Thermal power stations operate at elevated temperatures and pressures in order to attain maximum available steam energy. At these high temperatures creep becomes a dominant mechanism that needs to be considered. However, for many components, the locations where peak stresses occur are unreachable to apply the commonly used Non-Destructive Testing (NDT) techniques. This encourages the use of Finite Element Analysis (FEA) to better predict the creep state in these complex components. Commonly, creep damage models are used in conjunction with accelerated creep tests to develop material models that can be implemented into a FEA to determine failure. These approaches are often infeasible for industrial decision-making, leaving a gap for more accessible commercially available models to be developed. This paper focuses on using openly available creep data from the Japanese National Institute for Material Science (NIMS). A creep strain model capable of modelling only the primary and secondary creep regimes was then chosen from the ANSYS database to fit this data. In order to fully characterise the experimental data a multi-creep-model approach was adopted that uses a family of creep models, instead of a single creep material model, to characterise the probable range of responses. This methodology was applied to an industrial application, namely an Intermediate Pressure (IP) valve operating under creep-prone conditions. The multi-creep-model approach was incorporated into FEA to analyse the variation in stress distributions. It was interesting to see that a variation of 153% in the creep strain models only resulted in a 21% variation in the relaxed stress. Worst case scenario life time calculations were then conducted using both a time-based Larson-Miller approach and a strain-based ASME code approach. Both sets of results showed that, for the specific component of interest, creep rupture lifetimes were in excess of 3000 years. It was therefore noted that, for the IP valve of interest, the operating temperature and pressure combination were such that no worrisome creep damage occurred. In conclusion, for the specific component analysed, the operating conditions are such that creep based failure will not occur. / Dissertation (MEng)--University of Pretoria, 2017. / NRF / EPPEI / Mechanical and Aeronautical Engineering / MEng / Unrestricted

The purpose of this thesis is to prove that the use of compliant mechanisms in gearbox applications is viable. Compliant mechanisms are developed for their implementation in Scania’s hybrid asynchronous gearboxes. These mechanisms are presented as a replacement for the latch assembly currently in use to hold the position of the gear-shifting elements. The objective is to implement a compliant mechanism in order to avoid wear and increase the life cycle within the given constraints, as well as to have a better understanding of this kind of mechanisms. The presented literature study shows that bistable and tristable compliant mechanisms are the most suitable ones for this application. Titanium alloys, tool steels, and bulk metallic glasses are discussed as the best material options for compliant mechanism manufacturing. A mechanism idea generation and selection process is conducted. Finite Element Analysis (FEA) is developed with the chosen bistable and tristable compliant mechanism ideas. The tristable concept results on being inappropriate for this application, as it does not fulfil the volume and positioning constraints. The bistable device is proven to be suitable, and further analysis is carried out to study its fatigue resistance and show that it fulfils all the requirements, solving the weaknesses of the latch and absorbing the impact in the shaft. Additive manufacturing methods and injection moulding are found to be incompatible with the designed mechanisms. That is why the chosen bistable mechanism is designed to be made out of different parts. Future work is presented to strengthen the weaker points of this project.

