1,721,095 research outputs found
Full-field analysis of the dynamic behaviour of thermally stressed panels
Get PDFThis thesis details the research conducted over the course of three years under funding from the European Office of the United States Air Force (EOARD) and the Engineering and Physical Sciences Research Council (EPSRC) as a part of a long-standing effort to collect high-quality experimental data which can be used in the development and validation of predictive computational mechanics models. The focus of this study is on the acquisition of full-field displacement and temperature data when thermally and thermo-mechanically loading aerospace grade material panels as a means to study the effect of non-uniform temperature distributions on their dynamic behaviour at a component level (macroscale). The inclusion of this data in the development of a robust predictive model has also been investigated. To that end, a review of the existing literature is provided which highlights the current knowledge gaps in the modelling and experiments on the thermal and thermo-vibratory loading of panels, as well as the state-of-the-art in full-field data analysis.
Initially, a finite element (FE) model was developed and compared to predictive and experimental data available in literature. This allowed for an investigation into the best practices to adopt in the development of a computational mechanics model with temperature-dependent material properties. It was found that a successful representation of experimental conditions strongly depends on the effective depiction of the thermal load and initial shape of the component.
Then, a thin plate with free edges and constrained about its centre was heated using quartz lamps arranged in two different configurations and mechanically loaded using a shaker. Experimental modal analysis was used to acquire the resonant frequencies and mode shapes of the plate. Mode shapes were studied by exciting the plate to its first eleven resonant frequencies and acquiring displacement data using a Pulsed Laser Digital Image Correlation method (PL-DIC). Infra-red imaging was used to acquire temperature maps across the specimen. Experimentally-acquired temperature maps and measurements of the plate’s initial shape were included in a temperature-dependent FE model, developed according to the findings in the preliminary study, previously described. For the first time, experimental results showed the resonant response of the plate to strongly depend on the temperature distribution across the structure, correlating well with past predictive work in the literature. This was supported by the results from the finite element model, which were validated against experimental data and found to yield reliable predictions.
The influence of temperature distribution in the deformation of panels was further investigated using a 1 mm plate with reinforced edges. The geometry was designed to emulate an aircraft’s skin with the reinforced edges performing the function of stringers and ribs. High temperatures were achieved using quartz lamps arranged in various configurations with controllable power output. PL-DIC was used to measure surface displacements and a commercially-available micro bolometer mapped the temperature distribution across the plate. Deflection results for the reinforced plate showed it to behave as a dynamic system that buckles out-of-plane when heated before relaxing to a steady state. It was demonstrated that the out-of-plane displacement experienced by the plate is strongly influenced by the in-plane spatial distribution of temperature
Integrated Modelling and Testing of Engineering Structures: A Validation Approach for Multi-physics Simulations
Get PDFThe aim of this project was to develop an extended approach for the validation of multi-physics models, using quantitative full-field data obtained from a carefully designed validation experiment. This work was performed to meet one of the technological gaps identified for an Integrated Nuclear Digital Environment (INDE), which was the need to develop a multi-scale and multi-physics validation method.
An enhanced multi-physics process flowchart containing three different physics domains, thermal, mechanical, and thermomechanical domains, has been successfully developed, which details the approach for validation of multi-physics models using quantitative experimental data. The new enhanced process flowchart was built from the flowcharts in the ASME and CEN V&V guides, with a focus on the design of validation experiments. As part of this research's objectives, the processes in the new flowchart were successfully tested using an industrial case study and carefully designed experiments that captured each physics component.
Three experiments, thermal expansion, three-point bending, and combined thermal expansion and three-point bending experiments, were successfully designed and tested to explore the processes of the enhanced validation flowchart. The comparison between the results obtained from the simulations and the measurements indicated a good level of agreement. A high degree of linearity was observed for the load versus displacement relationships for the physical experiments and the simulations. The superposition principle was employed to mathematically combine the data from the single physics domains to account for the overall effect of the multi-physics domains. The results obtained from the superposition of different physics data were in good agreement with those obtained from the multi-physics data, indicating that the multi-physics experiment successfully captured the boundary conditions of the individual physics domain. In this research, a system calibration experiment was performed for each experimental setup (thermal, mechanical, and combined thermal-mechanical experiments) to determine the uncertainty associated with each physical measurement. The results determined from the calibration experiment was used to determine the allowable or acceptable scatter in the comparison plots
Determination of Residual Stress in Components Manufactured using Laser Powder Bed Fusion (L-PBF)
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From measurement and uncertainty fields to features: a statistical approach for model validation.
