| 1. |
- Andersson, Tobias
(författare)
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Mechanical Behaviour of Adhesive Layers : Methods to Extract Peel and Mixed Mode Properties
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical Behaviour of Adhesive Layers Methods to Extract Peel and Mixed Mode Properties TOBIAS ANDERSSON Department of Applied Mechanics Chalmers University of Technology ABSTRACT This thesis is concerned with methods to extract material properties of thin adhesive layers loaded in peel and in mixed mode. The first part of the thesis is devoted to an experimental method to determine the complete stress-elongation relation (or cohesive law) for an adhesive layer loaded in peel using the DCB-specimen. The method is based on the concept of equilibrium of the energetic forces acting on the specimen. Two sources of energetic forces are identified: the start of the adhesive layer and the positions of the two acting loads. By use of the concept of equilibrium of energetic forces, it is possible to measure the energy release rate of the adhesive layer instantaneously during an experiment. The complete stress-elongation relation is found to be the derivative of the energy release rate with respect to the elongation of the adhesive layer at its start. By this procedure, an effective property of the adhesive layer is measured. The validity of the approach is investigated by experiments where the adherends deform 1) elastically and 2) plastically. It is found that a unique stress-elongation is obtained when the adherends deform elastically. The same relation cannot be used to predict the experiments where the adherends deform plastically indicating that the approach has limited applicability. The second part of the thesis is concerned with the development of a mesomechanical finite element model of a thin adhesive layer loaded in mixed mode. The model is calibrated to previously performed experiments. In these, the adhesive layer is loaded in monotonically increasing peel or shear. An in situ SEM-study is also performed and used to guide the modelling and calibration. The purpose of the mesomechanical finite element model is to facilitate the development of constitutive laws for adhesive layers. The modelling is based on Xu and Needleman’s method where all continuum finite-elements are surrounded by interface elements that allow for the development of micro cracks. Thus, this enables the modelling of the entire process of degradation and fracture of the adhesive layer. A genetic algorithm is developed for the calibration. The simulations are shown to be in reasonably good agreement with the experiments. Keywords: adhesive layer, stress-elongation relation, J-integral, energetic force, experimental method, RVE, interface elements, genetic algorithm
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| 3. |
- Bengtsson, Magnus, 1973
(författare)
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Quality-Driven Production of Aggregates in Crushing Plants
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aggregate quality is of great importance, and in this context it is essential to understand how various process parameters influence the product quality characteristics. Thus, there is a need for aggregate producers to improve their process knowledge.Research has led to an improved understanding of how different process parameters affect the outcomes of specific comminution processes. In aggregate production, breakage models are one example of such a research result. These models have been refined over the years. The breakage models that have been presented to date can accurately predict the size distribution of aggregates. On the other hand, they cannot normally predict any other material characteristics such as shape and strength. This thesis outlines the research to date, which has focused on a frequently discussed quality parameter, namely the shape of the product. Knowledge of various methodologies is required to measure this parameter. A novel method for measuring the shape of fine aggregates is presented. The rheology test, the methodology of which was originally based on a New Zealand standard, can be correlated with other data to measure particle shape in accordance with the F-aspect method. The correlation between these methods is very good, and since the rheology test is easy to use, it is suitable for a quarry or a basic test facility.The cone crusher is the most commonly used tertiary stage crusher today. One of its great advantages is that it produces a relatively low amount of fines. However, it only produces acceptable particle shapes for certain fraction sizes. The vertical shaft impact crusher (VSI) creates particles of a very satisfactory shape across all fractions, including fine aggregate, as confirmed by studies using the aforementioned rheology test.This thesis presents two models for predicting shape in crushed aggregate from a cone crusher. The first model is empirical, and my conclusion is that there are four dominating parameters affecting shape: the average feed size, the closed side setting (CSS), throw and eccentric speed. To further investigate how compressive breakage affects particle shape, an inter-particle breakage model is introduced. This model simulates a specific crushing stage inside the crusher. Simulation results show that the outcome from a crushing event can be predicted with good accuracy. A VSI model for describing particle size and shape is presented in this work. It is proposed that the dominating breakage mechanism in a VSI equipped with a rock box is of abrasive character. Abrasive breakage can appropriately shape all particle sizes, assuming the crusher is operated at a proper velocity.The empirical cone crusher model and the VSI models are combined in a mathematical simulation in order to determine how a crushing plant should be operated to maximise both yield and aggregate quality. The results show that there exists a set of rules to determine the crusher and screen settings that will achieve the best performance in terms of product yield and quality requirements.