<div>The author aims to investigate the current design provision for seismic and foundation settlement design of aboveground open-top storage tanks using finite element analysis. The thesis is divided into two independent but closely related studies: (1) seismic analysis of open-top storage tanks with flexible foundation and (2) fitness-for-service of open-top storage tanks subjected to differential settlement.</div><div><br></div><div>The present seismic design provisions in American Petroleum Institute’s storage tank standard API 650 (2013) assumes the tank foundation is rigid and therefore, ignores the effect of uplift during a seismic excitation. In the first study, the objective was to quantitatively critique rigid foundation assumption and conclude if the assumption is acceptable or not for a given tank geometry. Tanks with three different height to diameter ratio (H/D), i.e aspect ratios, of 0.67, 1.0 and 3.0 representing broad, nominal and slender geometry, respectively, were modelled having both rigid and flexible foundations. The flexible foundation was modelled with series of non-linear compression only springs. Additionally, for each tank model two different hydrodynamic pressure distribution suggested by (1) Housner and (2) Jacobsen-Veletsos were applied which are used by API 650 and Eurocode 8, respectively. Geometric non-linear analysis with non-linear material properties was conducted (GMNA) using Riks algorithm in Abaqus finite element analysis (FEA) program. The hoop stresses, longitudinal stresses, uplift and buckling capacity of each rigid foundation tank model were compared with its respective flexible foundation tank model and corresponding API 650 rule based provisions. It was observed that the assumption of rigid foundation from design point of view is acceptable for the broad tank, however, for the nominal and slender tanks this assumption is not acceptable. The buckling capacity of nominal and slender tanks having flexible foundation are significantly lower compared to rigid foundation. Therefore, the effect of uplift should not be neglected for design purposes for nominal and slender tank geometries.</div><div><br></div><div>In the second study, an alternative method for evaluating the structural integrity of storage tank subjected to differential settlement is proposed. The limitations of the existing method in API 653 (2014), currently used in the industry are highlighted. The tank settlement is measured underneath</div><div>12</div><div>the tank bottom along the tank circumference at discrete locations. The settlement can be transformed into a Fourier series by combining different harmonic components. In the existing API 653 method there is no distinction between the effects of different harmonic components whereas in the proposed method the effects of first five harmonic components are individually accounted and the cumulative damage is evaluated. The proposed method is formulated based on FEA conducted on twenty-one different tank models with each having different tank geometry. The limiting settlement value for each harmonic wave number is found for a given tank geometry by conducting GMNA using Riks algorithm, and a generalized trend is found for each harmonic wave number. The proposed method is further validated by performing numerous FEA simulations. The simulations were conducted for several tank models subjected to four representative actual measured settlement data. A set of tank models used in the validation was generated using random tank geometries and design parameters to have a blind test of the proposed method. Finally, a comparison is made between allowable settlement based on the API 653 method, the proposed method and the FEA. It was observed that the proposed method consistently results in conservative results compared to FEA. In contrast the API 653 method does not always result in conservative results. For some measured settlement data, the API 653 method gives overly conservative values and for others it gives non-conservative values. Moreover, the API 653 method is based on the beam theory which may not capture the true shell behavior. Therefore, the API 653 method requires modifications. The proposed method on the other hand is consistent and is based FEA which can capture the true shell behavior as it is formulated using shell theory. Therefore, it is recommended that the existing method in API 653 shall be replaced with the proposed method to determine the fitness of tank under differential settlement.</div>

Recently, the railway pavement structure system, as an integral part of the transport infrastructure, has been under fast development in some countries such as China, Turkey, and some European Union countries, particularly for the use of high-speed trains. In designing and constructing the railway pavement structure, it is necessary to take into account the infrastructure demand of the High-Speed Railway Lines (HSRL). Compared to traditional railway trains, HSRL can cause more significant problems to the ballast or base layer of commonly used ballasted railway pavements. The deteriorated ballast or base layer may further result in substructure degradation that may cause safety issues and catastrophic accidents. As a consequence, heavy goods or high-speed trains will affect railway efficiency. As a countermeasure, a railway pavement structure may be reinforced by geosynthetic materials in the ballast or base layer. In the literature, however, there is still a need to quantify the effect of geosynthetic materials, geogrid in particular, on the mechanical responses of railway pavement structures to HSRL loads, which is necessary knowledge in supporting the selection of appropriate material and placement location of geogrid. Therefore, the goal of this study is to investigate how a geogrid reinforcement layer can change the essential characteristics of a ballasted railway pavement structure, with focus on the material type and placement location of geogrid that can help minimize the rate of deterioration of the railway pavement structure system. This research attempts to validate the advantage of geogrid reinforcement through numerical simulation in a realistic railway setting. All technical literature on the use of geogrids in the railway system has been studied. A three-dimensional (3D) finite element model was constructed for the numerical simulation, in which three different types of geogrid placed at two different locations (i.e., within the ballast layer, between the ballast and the sub-ballast layer) within a railway pavement structure were analyzed under a range of vertical wheel loads. Therefore, four possible applications of geogrid reinforcement systems (G0: no-reinforcement; G1: reinforced with geogrid having the lowest density and Young’s modulus; G2: reinforced with geogrid having the intermediate Young’s modulus and density; G3: reinforced with geogrid having the highest density and Young’s modulus) were modeled to represent different situations in ballasted railway systems. Railway mechanical responses, such as vertical surface deflection, maximum principal stress and strain, and maximum shear stress were analyzed and compared among the four geogrid reinforcement scenarios and under four vertical wheel load levels (i.e., 75, 100, 150 and 200 kN). The advantages of such geosynthetics in ballast are indicated by result difference in the mechanical responses of railway pavement structures due to the use of different geogrid materials. The results also show that the reinforced structures have lower vertical surface deflection, lower maximum shear stress at the interface of sleeper and ballast, and maximum principal stress at the bottom of the ballast layer than a non-reinforced railway pavement structure. Consequently, the addition of geogrid into the ballast layer, and between the ballast and sub-ballast layer has been shown to reduce critical shear and principal stresses and vertical surface deflection in a ballasted railway pavement structure. Besides that, the results of the analysis confirm that geogrid reinforced layers exhibit higher resistance to deformation than the non-reinforced layers.

