| 61. |
- Torres, Hector
(författare)
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Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS2 and WS2. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS2 to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS2 to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.
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| 62. |
- Tran, Anh Tuan
(författare)
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Resistance of cold-formed high strength steel sections : Effect of cold-formed angle
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cold-formed steel members are increasingly used in industrial and civil construction. Their use allows optimised member cross sections and shapes, thereby reducing the amount of steel used as well as the weight of the structures and, consequently, reducing harmful effects on the environment. Cold-formed members are manufactured using various methods such as cold rolling or press braking. In the cold rolling method, the cold-formed member is made by passing a flat steel sheet through a series of deformation stages. In the press braking method, the cold-formed member is produced by bending a flat steel sheet along its length. Cold-formed steel circular and polygonal sections have been used for wind turbine tubular towers. In order to harness the maximum amount of the wind's kinetic energy, the height of wind turbine tubular towers has significantly increased over the last few decades. Using high strength steel material has proven to be a feasible solution to the problem of the increasing height of wind turbine towers. The use of high strength steel material for cold-formed members significantly improves their properties. It enables thinner, longer and stronger structures. Moreover, the quantity of steel material required for building cold-formed steel structures is considerably reduced, producing a beneficial effect on the environment.This thesis describes experimental and numerical investigations of cold-formed high strength steel sections and the effects of cold-formed angles on their properties. The effect of cold-formed angles on the properties of high strength steel was studied using tensile coupon tests. Coupon specimens with different cold-formed angles and different thicknesses were considered. Experimental results revealed that the cold-formed angle has a significant effect on the material properties. Furthermore, the cold-formed angle dependencies of the yield and tensile strengths were determined, and the strengths obtained with/without considering the influence of the cold-formed angle were compared. The yield and tensile strengths both decreased with increasing coldformed angle. The behaviour of cold-formed high strength steel angles was also investigated. Thirty-six specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160°, and 170°) and different thicknesses (4 mm and 6 mm) were used for the investigation. Test results indicated that the resistance of the cold-formed angles significantly decreases, by 84%, with increasing coldformed angles from 90° to 170° (reducing influence of cold-forming). Moreover, a cold-formed angle significantly affects the failure modes of the angles. Experimental and numerical studies of cold-formed high strength steel circular and polygonal sections were carried out. A total of 32 cold-formed high strength steel circular and polygonal specimens, with and without openings, were tested under uniaxial compression. Initial geometric imperfections of the specimens were detected by using a 3D laser scanning method. Finite element analysis (FEA) models were compared with, and calibrated against, test results. The FEA results agreed well with the experimental results.
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| 63. |
- Wolf, Veronika
(författare)
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Analysis of Arctic ice cloud properties using in-situ and remote sensing measurements
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cirrus clouds play an important role in the radiation balance of the atmosphere as theycan have a warming and cooling effect. The resulting net radiation effect depends ontheir micro- and macrophysical properties such as particle size, shape, and numberconcentration. The net warming or cooling effect of clouds is still one of the biggest uncertainties, for example in climate models. For weather and climate models and remotesensing retrievals, precise knowledge of micro- and macrophysical cloud propertiesis therefore necessary. This is true in particular for Arctic cirrus clouds, where we still lack data and need better understanding. Yet, climate change affects high latitudes stronger than other regions. Thus, more knowledge about micro- and macrophysicalproperties of Arctic cirrus clouds is needed urgently.The focus of this thesis is on the retrieval of physical particle properties of Arcticice clouds. Balloon-borne in-situ measurements with concurrent lidar measurementshave been performedinKiruna in winter. The Balloon-borne IceCloud Particle Imager(B-ICI) takes images of ice particles directly inside the cloud. After recovery, the imagesare analysed to gain information about particle shape, size, area, and number concentrationand to determine the extinction coefficient.Whenever possible, concurrent lidar measurements have supplemented the balloonbornemeasurements. Due to balloon drift, there is a spatial and temporal distance betweenB-ICI and lidar. Hence, both instruments do not sample a cloud at the same timeand place. Taking into account the wind