FRACTURE AND FATIGUE CRACK GROWTH BEHAVIOR OF TWO KINDS OF LINEPIPES

摘要

Fracture and fatigue failure are important failure forms for long-distance pipeline. Fracturernprocess always includes four steps: crack initiation, stable propagation, unstable propagation andrncrack arrest. J resistance curve (J_R) and crack arrest toughness (K_(Ia)) can be used to describernfracture process in this paper. Resistance curves at different temperatures ( -60, -40 , -20, 0, 20℃)rnwere tested to determine X52 ERW and X60 SSAW linepipes’ crack resistance ability. Inrnaddition, to prevent gas pipeline from fracture, crack arrest toughness of linepipe should meetrncertain requirements. So crack arrest tests at different temperatures were performed too. In orderrnto study brittle fracture feature of linepipe, wide-plate tension test of base metal and weld wererncarried out to determine the fracture toughness K_c of the material. The result showed that X seriesrnlinepipes possessed excellent brittle fracture resistance within the test temperature (20 ~ -60℃) .rnFull-scale burst tests with defect and defect-free pipes were carried out to determine its burstrnpressure and study the effect of defect on pipe's strength and toughness.rnIn addition, the paper has made research on fatigue crack growth rate of SSAW and ERWrnpipes. Measurement of fatigue crack growth threshold ( ΔK_(th)) and fatigue crack growth rate (da/dN)rnwas performed with same specimen. Test results showed that, as to SSAW pipes, the weld got thernhighest ΔK_(th), then the heat affected zone (HAZ) was followed, where the pipe base metal got thernlowest. In Paris equation, da/dN = A( ΔK)~m, the index m in equations of the three pipes were allrnbetween 3.1 and 4.3. To simulate the effect of transmitting medium pressure fluctuation inrnpipeline on pipe's fatigue property, stress ratio R = 0.1 and 0.6 were chosen to carry out da/dN test.rnIt showed that fatigue crack growth rates increased as R increased. SEM analysis on fatiguernfracture surface showed that, near the fatigue crack growth threshold zone, it was quasi-cleavagernfracture and some inter-granular fracture, secondary crack could be found in some area too.rnWhereas, in fatigue crack growth zone, it was almost quasi-cleavage fracture, and fatigue striationrnformation could be found in some area.