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| 4. |
- Biel, Anders
(författare)
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Mechanical behaviour of adhesive layers : experimental methods, cohesive laws, and fracture mechanics
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Adhesive joining is today viewed as one of the key technologies to achieve decreased emissions in the automobile industry. To decrease weight, optimal material selection often results in different materials for different parts. This leads to the necessity to join mixed material. Here, the use of adhesives is the most promising joining technology. For a rational design process, good models for strength analysis of adhesively joined structures are essential. With cohesive modelling, fracture of the adhesive layer is modelled with a stress-deformation law. This law - often denoted a cohesive law - gives the traction exerted on the adherends due to the deformation of the adhesive layer. This thesis is concerned with experimental methods to measure cohesive properties of engineering adhesives and standardized methods to measure the fracture energy of adhesives. A new method to measure cohesive laws is developed. With this method, the cohesive law of an epoxy adhesive is measured in shear. In peel loading, with elastically deforming adherends, the cohesive law is shown to be independent of the geometry of the specimen. If the adherends deform plastically the fracture energy increases. Experiments are performed in order to determine the temperature dependence of the cohesive layer for an epoxy adhesive. It is shown that the peak stress is strongly dependent on the temperature while the fracture energy shows only small temperature dependence. Experiments are also performed to study the influence of strain rate in peel and shear loading. The experiments show that the peak stress increases with an increasing strain rate and that the fracture energy increase in peel loading and decreases in shear with increasing strain rate. A new method to experimentally determine the relation between damage and plasticity in the adhesive during the fracture process is developed. For the present adhesive, it is shown that only minor plasticity occurs during the fracture process in peel loading. For peel, several commonly used methods to evaluate the fracture energy using the double cantilever beam specimen are critically studied. For some methods the error in evaluated fracture energy is larger than 40 %. It is shown that the evaluated fracture energy is more dependent on the choice of method than on the cohesive properties of the adhesive layer.
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| 5. |
- Cederfeldt, Mikael, 1975
(författare)
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Planning Design Automation : A Structured Method and Supporting Tools
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The demand for customised products that meet different markets and different customers is steadily increasing. Also, the demand for shorter lead times for the delivery of these customised products puts strains on design departments whose work tends to become increasingly repetitive. At the same time, designing variants takes time from innovative, original design, and/or problem-solving tasks. A powerful tool in the endeavour to cut lead times, workloads, and ultimately costs in order to become more competitive in an increasingly globalised market is Design Automation. Automating tedious and repetitive design tasks will free the designers to focus on the tasks that require skill, creativity, intuition, and cooperation to be solved. Consequently, seeing a need for design automation systems is not difficult. What becomes a lot more difficult is identifying the type, scope, and format of the system implementation, as well as the actual design tasks and activities to support or automate. Therefore, there is a need for structured and systematic approaches for the realisation and implementation of design automation systems. This research work is aimed at presenting such approaches, methods, and aids. It also addresses the importance of identifying the exact tasks to be automated. This has to be done in order to find the method and implementations best suited for solving the tasks, something that is especially important for companies whose human and financial resources might not allow them to invest in a system with functionality that vastly exceeds their actual needs.The contribution of this work is a structured method for planning for design automation implementation. First, the design process is discussed from an automation perspective. Following this is a presentation of a framework of design automation. This framework has the purpose of serving as a common base for consensual discussions about design automation. In addition, it supports the setting-up of system specifications. The framework is followed by the introduction of a set of identifiers of system needs and potentials, focusing on the existing processes that need to be broken down and identified in order to specify the tasks to be automated. Following this is a set of criteria of system characteristics, focusing on properties of the intended system implementation. Finally, some realisation and implementation issues are addressed and exemplified through a number of pilot system implementations.The presented method for planning design automation, together with the presented framework of design automation, provides implementers with issues to address regarding potential, need, scope, and format of system implementations. Further, it supports the weighing of desired system characteristics in order to find the right balance between system complexity and functionality.
