Fatigue Behavior of Line Pipes Subjected to Severe Mechanical Damage

Fatigue tests were carried out on ERW line pipes 200 mm and 300 mm in diameter with a severe gouge in dent type defect. Hoop stress fluctuation between 15% and 30% of the yield stress of the pipe materials was applied by cyclic internal pressure loading. By the parameter Q, a function of defect size and fracture toughness, fatigue behavior could have been well explained. Fatigue behavior differed above and below the threshold Q (Q_th. When Q<Q_th, crack propagation was initially associated with ductile crack growth, and therefore, fatigue life, N_f, decreased to less than 1000 cycles. On the contrary, N_f was larger than 1000 cycles when Q≧Q_th. N_f was predictable with a power law equation incorporating dent depth, gouge depth, and stress amplitude when Q≧Q_th. A gouge in a dent with Q<Q_th, requires immediate repair or replacement.