Near- threshold Fatigue Crack Growth Behavior and Crack Closure of Natural Gas Pipeline Steels

The near-threshold fatigue crack growth behavior and crack closure of three pipeline steels, - X65, X70 and X80 steels, - were investigated in air at a load ratio (R) of 0.1. With increase in the grade of pipeline steel, the average grain size of ferrite decreased and yield and tensile strength increased. In the near-threshold regime, the X80 steel had the slowest fatigue crack growth rate while the X65 steel had the fastest fatigue crack growth rate. The fatigue crack growth rate of the X70 steel was in-between those of the X65 and X80 steels in the near-threshold regime. For the X65 and X70 steels, it had been shown that the crack closure levels (K-cl/K-max) were 0.1 and independent of stress intensity factor range (Delta K). In contrast, the crack closure level of the X80 steel was 0.54 at the fatigue threshold mid decreased with increasing Delta K. From analysis of fracture surface roughness, it was found that the factors characterizing roughness-induced crack closure for three pipeline steels in air were average fracture surface roughness, R-a, and the effective range of the crack tip opening displacement, (Delta CTOD)(eff), It is concluded that the crack closure phenomenon of the X80 steel in the low Delta K region is roughness-induced.