The effect of Impact stresses on the fatigue properties of specimens cut from a tubular steel pile which had been subjected to a large number of impact loads has been evaluated The results demonstrate that the tensile fatigue endurance of specimens, incorporating butt welds which have been subjected to impact stresses, was greater than that incorporating normal butt welds by a factor of four The Inverse slope of the fatigue relationship (S-N curve) was approximately six, indicating either a dependency on fatigue Initiation rather than propagation or crack growth Into an increasing tensile residual stress field The beneficial results can be explained largely by the lower values of axial residual stresses which existed In the butt weld after piles had been subjected to impacting, compared to the as-welded state
The use of tubular steel piles driven into the sea bed to secure or anchor offshore platforms or berthing facilities is practiced world-wide Impact driving In excess of 10000 blows may be used to install the piles and, during installation, the local instantaneous dynamic stresses often exceed the static yield stress of the steel The effect of impact stresses on the fatigue life of piles had not been investigated prior to commencement of this research project It was thought to be deleterious and that the Miner's rule (Miner, 1945) contribution of each driving cycle should be considered in fatigue life calculations This requirement obviously reduced the overall fatigue life resulting in a commercial disadvantage for such plies In the first stage of an investigation earned out to determine the justification of this approach Priest and Gaunt (1989) reported that there was no significant difference between the fatigue properties of thin walled tubular butt welded specimens which had been subjected to simulated pile driving conditions and those which had not.
