Multi-objective Optimization of Tube Hydroforming Using Hybrid Global and Local Search

An investigation of non-linear multi-objective optimization is conducted in order to define a set of process parameters (i.e. load paths) for defect-free tube hydroforming. A generalized forming severity indicator that combines both the conventional forming limit diagram (FLD) and the forming limit stress diagram (FLSD) was adopted to detect excessive thinning, necking/splitting and wrinkling in the numerical simulation of formed parts. In order to rapidly explore and capture the Pareto frontier for multiple objectives, two optimization strategies were developed: normal boundary intersection (NBI) and multi-objective genetic algorithm (MOGA) based on the concept of dominated solutions . The NBI method produced a uniformly distributed set of solutions. For the MOGA method, a stochastic Kriging model was used as a surrogate model. Furthermore, the MOGA constraint-handling technique was improved, Kriging model updating was automated and a hybrid global-local search was implemented in order to rapidly explore the Pareto frontier. Both piece-wise linear and pulsating pressure paths were investigated for several case studies, including straight tube, pre-bent tube and industrial tube hydroforming. For straight tube hydroforming, the optimal load path was obtained using the NBI method and it showed a smaller corner radius compared to that predicted by the commercial program LS-OPT4.0. Moreover, the hybrid method coupling global search (MOGA) and local search (sequential quadratic programming: SQP) was applied for straight tube hydroforming, and the results showed a significant improvement in terms of the stress safety margin and reduced local thinning. For a commercial refrigerator door handle, the MOGA method was utilized to inversely analyze the loading path and the calculated path correlated well with the production path. For a hydroformed T-shaped tubular part, the amplitude and frequency of the pulsating pressure were optimized with MOGA. Thinning was reduced by 25% compared with experimental results. A multi-stage (prebent) tube hydroforming simulation was performed and it indicated that the reduction in formability due to bending can be largely compensated by end feeding the tube during hydroforming. The loading path optimized by MOGA showed that the expansion into the corner of the hydroforming die increased by 16.7% compared to the maximum expansion obtained during experimental trials

Finite element and mechanobiological modelling of vascular devices

There are two main surgical treatments for vascular diseases, (i) percutaneous stent deployment and (ii) replacement of an atherosclerotic artery with a vascular graft or tissue engineered blood vessel. The aim of this thesis was to develop computational models that could assist in the design of vascular stents and tissue engineered vascular grafts and scaffolds. In this context, finite element (FE) models of stent expansion in idealised and patient specific models of atherosclerotic arteries were developed. Different modelling strategies were investigated and an optimal modelling approach was identified which minimised computational cost without compromising accuracy. Numerical models of thin and thick strut stents were developed using this modelling approach to replicate the ISAR-STEREO clinical trial and the models identified arterial stresses as a suitable measure of stent induced vascular injury. In terms of evaluating vascular graft performance, mechanical characterisation experiments can be conducted in order to develop constitutive models that can be used in FE models of vascular grafts to predict their mechanical behaviour in-situ. In this context, bacterial cellulose (BC), a novel biomaterial, was mechanically characterised and a constitutive model was developed to describe its mechanical response. In addition, the interaction of smooth muscle cells with BC was studied using cell culture experiments. The constitutive model developed for BC was used as an input for a novel multi-scale mechanobiological modelling framework. The mechanobiological model was developed by coupling an FE model of a vascular scaffold and a lattice free agent based model of cell growth dynamics and remodelling in vascular scaffolds. By comparison with published in-vivo and in-vitro works, the model was found to successfully capture the key characteristics of vascular remodelling. It can therefore be used as a predictive tool for the growth and remodelling of vascular scaffolds and graft

Desenvolvimento de metodologias para identificação de parâmetros e otimização de forma em simulações numéricas de processos de conformação plástica

