The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Strain-Based Approach to Fatigue Strength Assessment of Laser-Welded Joints

Heikki Remes

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Engineering and Architecture for public examination and debate in Auditorium K216 at Helsinki University of Technology (Espoo, Finland) on the 7th of March, 2008, at 12 noon.

Dissertation in PDF format (ISBN 978-951-22-9190-8)   [10299 KB]
Dissertation is also available in print (ISBN 978-951-22-9189-2)


New laser-based welding technology leads to differences in the geometrical and mechanical properties of welded joints as compared to those encountered in conventional arc-welded joints. The present work concentrates on the effects of these properties on the fatigue resistance of steel butt joints welded using laser-based methods. The investigation exploited both experimental and theoretical methods.

In laser-based joints, the mechanical properties within the joint varied strongly, affecting notch stresses and strains. Therefore, the response analysis based on Neuber's rule requires a separate structural and local approach. The structural analysis, based on the stress-strain curve of parent material, gives the loading for the weld notch. This is applied to the local analysis, where the stress-strain curve of the heat-affected zone (HAZ) is used.

The initiation life of macro cracks of laser-based joints was observed to form an essential part of the total fatigue life, suggesting that the mechanical properties of the material in the weld notch have a strong influence on fatigue resistance. The strain-based approach was applied with several discrete growth steps to model the propagation of short cracks up to the length of a macro crack in the weld notch. Within each step, the number of load cycles causing the increase of fracture was calculated with the Coffin-Manson formula. The length of a discrete growth step, which describes the damage zone, was based on the multiple of the averaged grain size of the HAZ material. This growth length also defines the averaging distance of effective notch stresses and strains. The approach was validated with the results of fatigue tests, using welded miniature and plate specimens.

The new theoretical approach with several discrete growth steps was especially relevant to laser-based joints with fine-grained microstructure in the HAZ. The short crack modelling was required to attain reliable predictions for the fatigue resistance of welded butt joints when the life of the macro crack initiation constituted a significant portion of the total fatigue life or the initiation life varied as the function of the applied stress range. The outcome of this important result explained the increase of the slope of S-N curves from the generally accepted value of 3.0 for arc-welded joints up to values above 10 for laser-based joints.

The main reason for the differences in the initiation process of fatigue crack between the laser-based and arc-welded joints was the dissimilar grain size in the HAZ. The average grain size of the laser-based joints was about seven times smaller than that of the arc-welded joint. The other parameters causing the difference were the hardness of the weld materials and the weld size, which affected the joint stiffness, and thus the notch stresses and strains.

Keywords: fatigue, macro crack initiation, strain-based approach, weld notch, butt joint, laser hybrid welding

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© 2008 Helsinki University of Technology

Last update 2011-05-26