Fatigue Performance of Friction Welds Manufactured Both in Air and Underwater

Friction welding is being performed offshore in environments where arc welding may be difficult and where fatigue performance is critical. Friction welding underwater with Remotely Operated Vehicles (ROVs) can greatly reduce the cost of a project compared with using divers and arc welding because the support vessel, which is the major cost component in such an operation, is smaller. This paper describes two different programs of experimental work in which the fatigue endurance of friction welds were found to be better than that which could be expected from arc welded joints of similar geometry. The first program involved experimental work done with 25mm diameter steel bars. It found that, in the as-welded condition, friction welds have high fatigue strength. Residual stress measurements showed that this was due to a beneficial residual stress distribution in which compressive stresses are present at the surface adjacent to the failure site. Further evidence of this was obtained by subjecting some specimens to thermal stress relief. The fatigue strength of the stress relieved specimens was reduced compared with the as-welded joints but nevertheless the fatigue strength of these specimens was still high. The second program involved fatigue tests on friction stud welds in which the friction welding equipment was deployed offshore by divers or ROVs. The test specimens were made up of 19mm diameter studs friction welded onto structural steel plate. As with the first program, the specimens showed high fatigue endurance with results approximating to a DNV Class C1 curve. In some of the tests, the studs were preloaded in tension and results from specimens that were preloaded to the correct value specified for the joint were all stopped as run-outs, with specimens remaining unbroken.