This doctoral thesis concerns fatigue of welded structures. Welding is one of the world’s most common joining methods and it is frequently used in several structural applications in many fields. Some examples are construction vehicles, loader cranes, trucks, busses, forestry and agricultural machines, bridges and ships. Since these structures are subjected to repeated loading, fatigue is the most common cause of failure. A novel numerical algorithm has been developed which assesses the welded surface and calculates and quantifies weld quality parameters and the presence of defects which are critical in fatigue applications. The algorithm is designed for implementation in serial production. It will provide robust and reliable feedback on the quality being produced, which is essential if high strength steels are utilized. Two welding procedures which can increase the weld quality in as welded conditions have been assessed. These procedures utilize welding in different positions and pendling techniques, which can be accomplished using the existing welding equipment. It was found that by using these methods, the fatigue strength can be increased compared to normal weld quality. Furthermore, two fatigue assessment methods ability to account for increased weld quality in low cycle and high cycle fatigue applications has been studied. One of these methods showed sufficient accuracy in predicting the fatigue strength with small scatter and also account for increased weld quality. When implementing thinner high strength steels, the overall stress level in the structure increase. Therefore, other locations such as the steel cut edges may become critical for fatigue failure unless they are not designed and manufactured with the same quality as the welded joint. The influence of surface quality on cut edges was studied and the fatigue strength was estimated using international standards and a fatigue strength model for cut edges. / <p>QC 20160613</p> / WIQ / LIGHTSTRUCT / ONWELD

This study has been conducted to establish a relation between welding quality and strength in various types of steel. This because specification of quality levels higher or lower than required leads to an increase in manufacturing cost. A procedure has been developed to achieve the above objective. In this procedure EN 25817, EN 288-1 and similar welding and nondestructive testing standards have been taken as a basis. Furthermore, effort has been exerted to ensure that welding is performed in a manner that reflects the actual conditions encountered in the industry to the extent possible. The same principles have been pursued in material selection, and the materials have been selected from the low-carbon manufacturing steel types (St37, St44 and St52) that are frequently used in steel construction, boiler manufacturing and similar manufacturing areas. The welded pieces manufactured in accordance with the established procedure have been tested through radiographic and ultrasonic examination methods to check whether they conformed to the welding quality standards set in the procedure. v Quality levels B and C of the EN 25817 standard have been selected for this study. The sizes of potential defects of quality levels have been defined within this standard. In this study, plates with weld seam that has no defect have been used for Quality level B and plates with weld seam having gas pores in sizes specified in the relevant standard have been used for Quality level C. After this stage, the pieces have been subjected to mechanical tests and their strength values have been identified. Thereby, the association between welding quality and strength has been established, enabling us to specify which strength values can be achieved in specific welding quality levels.

Welding is a key process for heavy steel structures, but it is also a weak link in the structure since fatigue fractures in welds are a common cause of failure. This thesis proposes several changes in order to improve the fatigue properties in acost effective way, enabling reduced weight and reduced cost of welded structures. The main idea is to adapt the weld requirements and welding procedures to the load conditions of the weld. This approach ensures that the main focus in the welding process is the critical characteristics of the welds fatigue life properties. The fatigue life critical properties are most often related to the geometrical factors of the weld such as the radius at the weld toe or the penetration in the root. The thesis describes a holistic view of the subject and covers fatigue, weld quality, weld requirements and welding procedures. It becomes evident that the traditional way of working without a direct connection to fatigue is not the best. With an adaptation to the load conditions and fatigue, it is possible to enhance the fatigue life and reduce the welding cost. The main challenge is to connect the welding process, weld requirements and fatigue life properties. It is needed for an optimised welding process of heavy structures subjected to fatigue and toget a predictable fatigue life. Welds optimised for enhanced fatigue life properties are not necessary accepted according to the requirements in a current standard. Several welding procedures are proposed for improving the fatigue life properties of the weld, which indicate a high potential for enhanced fatigue lifeof fillet welds. The idea is to replace the "standard" fillet weld with three different weld types: (i) Welds with deep penetration, (ii) Welds with large weld toe radius and (iii) Welds produced with low cost. Together with customised requirements and reduced over-welding there is a vast potential for reduced weight, reduced cost and increased productivity.The main contribution of this thesis work is the cross-functional studies including design, analysis, production and quality control. This gives a framework for improvements supporting reduced cost and reduced weight of VIII welded structures without reducing the fatigue strength. Many shortcomings have been highlighted to change the welding from a state where welds are done in a way as they "always" have, by tradition, to a more contemporary situation where weld requirements and welding procedures are actively chosen to match the load conditions of the weld. This result in requirements and welding procedures which actually are connected to the fatigue properties as defined by the loading conditions, and where auditors with high probability can say that an accepted weld actually is better than a rejected weld.

