Discussion on Paper 11

DR G. EDWARDS, Shell Research KSEPL, The HagueMy question concerns the initial defect size one assumes in carrying out a fracture mechanics fatigue life prediction. The Paper implies that a defect of the size of the local weld notch stress field will develop rapidly. Would this therefore be a suitable initial defect size to assume for a conservative lifetime prediction? Does the Author have any feel for how the size of this notch region compares with typical defect sizes resulting from the welding process?

MR CLAYTON, in replyThe idea is that if a local region at the weld toe were to exceed yield stress, a crack would rapidly propagate through the region, and an elastic fracture mechanics analysis could be made using the yield zone size as the initial crack size for crack growth calculations. If this size is less than the size of defects which could be expected at weld toes (say 0.5 mm)then the initial crack size would clearly be this value rather than the yield zone size. Studies of yield zone size in welded joints are currently being made to determine the significance of the yield zone size.