Studies on the fatigue of branched connections related to the shakedown pressure

The concept of shakedown is that regions which plastically deform during initial loading subsequently adopt an elastic mode of deformation. High strain fatigue, i.e. up to 10,000 cycles, will be eliminated. Two branches of different thickness were welded to a common cylindrical vessel and hydraulically tested, simultaneously. The test pressure was the shakedown limit for the stronger branch and about 1.5 x shakedown for the weaker branch. The thinner branch failed by penetration through the weld near the crotch corner at 3,543 cycles. The thicker branch was given 10,000 cycles when, since no cracks were apparent, a starter notch was made in the crotch corner. Leakage occurred after another 10,678 cycles. This crack was also influenced by weld defects. Using fracture mechanics and a reported relationship between crack rate and stress intensity factors, some predictions of endurance at design pressure are derived for these branches. These lives are compared with the British Standard 1515 formula for fatigue avoidance and with the curves given in Section III of the ASME Boiler and Pressure Vessel Code. The former is considered conservative, while the ASME Code curve requires an additional safety factor as suggested by Jettam; A multiplication factor of 2 on the nominal stress concentration factor for welds is then considered satisfactory. It is tentatively considered that such junctions manufactured to Class 1 standard are satisfactory for daily cycling for 30 years, but are marginal for twice daily cycling for that time. This conclusion is tentative, since it depends critically on the size of weld defects present