (>0.9) between simulated and experimental results. Predictions showed excellent quantitative agreement with experimental data, with root mean square errors (RMSE) of 2.3 W for power draw and 2.1° for shoulder angle. The grinding kinetics of two ore types, a gold ore and a malachite copper ore (averaging 4.7% Cu), were characterized using the PBM. The specific breakage rate (S_i) for the malachite ore increased with particle size, reaching a maximum of 2.892 min⁻¹ at 2 mm, a phenomenon attributed to the reduced agglomeration tendency and lower surface energy of larger particles. Breakage distribution functions and the optimum feed size for efficient milling were established. Furthermore, investigations into binary ore blends revealed that overall grindability is non-linear and disproportionately influenced by the harder ore component, potentially leading to a broader product size distribution when ores of differing hardness are combined. The DEM-calibrated parameters and PBM-derived kinetic models generated in this study provide a robust, scalable foundation for the design and optimization of industrial grinding circuits, directly linking particle-scale mechanics to full-circuit performance. Keywords: Ball mill, Discrete Element Method, Population Balance Model, Milling kinetics. Doctor of Philosophy in Mechanical Engineering 2026-05-05T00:00:00Z http://localhost/xmlui/handle/123456789/6917 Contractor-Related Factors on Performance of Bridge Construction Projects in Kenya: Case Study of Kenya National Highways Authority Anyika, Joan Otike Bridge construction projects face more challenges in their implementation compared to road and building projects. This is due to their complexity and iterative nature of implementation. There is global evidence showing that bridge construction projects perform poorly in both developed and developing economies, and this is based on a myriad of factors such as client-related factors, environment-related factors, project design, and contractor-related factors. Out of these, contractor-related factors have been shown to have a great influence on the performance of construction projects, but less empirical evidence exists in terms of bridge construction projects. Therefore, this study examined the influence of contractor related factors (staff, site, financial, and managerial factors) on performance of bridge construction projects. The theory of construction management and the theory of constraints are the theoretical underpinnings for this research. Descriptive research design was implemented targeting 18 bridge construction projects implemented at KeNHA from 2012 to 2023. A total of 18 projects and 144 key informants represented the research population. Out of the 144 participants, 98 respondents were involved in the survey. In the total 18 projects, 144 respondents represented the population. The actual sample size based on the respondents was 98 consisting of clients (19), consultants (14), contractors (8), engineers (39), environment and social guards (2), project managers (4), stakeholders (7), subcontractors (2), technical advisors (3), and 1 inspector. A self-designed structured questionnaire was administered to the respondents. Descriptive, Pearson (r) correlation, and ordinal least squares regression analysis were performed. The findings indicated that contractor-related factors explained 19.7% of variation in performance of bridge construction projects and was statistically significant at the 95% confidence level. It emerged that staff and management factors had a positive effect on bridge construction projects’ performance while a negative effect of financial factors was confirmed. Additionally, site factors did not have any effect on performance of bridge construction projects. The study concluded that staff and management factors had a positive outcome on performance of bridge construction projects while financial factors had a negative outcome on performance of bridge construction projects. Site factors had no relationship with performance of bridge construction projects. It is recommended that bridge construction companies should adopt flexible working hours for contractors to enable them to work with minimal interruptions that are associated with bridge construction projects. Second, policymakers should create policies that guide contractors in provision of safe working environments for their staff. Third, it is recommended that contractors should be provided with training to develop their skills in decision-making during the implementation of bridge projects. Lastly, it is recommended that the national construction agencies have a category for bridge contractors. MSc in Construction Engineering 2026-03-11T00:00:00Z http://localhost/xmlui/handle/123456789/6913 Influence of Processing Parameters and Post Heat Treatment on Mechanical Properties of AISI 1025 Fabricated by Laser Engineered Net Shaping Tum, Elphas Kibet A 3D coupled thermomechanical model was developed to investigate the evolution of residual stress in AISI 1025 fabricated by Laser Engineered Net Shaping (LENS). The model, validated experimentally with an error margin of 4.24%, demonstrated reliable predictive capability. Process parameters strongly influenced mechanical performance, with optimal settings (270 W laser power and 7.1 mm/s scan speed) achieving a hardness of 169.17 HV, a relative density of 99.78%, and minimal porosity (0.91%). Residual tensile stresses was reduced to 131.83 MPa, accompanied by a refined microstructure. Post-processing revealed contrasting effects: quenching improved hardness by 22% due to martensite formation, but induced high tensile stresses (425 ± 14 MPa), whereas annealing at 850°C reduced residual stress by over 90%, improving crack resistance while lowering hardness by 25%. These findings highlight annealing as an effective strategy for stress relief, making AISI 1025 well suited for tool and die applications, particularly in filament production for Fused Deposition Modeling (FDM). MSc in Mechatronic Engineering 2026-03-09T00:00:00Z