While the fracture surface of the recent growth (at "B") is characterized by a clean quasi-cleavage topography, the zone of prior growth (at "A") was

Figure 6 Close-up view of the transition point between previous crack growth (A) and recent crack growth (B).

covered with a thick layer of corrosion product or deposit. This means that the crack was dormant for a certain amount of time before it was active again, which, during the most recent growth, resulted in the crack reaching the critical depth for unstable propagation. On the macroscopic level, the overall crack depth consisted, apparently, of six cycles of growth events, with well-defined arrest markings between them, as seen in Figure 7.

Figure 7 A macro-fractograph taken from the St. Norbert failure showing the various growth periods separated by crack arrest markings

For the St. Norbert case there is sufficient geotechnical data to suggest that the sliding of the clay soil on the river bank occurred in bursts when the water level in the river rose above a certain threshold. In the past several years, this threshold was surpassed in late spring when the run-off from the melting snow poured into the river. Similarly, in the case of the SNAM line in Italy, the pipe was found, from the readings of the strain gauges instrumented on the pipe, to undergo a period of extension at relatively high strain rate during the yearly rainy season [13].

Since the sliding movement of the soil does not reverse, the overall pattern of axial loading in pipelines associated with the land slide would, under ideal conditions without slippage between the pipe and the soil, be analogous to a monotonic tensile loading with a superimposed low-frequency wave component. When the total load is close to the yield point of the steel and is sustained for some time, straining due to low-temperature creep could generate sufficient plastic deformation at the crack tip for the growth to resume. For a line pipe steel, room-temperature creep can produce a strain rate in the order of 10-6 s-1 at a load close to the yield point of the steel [15]. Low-temperature creep-induced plasticity is a transient occurrence, and the strain rate in steels like linepipe at pipeline operating temperature decays to an insignificant level within 20 or 30 minutes of the initial loading. In one study [16], creep in an X-52 linepipe steel at 70 EF stopped within about 1000 seconds (~17 minutes) of loading to stress levels up to about 65 ksi. However, when the applied stress was held continuously at 95% UTS, creep continued to failure.

For linepipe steels exposed to a near-neutral pH environment, hydrogen-assisted plasticity can also occur, which may delay the exhaustion of the primary creep. In a recent review on pipeline SCC [17], Parkins pointed out, in his discussion of cyclic micro-plasticity, the relevance of the work on hydrogen-assisted creep by Oriani and Josephic [18]. In their creep measurement using a spheroidized mild steel, the rate of room-temperature creep of a prestressed wire, with a pre-strain of 5.5%, was found to increase dramatically when hydrogen gas fugacity was increased to 40 MPa. In fact, strain rates as high as 10-6 s-1 were reported after an increase in the hydrogen fugacity. However, it is unclear what level of cathodic charging is required to produce such a hydrogen fugacity in linepipe steels. For a 4340-steel polarized in a 0.1N NaOH solution at a potential of -1100 mV (SCE), the surface hydrogen fugacity is only about 0.1 MPa [19].

It is likely that some amount of hydrogen is produced in the course of crack propagation in the near-neutral pH environment, as a result of the cathodic reaction occurring on the crack flanks as well as at the crack tip. It is possible that this hydrogen could be sufficient to influence the low-temperature creep of linepipe steels. Relevant data in the existing open literature is limited on this subject. The nature of crack propagation in the case of axial cracking is not as clear. Because the crack growth rates under normal operating condition are very low, typically a fraction of a mm a year, corrosion products build up on the fracture surface, which obliterate much of the fractographic evidence necessary for post-mortem study. It is possible that the axial cracks also grow in a discontinuous manner, and each growth event is associated with a dynamic loading event such as a significant pressure fluctuation.  

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