A Modified Exponential Model for Predicting the Fatigue Crack Growth Rate in a Pipeline Steel Under Pure Bending

The present work proposes a fatigue crack growth rate (FCGR) model for steel pipelines subjected to sinusoidal loading using a modified exponential function. The modification in the exponential function is made for the non-dimensional parameter using the stress intensity range (ΔK) as the crack driving force. The acceptable values of ΔK for FCGR in stage-I ranged between 17.45-20.46 MPa√m, between 20.46-21.41 MPa√m for stage-II, and between 21.41-21.98 MPa√m for stage-III. A new correlation is also developed between the specific growth rate and the non-dimensional number. The modified exponential function predicted the FCGR within the acceptable values for all three stages in the radial direction. It shows the best performance for stage-I of FCGR and the lowest for stage-III. The microstructure envisages shallowed microvoids, while the striations and secondary cracks are mostly perpendicular to the FCG direction.