The IIW Recommendations allowances for improved welds by peening methods is assessed in terms of the SN Curve or FAT Class see red line on the left SN diagram. For steels with specified minimum yield stress below 350 MPa, the improvement factor is 1.3 in stress corresponding to a factor of 2.2 on life, it means from FAT 71 to FAT 90 or 30% increase in stress for our specific weld detail. The factor 2.2 on life comes from the assumed slope, m=3, of all the SN curves. Thus, (1.3) 3 = 2.2 would be the improvement in life in the SN Diagram. It is important to note that as all diagrams showing fatigue test results for UP improved welds demonstrated a less steep slope, m > 3, it would be reasonable to conclude the improvement in life it is far more than 2.2.
For example if we have slope m=5.5 as suggested in IIW Doc XIII-2362-11 overview of fatigue data for UP improved welds, the improvement in life would be (1.3) 5.5 = 3.7 times. As a result the 2.2 times life extension in life is a rather conservative estimate whereas an improvement of 4 times in life is closer to reality. The highest Fatigue Class which can be claimed for peened welds is FAT 125. The reason why we are allowed to use FAT 125 instead than FAT 90 is the fatigue test results reported in IIW Doc. XIII-2143-07 where FAT 151 was achieved for UP treated welds. The FAT 90 value is coming from 30% increase from stress range FAT 71 which gives 91 MPa and is then assumed to be FAT 90 according to IIW rules; hence an increase from FAT 71 to FAT 125 means a 76 % increase in stress. As a result and assuming m=3 we could claim an improvement in life: (1.76) 3 = 5.45 times.
For the case of the side shell longitudinal we assume a stress range of 250 MPa, see Fig on the left. The fatigue life for such a stress range in the as-welded condition, FAT 71, is 50.000 cycles according to Fig 15. According to IIW and assuming FAT 125 at a stress range of 250 MPa, would give us a fatigue life of 280.000 cycles. Please note again that the reason why we can assume and use FAT 125 for our calculations is the fatigue testing reported in IIW Doc XII-2143-07. The improvement is then 5.6 times in fatigue life for peening treated welds. This improvement is due to the IIW allowance to use FAT 125 for peened welds instead than FAT 71 as for as-welded. Whereas the max stress range in service is the same, assumed 250 MPa, the fatigue strength is different for treated and untreated welds. This is still a conservative estimate since FAT 151 > FAT 125, which verifies that the improvement achieved by the ultrasonic peening applied normally in ballast tanks.
Hence, the suggestion of a general 4 times fatigue life extension for UP improved welds seems to be in good agreement with current Fatigue Design Standards.