Needle peening to improve the fatigue performance of existing steel bridge welds

The problem of fatigue cracking in existing steel bridges is becoming an increasingly important one due to the natural aging of these structures, as well as the increasing traffic loads and volumes to which they are being subjected. One possible method of rehabilitating structures such as these that has received recent attention is the use of residual stress-based post-weld treatments, such as needle or hammer peening. A number of fatigue test-based studies have been performed over the years, demonstrating the effectiveness of these treatments in improving fatigue performance. Several industry guidelines and codes have started to acknowledge the positive effects of these treatments on fatigue performance. Despite these recent developments, a number of concerns remain regarding the implementation of post-weld treatments for improving fatigue performance. Two of the more important concerns are: 1) that effective procedures are lacking for controlling the quality of these treatments, and 2) that the beneficial compressive residual stresses introduced by these treatments may be undone by subsequent load histories that contain significant compressive overloads. This paper presents studies that were performed to investigate the potential of needle peening as a means of rehabilitating the welds in existing steel bridges. One of these studies demonstrates a possible quality control procedure for this treatment type. The other studies attempt to characterize, through testing and analysis, the impact of periodic compressive overloads on the fatigue performance of untreated and treated welds, as well as the effect of treating the weld part way through the planned service life