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| 7. |
- Ekwall, Daniel
(författare)
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Managing the risk for antagonistic threats against the transport network
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The World Trade Centre terror attack in 2001 changed the world and with it the conditions for logistics worldwide. The aftermath of the attack brought needed attention to the vulnerability of modern supply chains. This thesis addresses the antagonistic threats that exploit the vulnerability in a supply chain. Antagonistic threats are a limited array of risks and uncertainties and can be addressed with risk management tools and strategies. There are three key demarcations between antagonistic threats and other risks and uncertainties: deliberate (caused), illegal (defined by law), and hostile (negative impact, in this thesis, for transport network activities). This thesis makes a theoretical contribution to the usage of theories from criminology in supply chain risk management to handle antagonistic threats against the transport network. The recognition that antagonistic threats toward the transport network are a problem leads to verification of the research questions from the background and the theoretical framework. This is done to place or relate the research questions closer to the context. Furthermore, it leads to the conclusion that the answers may or may not contain competing and/or incompatible parts which differ depending on the perspective or viewpoint at the moment. One of the most important things to understand is that antagonistic threats toward freight always have been a feature in both business and politics. The different functions and goals for all stakeholders mean that all stakeholders and actors may use similar methods to manage antagonistic threats but the effects and consequences will change according to the circumstances. The system approach in this thesis is a soft-system thinking where reality is described in subjective terms and the whole system has the distinctive trait of vague or undefined boundaries between system components and the surrounding environment. Therefore, this thesis uses a complex system approach in which paradoxes and bounded rationality describes the system’s behaviour. This thesis defines the legal descriptions and criminal threats against and within supply chain management activities that entail both the systems context and boundaries. Managing of the antagonistic threats through the risk management perspective is separated into two sides, pre-event and post-event measures, which means the system needs to be robust and resilient, using logistics terms. It should be robust to automatically handle small risks (normally with high likelihood and low impact). The system also needs resilience in order to adapt, improvise, and overcome any disturbance greater than the system’s robustness can handle. Both robustness and its resilience can constitute of the full range of prevention, mitigation, and transferring tools and methods. Regardless of which perspective or viewpoint is chosen for analysing the problem, the same basic set of tools and methods are valid, but in practical use they need to be adapted to the actors’ needs and wants for managing their exposure to antagonistic threats.
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| 8. |
- Elgh, Fredrik
(författare)
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Computer-Supported Design for Producibility : Principles and Models for System Realisation and Utilisation
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For many products, the adaptation to customer specifications is essential and requires flexible product design and manufacture while maintaining competitive pricing. Engineering design is often concerned with striking a good balance between product properties, e.g. performance, and the resources required to manufacture and assemble the product. When different courses of action are to be evaluated, even seemingly small changes in customer requirements, product design, and manufacturing properties have to be handled with caution. Small changes can entail products with: low level of conformability with the manufacturing system, highly increased cost, and extended manufacturing lead-time. For most companies, the manufacturing system is a valuable asset that is more or less fixed and only minor adaptations are allowed. This implies that the product design has to be adapted to the manufacturing system to a large extent.Design for producibility (DFP) is the process in which a systematic method is used to reach the required functional properties of the product at the same time as good compliance with the manufacturing system is ensured. The DFP process usually needs to involve several persons simultaneously for the purpose of sharing information and knowledge. For many manufacturing companies, the collaboration between engineering design and production engineering is a critical issue and they have to improve their methods and tools for ensuring and enhancing producibility. This can be achieved by introducing computer-supported design for producibility. The present research is intended to contribute to the development and utilisation of different application systems that can be used as such computer support. The aim is to provide companies with support in application system development and to show how different application systems can be used in a systematic way as means to ensure and enhance producibility. The competitive advantages to gain from introducing computer-supported design for producibility are: product designs with high level of conformability with the production system, shortened manufacturing lead-time, and decreased manufacturing cost. This work contributes to the achievement of these advantages by introducing a framework with principles and models supporting application systems development. Three types of application systems are presented and their practical usefulness is examined, showing practitioners how producibility aspects can be assessed systematically. The main scientific and theoretical contribution of the work comprises: the descriptions concerning how to structure and describe the product and product-related information (manufacturing requirements, costs, process plans and production resources), the foundation of different information models, and the clarification of the models’ interrelationships. This is perceived as a contribution to a better understanding of the domains and how they relate to each other.Design for producibility (DFP) is the process in which a systematic method is used to reach the required functional properties of the product at the same time as good compliance with the manufacturing system is ensured. The DFP process usually needs to involve several persons simultaneously for the purpose of sharing information and knowledge. For many manufacturing companies, the collaboration between engineering design and production engineering is a critical issue and they have to improve their methods and tools for ensuring and enhancing producibility. This can be achieved by introducing computer-supported design for producibility. The present research is intended to contribute to the development and utilisation of different application systems that can be used as such computer support. The aim is to provide companies with support in application system development and to show how different application systems can be used in a systematic way as means to ensure and enhance producibility.
