Launchpad, July/August 2007
For a full list of proposed, active and closed projects see here
Further study of fatigue damage to girth welds from low stresses in the loading spectrum
Launched: February 2007
Cost per sponsor: £50 000 pa, two payments
Project leader: Yanhui Zhang
Fatigue design of welded joints is based on data obtained under constant amplitude loading in conjunction with Miner's rule to estimate the damage introduced by variable amplitude loading in service. However, Miner's rule can be unsafe, especially for service loading containing many low stress cycles below the constant amplitude fatigue limit. These issues were addressed in a previous project in the specific context of girth welded pipes. Full-scale fatigue tests were used to establish constant amplitude data for the girth welds, but the joints had to be cut into strip specimens for tests under variable amplitude loading. Significant problems of interpretation of the results arose because the strip specimens had a much higher fatigue strength in the important long-life regime than the full-scale joints. This highlighted the need for testing full-scale pipes under spectrum loading and, following trials at TWI, this is now possible in the resonance test rigs used previously for the constant amplitude tests. The current project will therefore concentrate on full-scale tests under realistic service loading histories with the aim of establishing new design guidance to enhance safety while reducing costs for pipes and risers.