Doutoramento em Engenharia MecânicaPor parte da indústria de estampagem tem-se verificado um interesse crescente em simulações numéricas de processos de conformação de chapa, incluindo também métodos de engenharia inversa. Este facto ocorre principalmente porque as técnicas de tentativa-erro, muito usadas no passado, não são mais competitivas a nível económico. O uso de códigos de simulação é, atualmente, uma prática corrente em ambiente industrial, pois os resultados tipicamente obtidos através de códigos com base no Método dos Elementos Finitos (MEF) são bem aceites pelas comunidades industriais e científicas Na tentativa de obter campos de tensão e de deformação precisos, uma análise eficiente com o MEF necessita de dados de entrada corretos, como geometrias, malhas, leis de comportamento não-lineares, carregamentos, leis de atrito, etc.. Com o objetivo de ultrapassar estas dificuldades podem ser considerados os problemas inversos. No trabalho apresentado, os seguintes problemas inversos, em Mecânica computacional, são apresentados e analisados: (i) problemas de identificação de parâmetros, que se referem à determinação de parâmetros de entrada que serão posteriormente usados em modelos constitutivos nas simulações numéricas e (ii) problemas de definição geométrica inicial de chapas e ferramentas, nos quais o objetivo é determinar a forma inicial de uma chapa ou de uma ferramenta tendo em vista a obtenção de uma determinada geometria após um processo de conformação. São introduzidas e implementadas novas estratégias de otimização, as quais conduzem a parâmetros de modelos constitutivos mais precisos. O objetivo destas estratégias é tirar vantagem das potencialidades de cada algoritmo e melhorar a eficiência geral dos métodos clássicos de otimização, os quais são baseados em processos de apenas um estágio. Algoritmos determinísticos, algoritmos inspirados em processos evolucionários ou mesmo a combinação destes dois são usados nas estratégias propostas. Estratégias de cascata, paralelas e híbridas são apresentadas em detalhe, sendo que as estratégias híbridas consistem na combinação de estratégias em cascata e paralelas. São apresentados e analisados dois métodos distintos para a avaliação da função objetivo em processos de identificação de parâmetros. Os métodos considerados são uma análise com um ponto único ou uma análise com elementos finitos. A avaliação com base num único ponto caracteriza uma quantidade infinitesimal de material sujeito a uma determinada história de deformação. Por outro lado, na análise através de elementos finitos, o modelo constitutivo é implementado e considerado para cada ponto de integração. Problemas inversos são apresentados e descritos, como por exemplo, a definição geométrica de chapas e ferramentas. Considerando o caso da otimização da forma inicial de uma chapa metálica a definição da forma inicial de uma chapa para a conformação de um elemento de cárter é considerado como problema em estudo. Ainda neste âmbito, um estudo sobre a influência da definição geométrica inicial da chapa no processo de otimização é efetuado. Este estudo é realizado considerando a formulação de NURBS na definição da face superior da chapa metálica, face cuja geometria será alterada durante o processo de conformação plástica. No caso dos processos de otimização de ferramentas, um processo de forjamento a dois estágios é apresentado. Com o objetivo de obter um cilindro perfeito após o forjamento, dois métodos distintos são considerados. No primeiro, a forma inicial do cilindro é otimizada e no outro a forma da ferramenta do primeiro estágio de conformação é otimizada. Para parametrizar a superfície livre do cilindro são utilizados diferentes métodos. Para a definição da ferramenta são também utilizados diferentes parametrizações. As estratégias de otimização propostas neste trabalho resolvem eficientemente problemas de otimização para a indústria de conformação metálica.The interest of the stamping industry in the numerical simulation of sheet metal forming, including inverse engineering approaches, is increasing. This fact occurs mainly because trial and error design procedures, commonly used in the past, are no longer economically competitive. The use of simulation codes is currently a common practice in the industrial forming environment, as the results typically obtained by means of the Finite Element Method (FEM) are well accepted by both the industrial and scientific communities. In order to obtain accurate stress and strain fields, an effective FEM analysis requires reliable input data such as geometry, mesh, non-linear material behaviour laws, loading cases, friction laws, etc.. In order to overcome these difficulties, a possible approach is based on inverse problems. In this work, the following inverse problems in computational Mechanics are presented and analysed: (i) parameter identification problem, that refer to the definition of input parameters to be used in constitutive models for numerical simulations, based on experimental data, and (ii) initial blank and tool design problem, where the aim would be to estimate the initial shape of a blank or a tool in order to achieve the desired geometry after the forming process. New optimization strategies in parameter identification problems that lead more efficiently to accurate material parameters are introduced and implemented. The aim of these strategies is to take advantage of the strength of each selected algorithm and improve the overall robustness and efficiency of classical optimization methodologies based on single stages. Deterministic algorithms, evolutionary-inspired algorithms or even the combination of these two algorithms are used in the proposed strategies. Strategies such as cascade, parallel and hybrid approaches are analysed in detail. In hybrid strategies, cascade and parallel approaches are integrated. Two different approaches are presented and analyzed for the evaluation of the objective functions in parameter identification processes. The approaches considered are single-point and FE analyses. The single infinitesimal point evaluation seems to characterize an infinitesimal amount of material subjected to all kind of deformation history. On the other hand, in all FE analysis codes, the constitutive model is implemented and accounted for in each element integration point. Inverse problems, such as blank and tool design, are presented and described. In the case of the initial blank optimization process the design of a carter is presented. Also related to the initial blank optimization process, a study of the influence of the initial geometry definition in the optimization process is conducted. This study is performed considering the NURBS formulation to model the blank upper surface that will be changed during the optimization process. In the case of the tool design problem, a two-stage forging process is presented. In order to achieve a straight cylinder after forging, two different approaches are analyzed. In the first one, the initial geometry of the cylinder is optimized and, in the other one, the shape of the first stage tool is optimized. To parameterize the free surface of the cylinder different methods are presented. Furthermore, in order to define the tool in this example, different parameterizations are presented. The optimisation strategies proposed in this work efficiently solve optimisation problems for the industrial metal forming

