Effect of overaload on fatigue crack propagation

Abstract

Material of pipes is one mainly of Steel for large diameter pipes, API-X65 steel pipes is a higher strength, tough, weldable steel for applications in the oil and gas industry. X65 Flange Steel meets fully the mechanical property requirements of the API 5L X65 line pipe specifications whilst maintaining a high degree of toughness and weldability. The effects of single peak tensile overloads have been reported in many investigations simply because this type of loading can lead to significant load interaction effects. Several mechanisms have been proposed to explain crack growth retardation, which include models based on residual stresses, crack closure, crack tip blunting, strain hardening, crack branching and reversed yielding. Single Edge Notch Bend (SENB) specimens, 12.72 mm thick, of the steel pipe API 5L X65 were subjected to crack growth period following stress ratio value and stress intensity factor range are given, the effect of an overload on the number of delay cycles and delay distance. In the retardation process, crack tip blunting is considered to play a dominant role. Delayed retardation due to an overload has been shown to occur at following overload ratio values: 1.8, 2.0, 2.17 and 2.4. Overload plastic zone size increases with increase in overload Ratio. The number of delay cycles increases nonlinearly with overload ratio.