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Post weld improvement methods that reduce the stress concentration atthe weld toe, remove weld imperfections and/or introduce local compressive stresses atthe weld toe can significantly improve the fatigue strength of a structure. In some cases,however, the degree of improvement is limited by the fatigue strength of other details orlocations in a component. The material strength and type of loading also influence theobserved fatigue crack behaviour. This study reports on fatigue crack patterns observedfor both constant amplitude (CA) and variable amplitude (VA) test results performed onhigh strength steel (fy = 700 and fy = 960 MPa) longitudinal non-load carrying welds.Some specimens were in the as-welded state while others were post-weld treated usingeither ultrasonic impact treatment (UIT) or ultrasonic peening (UP) equipment. Thesetreatment techniques are generally categorized as residual stress modificationprocesses, but they also reduce the local stress concentration near the weld toe. Failuremodes were significantly different for CA and VA loading and VA loading showed lessimprovement. The study shows that fatigue strength of a welded structure can besignificantly improved by ultrasonic peening, but that care must also be given to jointpreparation, quality control of the peening procedure and the type of loading expected.Improper peening procedures can also induce unexpected and undesirable fatiguecrack patterns that produce little improvement in the resulting fatigue strength.