TWI's fatigue laboratory has completed a series of trials in support of a scheme to lengthen the fatigue lives of existing London bridges by performing in-service peening.
Since the bridges were designed and built - some as late as the 1980s - traffic density and vehicular loading have increased. Some of the welded details examined on the bridges were not up to the mark and to counter that difficulty, certain critical regions of the welds were peened to enhance their fatigue lives.
TWI carried out experiments to assess the effectiveness and ease of operation of peening in service, using 2m long specimens loaded in a fatigue testing machine. The test piece was modelled on the top flange of a particular London bridge.
It has been found that peening applied at the weld toe can increase fatigue life by three to four times or more. If the toe of the weld is physically hammered hundreds of times, compressive residual stresses are introduced into the surface layer and inhibit fatigue cracking.
This project related to bridges, but peening can be equally effective on ships and offshore structures.
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