Nowadays there is a strong demand for lighter vehicles in order to increase the pay load. Through this the specific fuel consumption is decreased, the amount of greenhouse gases is lowered and the transport economy improved. One possibility to optimize the weight is to make the components from high strength steels and join them by welding. Welding is the main joining method for fabrication of a large proportion of all engineering structures. Many components experience fatigue loading during all or part of their life time and welded connections are often the prime location of fatigue failure.Fatigue fracture in welded structures often initiates at the weld toe as aconsequence of large residual stresses and changes in geometry acting as stress concentrators. The objective of this research is to increase the understanding of the factors that control fatigue life in welded components made from very high strength steels with a yield strength of more than 800 MPa. In particular the influences of the local weld toe geometry (weld toe radius and angle) and residual stress on fatigue life have been studied. Residual stresses have been varied by welding with conventional as well as Low Transformation Temperature (LTT) filler materials. The three non-destructive techniques Weld Impression Analysis (WIA), Laser Scanning Profiling (LSP) and Structured Light Projection (SLP) have been applied to evaluate the weld toe geometry.Results suggest that all three methods could be used successfully to measure the weld toe radius and angle, but the obtained data are dependent on the evaluation procedure. WIA seems to be a suitable and economical choice when the aim is just finding the radius. However, SLP is a good method to fast obtain a threedimensional image of the weld profile, which also makes it more suitable for quality control in production. It was also found that the use of LTTconsumables increased fatigue life and that residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welded parts.

In recent years a strong interest has been expressed to produce lighter structures.One possible solution to reduce the weight is to utilize high strength steels and use welding as the joining method. Many components experience fatigue loadingduring all or part of their life time and welded connections are often the prime location of fatigue failure. This becomes more critical in welded high strength steels as fatigue strength of welds does not increase by increasing the steel strength. A possible solution to overcome this issue is to use fatigue improvement methods.The main objectives of this project are, therefore, to increase understanding of the factors that control fatigue life and to investigate how the fatigue strength improvement methods; high frequency mechanical impact (HFMI) treatment and use of Low Transformation Temperature (LTT) consumables will affect fatigue properties of welds in high strength steels. In this regard, Gas Metal Arc Welding(GMAW) was used to produce butt and fillet welds using LTT or conventional fillers in steels with yield strengths ranging from 650-1021 MPa and T-joint weldsin a steel with 1300 MPa yield strength. The effect of HFMI on fatigue strength of the welds in 1300 MPa yield strength steels was also investigated. Butt and fillet welds in 650-1021 MPa steels were fatigue tested under constant amplitude tensile loading with a stress ratio of 0.1 while T-joints were fatigue tested under constant amplitude fully reversed bending load with a stress ratio of -1. The nominal stress approach was used for fatigue strength evaluation of butt and fillet welds whereas the effective notch stress approach was used in case of T-joints. Relative effectsof the main parameters such as residual stress and weld toe geometry influencing fatigue strength of welds were evaluated. Residual stresses were measured using X-ray diffraction for as-welded and HFMI treated welds. Neutron diffraction was additionally used to investigate the near surface residual stress distribution in 1300 MPa LTT welds.Results showed that use of LTT consumables increased fatigue strength of welds in steels with yield strengths ranging from 650-1021 MPa. For butt welds, the vii characteristic fatigue strength (FAT) of LTT welds at 2 million cycles was up to46% higher when compared to corresponding welds made with conventional fillermaterials. In fillet welds, a maximum improvement of 132% was achieved when using LTT wires. The increase in fatigue strength was attributed to the lower tensile residual stresses or even compressive stresses produced close to the weldtoe in LTT welds. Weld metals with martensite transformation start temperatures around 200 °C produced the highest fatigue strength. In 1300 MPa yield strength steel, similar FAT of 287 MPa was observed for LTT welds and 306 MPa for conventional welds, both much higher than the IIW FATvalue of 225 MPa. The relative transformation temperatures of the base and weldmetals, specimen geometry and loading type are possible reasons why the fatigue strength was not improved by use of LTT wires. Neutron diffraction showed that the LTT consumable was capable of inducing near surface compressive residual stresses in all directions at the weld toe. It was additionally found that there arevery steep stress gradients both transverse to the weld toe line and in the depth direction, at the weld toe. Due to difficulties to accurately measure residual stresses locally at the weld toe most often in the literature and recommendations residual stresses a few millimetre away from the weld toe are related to fatigue properties. However, this research shows that caution must be used when relating these to fatigue strength, in particular for LTT welds, as stress in the base materiala few millimetre from the weld toe can be very different from the stress locally at the weld toe.HFMI increased the mean fatigue strength of conventional welds in 1300 MPa steels about 26% and of LTT welds by 13%. It increased the weld toe radius slightly but produced a more uniform geometry along the treated weld toes. Large compressive residual stresses, especially in the longitudinal direction, were introduced adjacent to the weld toe for both LTT and conventional treated welds. It was concluded that the increase in fatigue strength by HFMI treatment is due to the combined effect of weld toe geometry modification, increase in surface hardness and introduction of compressive residual stresses in the treated region.It was concluded that the residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welds. This is based on the observation that a moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm, in fillet welds. Also, a higher fatigue strength was observed for HFMI treated conventional welds compared to as welded samples having similar weld toe radii but with different residual stresses.