Ultrasonic Peening for life extension and refurbishment
In recent years Classification Societies have issued recommendations in reference to the use of ultrasonic peening as a fatigue life extension method for weld connections. However, these recommendations still do not include any advice, with the exception of ABS see below, for the use of ultrasonic peening in connection to new design or increased design stress ranges.
“The finished shape of a weld surface treated by ultrasonic/hammer peening is to be smooth and all traces of the weld toe are to be removed. Peening depth below the original surface is to be maintained at least 0.2 mm. Maximum depth is generally not to exceed 0.5 mm.
Provided these recommendations are followed, when using the ABS S-N curves, a credit of 2 on fatigue life may be permitted when suitable toe grinding or ultrasonic/hammer peening are provided. Credit for an alternative life enhancement measure may be granted based on the submission of a well-documented, project-specific investigation that substantiates the claimed benefit of the technique to be used.”
“An exception may be made if the target design fatigue life cannot be satisfied by other preferred design measures such as refining layout, geometry, scantlings, and welding profile to minimize fatigue damage due to high stress concentrations. Grinding or ultrasonic peening can be used to improve fatigue life in such cases. The calculated fatigue life is to be greater than 15 years excluding the effects of life improvement techniques. Where improvement techniques are applied, full details of the improvement technique standard including the extent, profile smoothness particulars, final weld profile, and improvement technique workmanship and quality acceptance criteria are to be clearly shown on the applicable drawings and submitted for review together with supporting calculations indicating the proposed factor on the calculated fatigue life.”
Improvement Techniques other than Grinding of Welds
Improving fatigue life by using improvement techniques other than grinding is to be regarded as an exceptional measure. Such improvement techniques may be considered by the Society on a case by case basis. In such a case, the fatigue life improvement factor βif = 2.2 to be duly justified by the designer.