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Study of residual stresses around cold-expanded holes
Get PDFSplit sleeve cold expansion is one of the most widely used methods in the aerospace industry to enhance the fatigue performance of fastener holes in airframe structures. The initial motivation, which led to this research programme, was to develop an understanding of the behaviour of fatigue cracks emanating from cold-expanded holes, with a particular emphasis on the influence of these cracks on the surrounding compressive residual stresses. There are two strands of the research presented in this thesis: first being related to the study of hole deformation resulting from split sleeve cold expansion; and the second one focused on the fatigue behaviour of cracks emanating from cold-expanded holes.
The strain fields developed from cold expansion were measured using stereoscopic digital image correlation (DIC) technique in aluminium specimens of two different thicknesses giving thickness to hole diameter ratio of 0.25 and 1. The capability of DIC in providing full-field strain data was exploited to determine the shape and size of the plastic zones developed from cold expansion. The results showed that the existing split sleeve cold expansion process is not as effective in creating an axisymmetric compressive residual elastic stress zone around the fastener holes in thin as it is in the thick specimens. The thin specimens used in this investigation were equivalent in thickness to sheet material commonly used in an aircraft fuselage or wing skins and the results indicate that there is a need to review the use of cold expansion process using a split sleeve and mandrel for holes in thin sheets.
A simple approach utilising DIC was presented to analyse the strain fields resulting from cold expansion in stacked specimens. The results showed that stacking offers some improvement in the cold expansion of thin sheet components. They also demonstrated the workability of this approach which can be applied effectively to analyse cold expansion of fastener holes associated with a real joint configuration in an airframe.
The propagation of fatigue cracks initiating from the cold-expanded holes was investigated by employing the thermoelastic stress analysis (TSA) technique and their influence on the surrounding residual stresses was determined using synchrotron x-ray diffraction (SXRD) technique. A long-standing ambiguity in the literature regarding the potential relaxation of beneficial compressive residual stresses, as a result of fatigue crack propagation, was addressed; and it was established, from TSA and SXRD results, that the formation or propagation of a fatigue crack does not cause any significant relaxation of these residual stresses. The results also clearly identify the loading conditions under which the residual stresses are expected to relax. This information is important in improving the theoretical models for fatigue life assessment of cold-expanded holes. The results should also be useful for the engineers in the aerospace industry to realise the full potential of the cold expansion process and to utilise it more effectively in the manufacturing of airframes leading to improved fatigue endurance under different loading conditions
DEVELOPMENT OF A REGULATORY ENVIRONMENT FOR FUSION REACTORS
Get PDFThe growing interest in nuclear energy in response to the rising cost of energy has brought forward the potential of fusion energy. Nuclear fusion has been developed since the 1970s; however, the commercial realisation of fusion energy has only gained some significant progress in recent years. The need for an economic fusion design and a clear regulatory framework were emphasised across the industry.
An innovative methodology was developed to investigate the key differences in safety designs and regulatory requirements between nuclear reactor types by mapping nuclear safety features to regulatory fundamental principles. Safety features were extracted from safety cases of two research reactors, including a research fusion reactor (Joint European Torus) and a research fission reactor (Tsing Hua Open-pool Reactor), and a safety case of a commercial fission reactor (Hinkley Point C). A comparative analysis based on the maps has revealed differences between hazards profiles of fission and fusion reactors. Furthermore, the three maps formed a basis for the development of a regulatory safety map for a commercial fusion reactor. A predicted map was produced, which demonstrated a significant increase in the number of safety features compared in the commercial fusion reactor compared with research ones. Additionally, the map also revealed an order of priority in the safety designs of different design areas. Qualitative data using a semi-structured interviews were conducted to investigate the current practice in the safety design of a commercial fusion reactor, which was used to validate the prediction methodology.
The successful delivery of a fusion design relies on an effective design approach to overcome the complexity of fusion reactors. An investigation into the challenges in implementing concurrent engineering design process was conducted through a literature review. Three major challenges were identified: effective communication systems, implementation of design reviews and compatibility of tools. Key attributes and capabilities of an integrated digital environment were identified and examined for the potential in overcome these challenges. A case study was conducted in the STEP programme at UKAEA to study the current design process of a commercial fusion reactor. The study revealed that the use of simulations models and digital tools has helped reducing some level of these challenges; however, they still exist. It was highlighted that an integrated digital environment is needed to effectively implement a concurrent design approach
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