抄録
Very low cycle fatigue strength of welded joints of HT570 steel with weld defects was investigated to study the acceptable size of weld defects in girth welds of gas pipelines under cyclic displacement of land by earthquake. Butt welded joints containing incomplete penetration, porosities, lack of fusion in the intermediate pass, and crack in penetration bead were manufactured and tested under strain controlled conditions.
All specimens tested in this study satisfied the fatigue design curve for girth welds of gas pipelines. The fatigue strength of specimens with weld defects generally decreases with the increase of equivalent defect size. The shape of cracks initiated from defects is affected by the reinforcement, and the surface crack propagates rapidly along the weld toe of the penetration welds and passes through the width of the specimen. The relationship between the J-integral range and the crack propagation rate under very low cycle fatigue load is almost the same as the extension of the relationship in low J-integral range. The crack propagation analysis which regards defects as cracks leads to the relationship between the size of defects and the number of cycles passing through the thickness. The critical size of defects in welds of gas pipeline under cyclic displacement is proposed by the analysis. The critical crack size of the surface defect could be applicable to the buried defect because the surface defects give a more conservative evaluation than buried defects. The analytical results give conservative estimations compared to experimental results.
