Fatigue strengthening of welds in light rail structures

London's Docklands Light Railway (DLR) has been in service for 20 years and has previously been strengthened when it was upgraded from one-car trains to two-car trains in 1990. It is now to be upgraded again to carry three-car trains to be run at higher frequencies and is required to have a fatigue life of 120 years from the year 2010, namely for the structures to remain serviceable until the year 2130. Some of the welds connecting stiffeners to main beams, which were found to be under strength in fatigue, were strengthened by ultrasonic impact treatment (UIT). A programme of fatigue tests has been carried out to evaluate the extent of the improvement and confirm that it is adequate. The tests were on tensile test pieces having non-load-carrying fillet welded attachments relevant to the (BS5400 class F2) under strength welds. Test pieces were pre-fatigued to simulate 20 years' service and treated by ultrasonic impact. The UIT improved the class F2 fillet welds to a performance between class D and class E. There is a 97·5% probability that performance is improved by a factor of 3 on life and the value of σ_o (the constant amplitude non-propagating design stress range) is raised from 35 to 50 N/mm². The action of UIT is to halt crack propagation from the weld toe and propagation from the root is slowed as it passes through the treated zone. Furthermore, UIT is effective on existing fatigue cracks having an estimated surface length of up to 8 mm and depth of 1·5 mm. Experiences gained from the experimental programme enabled an improved specification for UIT quality control to be developed. These data were used in support of the design work to upgrade the DLR structures and the UIT was successfully applied to the under-strength welds under live loading.