Cluster:
Applicability of Strength Assessment Approaches of Cyclic-Loaded Welded Structures

Field of research:
Fatigue Strength; Ship Structural Design / CAD

Duration:
01.09.2005 – 31.10.2008

Project leader: 
Prof. Dr.-Ing. W. Fricke

Funded by: 
AiF (via CMT), FKZ 14517N – 6 partners

SUMMARY:
Within the cluster project “Applicability of strength assessment approaches to cyclic loaded welded structures”, a project with experimental and numerical investigations on “stiffened plate components of steel ship structures” are presented. The objective of the project was the validation and/or modification of different fatigue strength assessment approaches on the basis of investigations on typical welded test models from ship structures. The influence fac-tors on the fatigue strength of the test models and corresponding small-scale specimens representing the critical areas should be recorded as far as possible. Together with the results of the other cluster projects, which cover structures from different technical branches, the existing fatigue assessment approaches, i. e. the nominal, structural hot-spot and the notch stress approaches, should be harmonized and, if necessary, improved.

In the project presented her, two types of test models were investigated. The first concerns web frame corners in ro/ro ships, where the connection of the flanges with fully-penetrated welds was considered to be prone to fatigue. Therefore, a cruciform joint was taken as small-scale specimen. The fatigue lives obtained in the fatigue tests of cruciform joints correlate well with all fatigue assessment approaches, as long as the effects of pre-deformations are taken into account, particularly the axial misalignment. Three larger test models were investigated with constant amplitude loading. Crack initiation was unexpectedly found not at the cruciform joint of the flanges, but at the ends of the adjacent cut-out for welding, in spite of the fact that the structural hot-spot stress as well as the notch stress were higher at the cruciform joint. Refined investigations showed that both the local weld profile as well as different residual stresses affect the structural behaviour. Nevertheless, the structural hot-spot stress as well as the notch stress approach are suitable for the fatigue assessment of the detail. However, the nominal stress approach is non-conservative because the effective breadth of the flange is obviously far reduced despite its high slenderness.

The second test model concerns an intersection between a holland profile and a transverse web, where the ends of the stiffener attached on the bulb are critical. As small-scale specimen, a longitudinal stiffener welded on the bulb of a holland profile was chosen, which showed under 4-point bending a unexpectedly low fatigue strength. In total, five larger test models of the intersection were tested, three with constant and two with variable load amplitude. Measurements and numerical analyses showed a very high structural stress concentration factor which is obviously affected by the thickness of the bulb. Both the structural hot-spot stress as well as the notch stress approaches are well-suited for the fatigue strength assessment using these types of computed stresses. This is also true for the nominal stress approach, for which the construction rules already state a relatively low fatigue class. Also the live assessments for the tests with variable amplitudes remained on the safe side.

Altogether. the results of the investigations confirm the existing approaches for fatigue strength assessment, although some weaknesses of the nominal stress approach were identified, which include reduced effective breadths of flanges at intersections and longitudinal stiffeners ob bulbs of profiles. In addition it has been shown that effects of the local weld geometry and favourable residual stresses sometimes result in unexpected findings, where cracks do not appear at the expected location.

FINAL REPORT:

• Fricke, W.; A. von Lilienfeld-Toal und H. Paetzold (2009): Versteifte Plattenstrukturen im Stahlschiffbau – im Cluster Anwendbarkeit von Festigkeitskonzepten für schwingbelastete, geschweißte Bauteile. CMT-Forschungsbericht 14/2009, Center of Maritime Technologies Hamburg.

FURTHER PUBLICATIONS:

• Fricke, W.; von Lilienfeld-Toal, A.; Paetzold, H. (2012): Fatigue strength investigations of welded details of stiffened plate structures in steel ships. Int. J. Fatigue 34:1, 17 – 26.
• Fricke, W.; T. Gosch, A. von Lilienfeld-Toal, H. Paetzold (2008): Schwingfestigkeitsuntersuchungen an Rahmenecken von Ro/Ro-Schiffen. Jahrbuch der Schiffbautech. Ges., Bd. 102 (erscheint 2009).
• Fricke, W.; H. Paetzold und J. Rörup (2009): Betriebsfestigkeitsverhalten von Spantdurchfüh-rungen an HP- und Winkelprofilen. Hansa, Heft 5/2009.
• Fricke, W.; von Lilienfeld-Toal, A. und Paetzold, H. (2009): Versteifte Plattenstrukturen aus dem Stahlschiffbau. In: Festigkeit geschweißter Bauteile – Anwendbarkeit lokaler Nachweis-konzepte bei Schwingbeanspruchung, DVS-Berichte Bd. 256, DVS-Media GmbH, Düsseldorf.
• Fricke, W.; von Lilienfeld-Toal, A. and Paetzold, H. (2009): Fatigue investigation of a complex ship structural detail. Proc. 2nd Int. Conf. on Material and Component Performance under Variable Amplitude Loading (Ed. C.M. Sonsino and P.C. McKeighan), Vol. II, 681-691, Deutscher Verband für Materialforschung und –prüfung (DVM), Berlin
• Fricke, W.; A. von Lilienfeld-Toal, H. Paetzold und J. Rörup (2009): Betriebsfestigkeitsver-halten von Spantdurchführungen an HP- und Winkelprofilen. Jahrbuch der Schiffbautech. Ges., Bd. 103 (erscheint 2010).
• Fricke, W. and Paetzold, H. (2009): Full-Scale Fatigue Tests of Ship Structures to Validate the S-N Approaches for Fatigue Strength Assessment. IIW-Doc. XIII-2279-0