A novel approachfor assessing the fatigue strength of ultrasonic impact treated welded structures.

It is commonly observed that complex fabricated structures subject tofatigue loading fail at the welded joints. Some problems can be corrected by proper detail design but fatigue performance can also be improved using post-weld improvement methods. In general, improvement methods can be divided into two main groups: weld geometry modification methods and residual stress modification methods. The former remove weld toe defects and/or reduce the stress concentrationwhile the latter introduce compressive stress fields in the area where fatigue cracks are likely to initiate. Ultrasonic impact treatment (UIT) is a novel post-weld treatment method that influences both the residual stress distribution andimproves the local geometry of the weld. The structural fatigue strength of non-load carrying attachments in the as-welded condition has been experimentally compared to the structural fatigue strength of ultrasonic impact treated welds. Longitudinal attachment specimens made of two thicknesses of steel S355 J0 have been tested for determining the efficiency of ultrasonic impacttreatment. Treated welds were found to have about 50% greater structural fatigue strength, when the slope of the S-N-curve is three. High mean stress fatigue testing based on the Ohta-method decreased the degree of weld improvement only 19%. This indicated that the method could be also applied for large fabricated structures operating under high reactive residual stresses equilibrated within the volume of the structure. The thickness of specimens has no significant effect tothe structural fatigue strength. The fatigue class difference between 5 mm and 8 mm specimen was only 8%. It was hypothesized that the UIT method added a significant crack initiation period to the total fatigue life of the welded joints. Crack initiation life was estimated by a local strain approach. Material parameters were defined using a modified Uniform Material Law developed in Germany. Finite element analysis and X-ray diffraction were used to define, respectively, the stress concentration and mean stress. The theoretical fatigue life was found to have good accuracy comparing to experimental fatigue tests.The predictive behaviour of the local strain approach combined with the uniformmaterial law was excellent for the joint types and conditions studied in this work.

The determination of EONIA rate: The impact of the financial crisis and the effects of the changes in the monetary policy implementation framework

EONIA is a market based overnight interest rate, whose role as the starting point of the yield curve makes it critical from the perspective of the implementation of European Central Bank´s common monetary policy in the euro area. The financial crisis that started in 2007 had a large impact on the determination mechanism of this interest rate, which is considered as the central bank´s operational target. This thesis examines the monetary policy implementation framework of the European Central Bank and changes made to it. Furthermore, we discuss the development of the recent turmoil in the money market. EONIA rate is modelled by means of a regression equation using variables related to liquidity conditions, refinancing need, auction results and calendar effects. Conditional volatility is captured by an EGARCH model, and autocorrelation is taken into account by employing an autoregressive structure. The results highlight how the tensions in the initial stage of the market turmoil were successfully countered by ECB´s liquidity policy. The subsequent response of EONIA to liquidity conditions under the full allotment liquidity provision procedure adopted after the demise of Lehman Brothers is also established. A clear distinction in the behavior of the interest rate between the sub-periods was evident. In the light of the results obtained, some of the challenges posed by the exit-strategy implementation will be addressed.