Public defence, Mechanical Engineering, M.Sc. Matti Rautiainen

Title of the thesis: Traction force approach for fatigue assessment of complex welded connections

Thesis defender: Matti Rautiainen
Opponent: Prof. Johan Maljaars, Eindhoven University of Technology, The Netherlands
Custos: Prof. Heikki Remes, Aalto University School of Engineering

Structures optimised in line with the principles of sustainable development can simultaneously reduce manufacturing costs and carbon dioxide emissions generated during manufacturing and throughout their service life. From a design perspective, however, such complex structures pose considerable challenges, as the time required to determine their fatigue life, the complexity of the analyses and the associated costs all increase significantly. This doctoral dissertation investigated predictive modelling of the fatigue life of welded steel structures in cases where the aim is to manufacture welded joints with as few fabrication steps as possible.

Limited research has been conducted on the fatigue strength of complex three-dimensional welded structures and on simplified fatigue design models, with research focusing on more detailed predictive models instead. However, applying detailed models to large structures, such as cruise ships, can be difficult in terms of design turnaround times, workload and accuracy of the required data. The aim of this study was to develop an optimal fatigue design method for complex structures. The traction force approach developed in the study simplifies the stress state in a structure, while allowing increased modelling error. This error was found to remain within acceptable limit subject to certain limitations. Full-scale tests showed that local nominal stress was a suitable stress value for determining the fatigue strength of the structure. However, further testing is required before the results can be widely applied.

This doctoral dissertation concludes that more detailed calculation methods do not guarantee reliable fatigue life predictions for joints in complex structures. Instead, the results support the simpler local nominal stress method, where the stress value required for fatigue design can be determined efficiently using the traction force approach developed in this research. 

Key words: Fatigue, welded connection, weld root, local nominal weld stress

Thesis available for public display 7 days prior to the defence at Aalto University's public display page. 

Contact information: matti.rautiainen@aalto.fi