For fatigue design of welded hollow structural sections connections, the “hot spot stress method” in CIDECT Design Guide 8 is widely used. This method forms the basis of various national and international design standards. This thesis sought to address some contemporary design issues where the existing approaches cannot be directly applied. Modified design approaches were proposed for various practical design details. For galvanizing of welded tubular steel trusses, sufficiently large holes to allow for quick filling, venting and drainage must be specified. These holes, quite often specified at the hot spot stress locations, will inevitably affect connection fatigue behaviour. In Chapter 1, six rectangular hollow section (RHS) connections were tested under branch axial loading. The stress concentration factors (SCFs) obtained from the experimental investigation were compared with those calculated using the formulae in CIDECT Design Guide 8. It was shown that the predictions based on the current formulae were unsafe. Hence, finite element (FE) models were developed and validated by comparison with the experimental data. A subsequent parametric study was conducted, including 192 FE models with different hole locations and non-dimensional parameters [branch-to-chord width (β), branch-to-chord thickness (τ), and chord slenderness (2γ) ratios]. SCF formulae for RHS connections with vent/drain holes at different locations were established based on the experimental and FE data. In Chapter 2, by modifying the 192 parametric models in Chapter 1, FE analysis was performed to examine the existing SCF formulae in CIDECT Design Guide 8 for RHS T-connections under branch in-plane bending. The parametric study showed that the existing SCF formulae can lead to unsafe predictions. Critical hot spot stress locations were thus identified. The effects of both branch in-plane bending and chord loading were studied. New design formulae that take the vent and drain holes into account were proposed. The design rules in CIDECT Design Guide 8 assumes sufficient chord continuity on both sides of connection. Therefore, the existing formulae cannot be directly applied to RHS-to-RHS connections situated near a truss/girder end. Chapter 3 sought to develop new approach for calculation of SCFs in such connections. 256 FE models of RHS-to-RHS X-connections, with varied chord end distance-to-width (e/b0) and non-dimensional parameters were modelled and analyzed. The analysis was performed under quasi-static axial compression force(s) applied to the branch(es) and validated by comparison of strain concentration factors (SNCFs) to SNCFs obtained from full-sized connection tests. For all 256 connections, SCFs were determined at five critical hot spots on the side of the connection near the open chord end. The SCFs were found to vary as a function of e/b0, 2γ and β. Existing formulae in CIDECT Design Guide 8 to predict SCFs in directly welded RHS-to-RHS axially loaded X-connections were shown to be conservative when applied to a connection near an open chord end. SCF reduction factors (ψ), and a parametric formula to estimate ψ based on e/b0, 2γ and β, were derived. For RHS-to-RHS connections situated near a truss/girder end, reinforcement using a chord-end cap plate is common; however, for fatigue design, formulae in current design guidelines [for calculation of SCFs] cater to: (i) unreinforced connections, with (ii) sufficient chord continuity beyond the connection on both sides. Chapter 4 sought to develop definitive design guidelines for such connections. The parametric models in Chapter 3 were modified to simulate such connections. Existing SCF formulae in CIDECT Design Guide 8 were shown to be inaccurate if applied to cap plate-reinforced end connections. SCF correction factors (ψ), and parametric formulae to estimate ψ based on e/b0, β, τ and 2γ, were derived. The same methodology was used in Chapter 5 to study the SCFs in square bird-beak (SBB) and diamond bird-beak (DBB) tubular steel X-connections situated at the end of a truss or girder. A comprehensive parametric study, including 256 SBB and 256 DBB connection models, covering wide ranges of chord end distance-to-width (e/b0) and non-dimensional parameters, was performed. Two sets of correction factor (ψ) formulae for consideration of the chord end distance effect were derived, for SBB and DBB X-connections, respectively. / Graduate

