Fatigue Strength of a Longitudinal Attachment Improved by Ultrasonic Impact Treatment

It is commonly observed that complex fabricated structures subject to fatigue loading fail at the welded joints. Some problems can be corrected by proper detail design, but fatigue performance can also be improved using post-weld improvement methods.

In general, improvement methods can be divided into two main groups: weld geometry modification methods and residual stress modification methods. The former remove weld toe defects and/or reduce the stress concentration, while the latter introduce compressive stress fields in the area where fatigue cracks are likely to initiate.

The ultrasonic impact treatment method belongs to the class of residual stress improvement methods. The method makes use of an ultrasonic carrier frequency to accelerate hardened tools that, in turn, impact the weld toe. The fatigue strength of non-load carrying attachments in the as-welded condition has been experimentally compared to the fatigue strength of ultrasonic impact treated welds.

Longitudinal attachment specimens made of two thicknesses of steel S355 J0 have been tested for determining the efficiency of ultrasonic impact treatment. Treated welds were found to have about 50% greater fatigue strength, when the slope of the S-N-curve is three. High mean stress fatigue testing based on the Ohta-method did not decrease the degree of weld improvement due to UIT. This indicated that the method could be also applied for large fabricated structures operating under high reactive residual stresses equilibrated within the volume of the structure.

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