Browsing by Author "Daniel Tilahun (Assoc. Prof.)"

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    Failure Analysis of Switch Rails and Crossings Towards Maintenance Improvement: A Case Study of Addis Ababa Light Rail Transit
    (2024) Ruhama Minwuyelet; Daniel Tilahun (Assoc. Prof.)
    Railway systems are such a complex transportation systems that consists several components like rails, switches, crossings, check rails, turnout carriers, and some other components. Maintenance of railway “switch and crossing” (S&C) systems is critical for effective and safe train operations. The material degradation and geometry optimization of switches and crossings should be considered for an efficient operation of railway system. The failures of railway tracks are an unavoidable phenomenon that affects the operation intensively. AALRTS rail material is 50 Kg/m U71Mn and the frog is Hadfield steel. Previously different failure assessment and investigation researches have been carried out, however, failure investigation techniques need to be updated frequently and assessed because the problem still exists. Markov chain model was implemented for statistical analysis of critical failures and the output results are “Mean Time To Failure” for both critical and disastrous failures. Based on the results it is possible to recommend that increasing the number o f “Ultrasonic Inspection Cars” test from 3 to 5 or increasing the test interval from 122 days to 73 days per year will minimize “Mean Time To Failure” from 3.1 years to 1.6 years. The mean time to failure results can be an input for a strategic track maintenance planning. “Failure mode, effects, and criticality analysis” (FMECA) were implemented to identify the most critical failure mode with higher risk. The welded rail specimen`s quality, hardness, and microstructural features were evaluated at different cooling rates experimentally. To identify and assess the microstructure feature and hardness of rail welding through different cooling rates three major NDT tests have been employed. Increasing the number of tests of inspection or the inspection interval will minimize the mean time to failure. Generally, all the non-destructive test results demonstrate that there is a noticeable defect on the welded rail cooled at 6°C/s. Comparatively fewer defects were observed on the welded rail cooled at 3°C/s; while acceptable defects were manifested on the one cooled at 2°C/s. The minimum cooling rate can be achieved through both preheating and post-heating process. From the switch panel, “Failure mode, effects, and criticality analysis” (FMECA) results “gauge corner spalling” failure mode was with the highest risk priority number so that its improvement has a great influence on the maintenance efficiency.Additionally, from the detail results of failure mode, effects, and criticality analysis (FMECA) of turnouts; failure modes under high risk category need special attention during maintenance planning and need improvement of rectification techniques. From the results of the analysis six failure modes have been laid under high risk categories whereas two failure modes have been laid under moderate risk categories and four failure modes have been laid under low risk categories. As a conclusion cooling of rail welding`s at 2°C/s cooling rate will give the material good micro-structural feature and better weld quality relatively. This minimum cooling rate 2°C/s achieved by uniform and optimum preheating and post-heating temperatures. Finally, the researcher recommend a controlled cooling rate for welding quality improvement and maintenance efficiency increment.
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    Influence of Nanoclay on Interlaminar Fracture Characterization of GFRP, and Intralaminar Fracture Characterization of SIMS & Glass Mat Reinforced Thermoplastics Materials
    (Addis Ababa University, 2020-06) Mulugeta Habtemariam; Belingardi, Supervisor: Giovanni (Prof.); Daniel Tilahun (Assoc. Prof.); Ermias G. Koricho (Asst. Prof.)
    The needs of lightweight and customizable structural materials instigate researchers to conduct rigorous materials characterization, with particular attention toward failure mechanisms and safety standards, and to study the materials’ development. Furthermore, the currently available structural and semi-structural composite materials made up of fiber-reinforced plastic, that are presently considered to keep the environmental regulations, require multidimensional examination and analysis because of its heterogeneous nature a non-isotropic behavior. Consequently, this research aims to improve the knowledge on specific lightweight material and to contribute the confidence that toward their use in spite of its poor nature and suspicious to fail, focusing on some fundamental material properties, such as interlaminar and intralaminar behaviors. The research started from failure characteristics and mechanisms, followed by the analysis of the modifications adopted to enhance its structural properties in advanced level, and finally report the adopted experimental characterization procedures and discuss the main findings. The first part of the study has been focused on composite materials with special targets of enhancement and the structural behavior of these materials was experimentally characterized. The material was manufactured with a plain-woven S-glass fiber-reinforce plastic. The material modification was obtained by adding nanoparticles to the matrix; therefore a nano-modified composite was developed by the appropriate combination of epoxy and nanoclay family particles, Cloisite 20B. Thus, the fundamental experimental work included the effect of nanoclay, Cloisite 20B inclusion on the mechanical behavior of a woven type glass fiber reinforced plastic (GFRP) composite. Specifically, the study examined the effect of nanoclay, added with various weight percentages, on the tensile, compressive strengths, and modulus of elasticity of GFRP in both weft and warp directions. Results showed that depending on the warp and weft directions, the inclusion of nanoclay, Cloisite 20B, altered the mechanical behavior of GFRP. The advanced investigations focus on the interlaminar characteristics of the material. In this work, the effect of meticulous nanoclay, Cloisite 20B, inclusion on the interlaminar fracture toughness glass fiber reinforced plastic composite was investigated using careful experimental procedure. Afterwards the study moved to the fracture mechanics behavior, with particular reference to the mode-I interlaminar behavior. Tests were conducted based on a double cantilever beam (DCB) specimen using the specific American Society of Testing Materials standard (ASTM D5528). Results showed that the inclusion of nanoclays improved the interlaminar fracture toughness of the GFRP composite in the range of 12.65% and 54.07% relative to pristine, with progressive percentage increment of the nanoclays weight percentage content (from 0.5 to 2%). Therefore, the dissemination of this experimental research results contributes to overlook a better understanding of nanoclay fillers and their contribution to mechanical behaviors; this can lead to a better design of novel structural composites. Moreover, it guides how Cloisite fillers contribute to improve the delamination resistance with this special composite material having a better retardant flame propagation property that can be relevant for some structures. The second part dealt with lightweight materials that are intended for the vehicle /automotive industry. The intralaminar behavior of the two types of materials, which were supplied by two international companies, was investigated. The fundamental behavior, impact, and special structural application studies of these two types of innovative materials were examined, once again with particular attention toward the impact response and the fracture nature of these materials. The first material type is semi impregnated micro sandwich structure (SIMS) and it is manufactured with two specific reinforcing fibers (carbon and glass). The other material belongs to the glass mat thermoplastic (GMT) family that has also two types: the conventional GMT and the GMT modified by adding unidirectional fibers (GMT-UD) to stiff the structure. For those materials, the intralaminar fracture and the nature of the crack behavior were experimentally investigated using compact tension specimen test. The intralaminar fracture toughness of each material was determined along with crack propagation behavior. As a result, the output of this research fills the gaps and it can contribute to having a full picture of the GMT and SIMS materials.