speed, it is possible to determine the time of lidarobservation at which the cloud segment probed by balloon was closest to the lidar. However, cloud homogeneity has to be assumed. The results from B-ICI are comparedto extinction coefficients and depolarization ratios obtained fromlidar measurements. Measurement results from B-ICI and lidar measurements are, despite the spatial andtemporal distance, very similar and thus comparable. Clouds consisting of small andcompact particles have a smaller extinction coefficient and depolarization ratio thanclouds which consist of large, complex-shaped particles.For each cloud that has been measured, the cloud origin, i.e. its formation processis determined with the help of back-trajectory modelling. With that, it can be studiedif particle properties depend on cloud origin. This analysis reveals that particle sizeand shape exhibit strong differences with respect to the formation process. If ice particleshave been formed via the liquid droplet phase, they can grow to large sizes andinto complex shapes. If, however, they have been formed directly from vapour or supercooledsolutions, they are smaller and most often compact in shape. Hence, it ispossible to predict the formation process if size distribution and predominant particleiiishapes are known. Or inversely, size distribution and shapes can be predicted by knowingthe formation process. To account for these differences, a new parametrization forparticle size distribution is given that depends on the formation process.While the cloud formation process is depending on temperature, supersaturation,and updraught speed, it should not depend on latitude. In fact, comparing results fromArctic measurements with measurements from other latitudes similarities are recognizable. For example, the new parametrization for particle size distributions of Arcticcirrus clouds depending on formation processes and an established parametrizationfor midlatitude cirrus depending on particle size are very similar. Thus, this thesis andthe attached papers will not just provide better information about the particle propertiesof Arctic ice clouds, it can also be used to improve weather and climate models forall latitudes.
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| 64. |
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| 65. |
- Xu, Johanna, 1989-
(författare)
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Structural Composite Lithium-Ion Battery : Effect of Intercalation induced volumetric changes on micro-damage
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The quest for lighter materials and structures to reduce climate impacts in the automotive industry has paved the way for multifunctional solutions. Mass saving on a system level can be achieved by materials or structures having more than one primary function, thus reducing the number of components used. Structural batteries are composite materials that simultaneously carry mechanical loads while delivering electrical energy. While carbon fiber is a commonly used reinforcing material in high-performance composite materials, it also possesses excellent lithium intercalation properties.Therefore,it is possible to use carbon fiber to develop structural batteries based onlithium-ion battery technology.Among several proposed solutions,the micro-battery employs the carbon fiber asa negative electrodeof the battery and also as a composite reinforcement material. The fiber is coated with a solid polymer electrolyte which works as an ion conductor and separator whilst transferring mechanical loads. The coated fiber is surrounded by additional matrix material acting asapositive electrode, composed of conductive additives, active electrode material and electrolyte. This assembly of materials allows thenecessary electrochemical processes to occur simultaneously, including electrochemical reactions at the surface of the active electrode material, mass transport within active electrode material by diffusion, mass transport in electrolyte by diffusion and migration, and electronic conduction.During electrochemical cycling the electrodes undergo volume changes as a result of lithium transport. The work in this thesis addresses the effects of volume changes on internal mechanical stress state in the structural battery. A physics-based mathematical model employing a number of coupled nonlinear differential equations has been set-up and solved numerically to investigate performance in the structural battery material. The resulting transient lithium ionconcentration distributions were used in combination with linear elastic analysis in order to assess the mechanical stresses in the fiber, coating and matrix caused by non-uniform swelling and shrinking of the micro-battery. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Linear elastic fracture mechanics hasbeen used to analyze radial matrix crack propagation and debonding at coating/matrix interface in both unidirectional (UD) and cross-ply laminate, under electrochemical load only and combined electrochemical and thermomechanical load. Results show that for cross-ply structural battery composite the sequence of macro-scale crack forming events differs from a conventional cross-plied composite, as well as froma UD composite battery laminate. The most likely course of failure events in a cross-ply laminate were found to be: 1) radial matrix crack initiation and unstable growth; 2)matrix/coatingdebond when the matrix crack hasacertain length.
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| 66. |
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