Ultra-high temperature concentrated solar thermal energy

Given the extremely high surface temperature of the Sun (~5778 K), solar radiation has the theoretical potential, in accordance with the second law of thermodynamics, to heat a receiver on Earth up to ultra-high temperatures (specified in this thesis as >1300 K). However, there is a gap between theory and practice, as contemporary solar thermal energy systems are still limited to temperatures below 900 K due to material and mechanical limitations. Running solar thermal energy at ultra-high temperatures promises greater energy conversion efficiencies for power plants by upgrading their basic cycles to include more advanced power cycles. Furthermore, the provision of solar thermal energy at ultra-high temperatures can unlock a wide range of energy-intensive industrial applications, including hydrogen and cement production, which can contribute to decarbonising sectors which are difficult to electrify. This thesis proposes a novel concept of an ultra-high temperature solar cavity receiver based on an optically exposed liquid metal heat transfer fluid, which flows down a corrugated back plate. The concept is investigated using a quasi-steady-state analytical energy model, in addition to a radiation-coupled Computational Fluid Dynamics (CFD) solution. The developed analysis methods are tailored to the proposed class of receivers, nonetheless, they can be generalised for broad solar receiver analysis or for analysing similar problems involving volumetric radiation absorption in other thermal applications. The concept is shown implementable at its absorptive cavity configuration with an overall (optical and thermal) receiver efficiency exceeding 70%. The proposed concept is a step towards narrowing the technological mismatch, in terms of temperature and scale, between state-of-the-art thermal energy storage and concentrated solar thermal at ultra-high temperatures. A characterisation of prospective ultra-high temperature receivers is presented, which involved a review of state-of-the-art solar thermal technologies with the purpose of identifying the existing challenges to operating at ultra-high temperatures. Based on this characterisation, the proposed receiver is designed to address the literature concerns. The proposed receiver concept involved novel engineering features, including the use of refractory containment materials and a transparent ceramic window to seal the aperture. Therefore, the conceptual investigation attempted to address possible concerns that might be introduced by the new features. Finally, the proposed receiver is demonstrated in a concentrated solar power plant application to emphasise, using quantitative terms, the benefits of operating the receiver at ultra-high temperatures for large-scale applications