Fatigue failure is one of the most critical issues in industry nowadays as 60 to 90 percent of failures in metals are due to fatigue. Therefore, different methods and approaches are developed to estimate the fatigue life of metallic parts. In this research, a case-hardened steel splined shaft is studied to estimate the fatigue life that the shaft will withstand before failure. The purpose of the research is to develop an advanced fatigue prediction method for splines.A static experimental test was performed on the splined shaft for analyzing the load-strain behavior of the shaft and determining the suitable load cases of the study. A dynamic test of pure torsional load was carried out to collect experimental results for validating the generated fatigue methods and investigating the failure behavior of the shaft. Stress analysis was performed on the part for investigating critical areas and the effect of the different spline teeth designs on the resulting stress. Two finite element models were analyzed using two software, MSC Marc software with a geometry of straight spline teeth and Spline LDP with an involute spline teeth model. DIN 5466-1 spline standard’s analytical solution was used for verification purposes. Stress and strain-based approaches were used to estimate fatigue life. The most suitable method was evaluated against experimental test results.The research findings show that the most critical stress areas on the shaft are the spline root fillet and relief. When the part fails due to fatigue the crack initiates at the root fillet and propagates to the relief. It is also shown that involute teeth spline gives higher stress than straight teeth for the same load due to less contact area.The conclusion of the research could be summarized in: the stress-based method (Wöhler curve) is giving good accuracy and proved a reliable method. While among six different approaches used of strain-based methods, four-point correlation method is giving the best correlation to test results. Hence, it is recommended to use four-point correlation method for fatigue analysis for its accuracy and for considering both elastic and plastic behavior of the material.

<p>This paper presents an experimental and numerical study of the behavior of steel wide flange sections subjected to loads causing compression buckling in the web. This research includes experimental investigation of the effects of load width and duration on web compression buckling. This data is then used to calibrate numerical models. Experimental investigations were conducted on specimens with load widths of approximately 2.5, 1.75, and 1.5 times their section depth. Loads sustained on the specimens had a magnitude of about 85% of the expected buckling strength to investigate creep effects near failure. Results of these experiments were used to calibrate numerical models for a parametric study.</p><p>The numerical parametric study examined 60 specimens of four wide flange sections, investigating the effects of loaded width and angle of load application on web compression buckling. The numerical models accounted for initial imperfections in the specimens by applying imperfections with a magnitude of 0.13*<i>t_w</i> to the first mode shape obtained from a linear perturbation analysis. This value of imperfection was chosen because it is the average imperfection measured in the experimental specimens and is likely a good representation of a typical wide flange section.</p><p>A prediction method is provided based on the data obtained from the numerical parametric study. This prediction method is derived from rectangular plate buckling solutions and considers the cases where the width of the concentrated load is a function of the section depth, and when the applied load is not orthogonal to the specimen. The current AISC 360-16 provisions do not directly address the influence of load width on the calculation of web compression buckling strength and refer to the design of compression members when the loaded width is greater than or equal to the section depth. The AISC approach was also evaluated and deemed conservative for design.</p><br>