The union of a set of materials by way of Resistance Spot Welding is designed so that once fused together, a substantial amount of intentional, external force must be applied to separate the contents. Therefore, Resistance Spot Welding is often the preferred fusion method in high-volume manufacturing processes. The result of Resistance Spot Welding however is the formation of a weld nugget which is not visible to the naked eye. Destructive and/or ultrasonic methods applied off-line must be used to determine the quality of each weld; both inefficient and expensive processes. The following research analyzes the data fed back during resistance spot weld sequences in-line and establishes a correlation between emitted characteristics and the final quality of a spot weld. The two characteristics researched to segregate weld quality are: the electrical sin wave signature and the acoustic sin wave signature produced during the welding sequence. Both features were discovered to have a direct correlation to the final quality of a weld once cured. By measuring and comparing these characteristics at the source, an opportunity is presented to decrease time and potential defects by confirming the quality of each weld in-process and at the source.

The development of high strength steel enables constructions to be made with less material than with previous used steel grades. The possibility of creating lightweight constructions with the same or even better mechanical properties is of high focus in the steel industry today. This can reduce productional costs as well as transportation related costs and have a less negative environmental impact. As steels are developed to be more durable, the processing of these steels grow more challenging and the use of conventional welding methods such as MIG/MAG-welding prove difficult (impossible for some steel grades) to obtain acceptable quality with single pass welds. The interest and use of laser-arc hybrid welding has increased during the last decade for its high welding speed and deep weld penetration. There are however still areas of the process that are not well documented. In this master thesis project, joining of laser cut high strength steel sheets with laser-arc hybrid welding has been tested to evaluate weld quality and process stability. The tests was performed on threesteel grades with increasing strength (S960QL, S1100QL and S1300QL). Steel sheets with different pre-welding preparations has been laser-arc hybrid welded and a documentation displaying resulting differences in weld geometry, microstructure, process stability and hardness has been created. For example, samples welded withthe mill scale still present was found not to have negative impact in terms of weld geometry and hardness when using laser-arc hybrid welding. Cooling rates was collected to aid the evaluation of microstructural composition in the welded area. The results of the work in this thesis project can be of aid in making the joining process of high strength steels more efficient.

With the growth in pipeline installations all over the world, there is a great demand for highly productive and robust welding systems. Mechanised pipe welding has been developed over the last 50 years and the present focus is towards development of automated pipeline welding systems. Pipeline welding automation is aimed at reducing costs and improving the installation quality. To attain fully automated pipe welding systems there is a need to rely on sensors and controls systems to mimic human like capabilities, such as visual inspection, in real time. The key aim of this work is to develop and evaluate methods of automatic assessment of weld bead shape and quality during narrow gap GMAW of transmission pipelines. This implies that the measured bead profile will be assessed to determine whether the bead shape will cause defects when the subsequent pass is deposited. Different approaches have been used to conquer the challenge that is emulating human reasoning, all with different objectives in mind. In spite of extensive literature research performed, very little information was found concerning the real time determination and assessment of bead shape quality and none of it was reported to be applied successfully to the pipeline industry. Despite the continuous development of laboratory laser vision systems commercial ones have been on the market for decades, some specifically developed for the welding application. Laser vision sensor systems provide surface profile information, and are the only sensors which can satisfactorily measure bead profile on a narrow groove. In order to be able to use them to automatically assess weld bead shape and quality, a deep understanding of their characteristics and limitations needs to be achieved. Once that knowledge was attained it was then applied to determine the best sensor configuration for this purpose. After that the development of human like judgment algorithms were developed to accomplish the aim that was set. Empirical rules were obtained from an experienced welder regarding the acceptability of bead shapes and were then applied in the developed system with good results. To scientifically evaluate and determine the rules to use in this system, further experiments would be required. The output of the system developed showed very accurate, reliable and consistent results that were true to the external measurements and comparisons performed. The developed system has numerous applications in the pipeline industry and it could easily be implemented on commercial systems.

The demand for high energy battery assemblies is growing in sectors such as transportation. Alongwith it is the need for reliable, efficient and cost-effective ways to electrically connect the batteries toensure their performance. Battery cells are most often put into modules or packs when produced forelectrically driven vehicles. The variable of greatest influence when welding battery packs is thecontact resistance between the cell and the connection tab. It is crucial to minimize this variable asmuch as possible to prevent energy loss in the form of heat generation. The purpose of this project is to conduct a comparative literature study of different weldingtechniques for welding batteries. The compared techniques are resistance spot welding, laser beamwelding and ultrasonic welding. The performance was evaluated in terms of numerous factors such asproduction cost, degree of automation and weld quality. All three methods are tried and proven to function in the production of battery applications. Eachmethod has separate strengths and limitations which makes them complement each other. Thus, it isimportant to look at several factors when deciding which welding technique is the most suitable forthe desired application. The scale of production, economical aspects as well as battery cell geometrywere concluded to be the most important in making this decision.