Reuse of steel and aluminium without melting

Carbon dioxide emissions must be dramatically reduced to avoid the potentially dangerous effects of climate change. The steel and aluminium industries produce large amounts of carbon dioxide, accounting for 6% of anthropogenic emissions. Previous studies have shown that in these industries there is limited scope for further improvements in energy efficiency. Material efficiency strategies can, however, further reduce emissions. This thesis focuses on materially efficient reuse without melting. A scoping study of current reuse found three opportunities, an examination of which forms the basis of this thesis: reusing components at end of product life; extending the lifespan of products; and reusing manufacturing scrap. The opportunity to reuse components has received little attention to date and there is no clearly defined set of strategies or barriers to enable assessment of appropriate component reuse; neither is it possible to predict future levels of reuse. This thesis presents a global assessment of the potential for reusing steel and aluminium components. A combination of top-down and bottom-up analyses is used to allocate the final destinations of current global steel and aluminium production to final products. A substantial catalogue has been compiled for these products characterizing key features of steel and aluminium components including design specifications, requirements in use, and current reuse patterns. To estimate the fraction of end-of-life metal components that could be reused for each product, the catalogue formed the basis of a set of semi-structured interviews with industrial experts. The results suggest that approximately 30% of steel and aluminium used in current products could be reused. Barriers against reuse are examined, prompting recommendations for redesign that would facilitate future reuse. In order to understand how product lifespans can be extended it must first be understood why products are replaced. A simple framework with which to analyse failure is applied to the products that dominate steel use, finding that they are often replaced because a component/sub-assembly becomes degraded, inferior, unsuitable or worthless. In light of this, four products, which are representative of high steel content products in general, are analysed at the component level, determining profiles of cumulative steel mass over the lifespan of each product. The results show that the majority of the steel components are underexploited – still functioning when the product is discarded. In particular, the potential lifespan of the steel-rich structure is typically much greater than its actual lifespan. Evidence from twelve case studies, in which product or component life has been increased, is used to tailor life-extension strategies to each reason for product failure, providing practical guidelines for designers. There is currently no commercial method of reusing small manufacturing scrap; however, previous research has demonstrated that extruded profiles can be created from small clean aluminium scrap, the scrap fragments solid-state welding together when extruded. In order to evaluate potential applications for these profiles four case studies are conducted in collaboration with aluminium producers and product manufacturers. It was found that strong and formable profiles could be produced from scrap. However, contaminated scrap sources, unreliable bonding and poor surface quality limited their potential for commercial use. No model exists for solid-state weld strength that is applicable to scrap extrusion. This prevents optimisation of the existing extrusion process and the development of new, potentially better, processes. Subsequently, this thesis presents a new model of weld strength as a function of relevant deformation parameters. The model is evaluated using a new experiment in which the deformation conditions can be varied independently. The experiments establish the basic relationships between deformation parameters and weld strength. The model correctly predicts these trends with predicted weld strengths typically lying within the experimental error range. The technical assessment of reuse presented in this thesis demonstrates the scope of potential change. If implemented, the opportunities presented would greatly increase the reuse of steel and aluminium, reducing the emissions emitted from liquid metal production in conventional recycling

Large space structures and systems in the space station era: A bibliography with indexes (supplement 03)

Bibliographies and abstracts are listed for 1221 reports, articles, and other documents introduced into the NASA scientific and technical information system between January 1, 1991 and June 30, 1991. Topics covered include large space structures and systems, space stations, extravehicular activity, thermal environments and control, tethering, spacecraft power supplies, structural concepts and control systems, electronics, advanced materials, propulsion, policies and international cooperation, vibration and dynamic controls, robotics and remote operations, data and communication systems, electric power generation, space commercialization, orbital transfer, and human factors engineering