The purpose of this master thesis was to study the dynamic response of the bridge Västerbron. The bridge is situated in Stockholm and is considered being of critical significance for the infrastructure. The thesis consists of both field measurements and analyses of a finite element model. A stochastic load model was created that is intended to simulate different realistic traffic situations based on parameters as velocity, vehicle type and amount of traffic. The traffic load model was implemented in a finite element model to study if the response was similar to the measurements. With comparisons of the dynamic properties the validity of the model can be assessed. Parameters as stiffness, mass and boundary conditions also often needs to be updated to describe the real behaviour of the bridge. With these updates a model can be created that could better predict problematic behaviour as fatigue. The field measurements were made with accelerometers and analysed in Matlab. The stochastic load model is also scripted in this environment. The FE-model was created using Python scripts that were implemented in BRIGADE/Plus. No conclusive results regarding the mode shapes of Västerbron could be found, however possible eigenfrequencies were identified and presented. The load model was implemented in the FE-model and the influence of different parameters were discussed. The results were consistent with structural dynamics theory and in the same order of magnitude as the measurements. This implies that the traffic load model can be used for further studies regarding dynamic analyses.

Magnetic field-assisted freeze-casting was used to create porous B4C ceramic preforms. An optimum slurry consisted of a mixture of B4C powders with 6 wt.% Er2O3 powder in an H2O-PVA solution and was cooled at a rate of 1 °C/min from room temperature to -30 °C resulting in porous green state ceramic preform with vertical channels. The Er2O3 powder was added to improve the magnetic response of the slurry. The preform was then sublimated to remove H2O and then sintered. The sintered ceramic preform was then infiltrated in the most vertically aligned channel direction with molten Al (A356) metal through a vacuum-assisted pump to create the metal matrix composite (MMC). Finite element analysis simulations were used to analyze and predict the anisotropic effect of B4C channel alignment on mechanical properties. The mechanical properties of the composite were then experimentally found via compression testing, which was compared with rule-of-mixtures and finite element modeling simulations, to analyze the effect of anisotropy due to magnetic field-assisted freeze-casting. This study reinforces the viability of cost-effective magnetic field-assisted freeze-casting as a method to create highly directional ceramic preforms, which can be subsequently metal infiltrated to produce MMCs with highly anisotropic toughness.

Although the concept of tensegrity structures was invented in the beginning of the twentieth century, the applications of these structures are limited, partially due to their low stiffness. The stiffness of tensegrities comes from topology, configuration, pre-stress and initial axial element stiffnesses.  The first part of the present work is concerned with finding the magnitude of pre-stress. Its role in stiffness of tensegrity structures is to postpone the slackening of cables. A high pre-stress could result in instability of the structure due to buckling and yielding of compressive and tension elements, respectively. Tensegrity structures are subjected to various external loads such as self-weight, wind or snow loads which in turn could act in different directions and be of different magnitudes. Flexibility analysis is used to find the critical load combinations. The magnitude of pre-stress, in order to sustain large external loads, is obtained through flexibility figures, and flexibility ellipsoids are employed to ensure enough stiffness of the structure when disturbances are applied to a loaded structure.  It has been seen that the most flexible direction is very much sensitive to the pre-stress magnitude and neither analytical methods nor flexibility ellipsoids are able to find the most flexible directions. The flexibility figures from a non-linear analysis are here utilized to find the weak directions.  In the second part of the present work, a strategy is developed to compare tensegrity booms of triangular prism and Snelson types with a truss boom. It is found that tensegrity structures are less stiff than a truss boom when a transversal load is applied. An optimization approach is employed to find the placement of the actuators and their minimum length variations. The results show that the bending stiffness can be significantly improved, but still an active tensegrity boom is less stiff than a truss boom. Genetic algorithm shows high accuracy of searching non-structural space. / QC 20110524