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Fatigue strength of welded structures at sub-zero temperatures
Project leader:
Moritz Braun
Funded by:
Stiftung Stahlanwendungsforschung (FKZ AVIF A 301)
DURATION:
1.1.2016-31.9.2019
PROJECT PARTNERS:
Dillinger Hütte, DNVGL, Flensburger Schiffbau-Gesellschaft, Mareval, Ramboll
DESCRIPTION:
Within the last years, it has been a continuously growing interest in exploration of oil and gas in the arctic region. Up to 25% of the undiscovered oil and gas resources are expected to be found in sea areas with at least seasonal ice coverage and harsh and cold environments (Gautier et al., 2009). As a result of the latter, structures exposed to cold climate are required to function reliably in their design conditions including exposure to low temperatures. Current regulations however do not cover the expected temperature range and thus the reliability of structures used in such conditions may be reduced. DNV GL provides some guidance down to -30°C (DNV, 2013), while NORSOK is limited to -14°C (Norsok, 2013). Consequently, there is a lack of guidance in structural behavior at low temperatures.
While the strength of steel increases under cold temperatures, the inherent risk of unexpected brittle fracture increases as well. All ferritic structural steels suffer from reduced fracture toughness at low temperatures due to the ductile-to-brittle transition behavior (DBT), which is characteristic for steels with body-centered cubic (bcc) crystal structure. At lower temperatures, the mechanism of stable crack growth behavior changes from plastic blunting and tearing to cleavage controlled brittle fracture. To this day, the transition behavior of welded structures is just barely understood. For this reason large parts, in particular fatigue critical once, are made of lower strength steel. By carefully assessing service conditions, fabrication processes and improving current international standards for ship and offshore structures, the sales of higher strength steel could significantly being increased.
OBJECTIVES:
This research project seeks to improve the applicability of higher strength steel in fatigue critical regions of ship and offshore structures. For this purpose the material behavior of base materials and welded structures made of higher strength steel will be analysed for temperatures up to -50°C by means of tensile, fracture toughness and SN tests. Based on the results state-of-the-art fatigue assessment methods will be checked and if need updated.
The research is focused on the following objectives:
1. To test the static and dynamic material properties for different structural details and material strengths at changing temperatures from room temperature to -50°C
2. Numerically simulation of the material behavior based on the test results
3. Verification of state-of-the-art fatigue assessment methods for welded structures at low temperatures like nominal stress, structural stress, notch stress and strain energy density methods
REFERENCES
Braun, M., Milaković, A.-S., Renken, F., Fricke, W., Ehlers. S. (2020). Application of Local Approaches to the Assessment of Fatigue Test results obtained for Welded Joints at Sub-Zero Temperatures. International Journal of Fatigue, https://doi.org/10.1016/j.ijfatigue.2020.105672
Braun, M., Müller, A.M., Milaković, A.-S., Fricke, W., Ehlers. S. (2020). Requirements for stress gradient based fatigue assessment of notched structures according to theory of critical distance. Fatigue Fract Eng M, https://doi.org/10.1111/ffe.13232
Braun M, Scheffer R, Fricke W, Ehlers S. Fatigue strength of fillet‐welded joints at subzero temperatures. Fatigue Fract Eng Mater Struct, 43 (2019)403-416. https://doi.org/10.1111/ffe.13163
A.-S. Milaković, M. Braun, T. Willems, H. Hendrikse, C. Fischer, S. Ehlers, Methodology for estimating OWT fatigue life under combined loads of wind, waves and ice at sub-zero temperatures, 4th International Conference on Ships and Offshore Structures, Cape Carnival, USA, 2019.
R. U. F. von Bock und Polach, M. Klein, J. Kubiczek, L. Kellner, M. Braun, H. Herrnring. State of the Art and Knowledge Gaps on Modelling Structure in Cold Regions in OMAE2019 – Proceedings of 38th International Conference on Ocean, Offshore and Arctic Engineering, Glasgow, Scotland, United Kingdom. https://doi.org/10.1115/OMAE2019-95085