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| 9. |
- Elmquist, Lennart, 1967-
(författare)
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Defect Formation during Solidification in Grey Iron Castings
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The presence of defects is of major concern in the production of grey iron castings. Some defects are merely an aesthetic problem while others can be detrimental for the performance of the component. Among the several possible defects, shrinkage porosity is one of the more challenging to understand and avoid. This type of defect can penetrate through the casting as a three-dimensional network. The purpose of this work has been to develop knowledge and understanding about the foundry process in order to increase product quality by eliminating or at least minimising the presence of shrinkage porosity in the production of vital engine components. This was done by developing fundamental knowledge concerning factors known to influence the presence of shrinkage porosity. Initially, an investigation was carried out in order to characterize shrinkage porosity commonly occurring in cylinder heads. Factors found to be important and contributing to the formation of shrinkage porosity were identified. It was found that this type of defect is formed at hot spots that have migrated during solidification. Knowledge of the migrating hot spot was used as a design criterion in the development of a geometry with the purpose of deliberately generating porosity. Based on this geometry a mechanism for the formation of this type of shrinkage porosity was proposed. The formation of shrinkage porosity depends on gases dissolved in the melt. In this work, their levels and variations in the melting process were investigated. It was shown how the solubility of oxygen depends on the temperature but not on the method used to produce the iron. On the other hand, the total amount of oxygen depends on the process but not on temperature. The discussion about gases also included hydrogen and nitrogen. The solidification of grey cast iron is complex. It is during the solidification that not only the final properties of the component but also the conditions for defect formation are established. Shrinkage porosity is formed due to volumetric changes followed by an inability to supply material to the contracting areas. The solidification of grey iron starts with nucleation and growth of the primary austenite followed by nucleation of eutectic cells. It was found that the microstructural constituents are interconnected. The importance of the macrostructure was also discussed and it was shown that shrinkage porosity can be found not only between eutectic cells but also between equiaxed crystals.
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| 10. |
- Engkvist, Josefin
(författare)
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Characterization of oxide scales formed on FeCrAl alloys at high temperatures
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the characterization of oxide scales formed on FeCrAl alloys at high temperatures. The effect of water vapour on commercial FeCrAl alloys oxidized in both dry and humid O2 has been investigated. Also, the effect of varying Al concentrations in model FeCrAl alloys was studied as well as the effect of using surface coatings in order to increase the oxidation resistance of FeCrAl alloys. The formed oxide scales were characterized using analysis techniques such as SEM, EDX, TEM, XRD, XPS, AES and SIMS. The main part of the work concerns the SEM, EDX, AES and SIMS techniques. The commercial FeCrAl alloys investigated follow the same trend in oxide formation independent of whether the alloy was conventionally cast or manufactured by powder metallurgy. In the as-received cold rolled condition the surface of the FeCrAl material shows a few nm thin native oxide. This native oxide is a mixture of oxides from Fe, Cr and Al, with a relatively high concentration of Cr. The oxide formation at elevated temperatures can be separated in a low temperature and a high temperature mode. At relatively low temperatures (500-600°C) a mixed oxide similar to the pre-existing native oxide forms, while at higher temperatures a thicker two-layered alumina scale forms. The outer and inner alumina layers are separated by a Cr-rich zone which is believed to be a remnant of the pre-existing native oxide and hence represents the original alloy surface. Accordingly the inner alumina layer is formed by oxygen inward diffusion while the outer alumina layer grows by cation outward diffusion. The inner alumina is composed of a-Al2O3, while the outer layer, during early stages, is composed of rapidly growing metastable polymorphs of alumina which with time transform to a-Al2O3. This phase transformation starts at the Cr-rich zone and progress outward. In presence of water vapour this transformation is inhibited due to hydroxylation of the surface of the metastable phases. Hence, a higher oxidation rate was observed in presence of water vapour. Model FeCrAl alloys having Al concentrations in the range 1.2-5 wt.% were investigated. At 900°C a minimum of 3.2% Al is needed in order to form a continuous alumina scale. Higher Al concentrations (= 4.4%) result in a relatively pure alumina scale and a slower oxidation rate. In order to evaluate the possibility to improve the oxidation resistance of FeCrAl a PVD SiO2 coating was applied on a commercial FeCrAl foil prior to oxidation. Parallel exposures of coated and uncoated samples were made at 1000°C and revealed that the SiO2 coating significantly reduced the initial oxidation rate of the FeCrAl foil. Further, metal organic CVD of a ZrO2 film in situ on an RE free FeCrAl alloy at 400 and 800°C showed that at the lower temperature mainly ZrO2 was formed while at 800°C an oxide scale composed of both ZrO2 and Al2O3 was formed. The results obtained illuminate the importance of advanced electron microscopy and surface analysis techniques in order to increase the understanding of high temperature oxidation of FeCrAl alloys and the oxidation mechanisms of this group of materials.
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