Axial cracking

Figure 1. Schematic of an axial crack on the inner wall-surface of a pipeline and the crack tip region under consideration.

We obtained full field solutions for an axial crack on the ID surface of a pipe under plane strain conditions. The pipe has an outside diameter of 40.64 cm and a wall thickness h = 9.52 mm. The depth of the axial crack is a = 1.9 mm (a/h = 0.2) (Fig. 2). In the absence of hydrogen, we assume a finite crack-tip opening displacement (CTOD), ha = 2 pm. 

In the MBL approach7 (see Fig. 5) we increased the stress intensity factor and the T-stress linearly from zero to K, =34.12 MPaVin and T /cr0 =-0.316 (Fig. 5a), respectively over 1 sec, and we then kept them constant. These magnitudes of the stress intensity factor Kt and T-stress are for the axial crack of depth a = 1.9 mm (a/h = 0.2) we analyzed in the full field formulation at 15 MPa hydrogen pressure. 

It should be noted that although the conclusions drawn above were for the case of an axial crack on the inner surface of the pipeline wall with a specific crack depth of a = 0.2/ the conclusions continue to be valid for crack depths that ensure SSY conditions in the pipleine7. Dadfamia et al.7 demonstrated this to be the case for a crack depth less than 0.4/ and hydrogen pressures up to 15 MPa. 

We solved the transient hydrogen diffusion initial/boundary-value problem coupled with the large strain elastoplastic boundary value problem for a pipe of an outer diameter 40.64 cm, wall thickness h = 9.52 mm, and with an axial crack of depth 0.2/i on the inner wall-surface. We obtained the solutions under hydrogen gas pressure of 15 MPa, material properties for an X70/80 type steel, and... 

M. Dadfamia, B. P. Somerday, P. Sofronis, and I. M. Robertson, On the Small Scale Character of the Stress and Hydrogen Concentration Fields at The Tip of an Axial Crack in Steel Pipeline Effect of Hydrogen-Induced Softening on Void Growth, Int. J. Mater. Res., to appear (2008). 

A PTFE-lined, stainless steel braided hose was used to transport sulphur trioxide at a chemical plant. The hose was attached to an external tank system and was insulated and heated. One day the hose failed and sulphur trioxide leaked and caused serious environmental problems. The failed hose had one long axial crack that had split the hose open, which allowed the liquid to be released. The engineering assignment in this case was to examine the cause of the failure. 

Plaisted, T.A., Amirkhizi, A.V., and Nemat-Nasser, S. (2006) Compression-induced axial crack propagation in DCEX2 polymer samples experiments and modeling. International Journal of Fracture, 141, 447 57. 

Both longitudinal (linear axial crack) and circumferential (arc of circle crack) breaks were simulated. 

A similar loading sequence caused failure in polyester fibres, but on some polyester fibres studied later by Oudet and Bunsell (1987) a low critical minimum load gave the same form of break. An important, and unexplained difference from nylon is that the axial cracks are closely parallel to the fibre axis. Consequently, the tails are extremely long. Fig. 9e,f. In one example, the crack had propagated beyond the final break zone, which was effectively a creep rupture failure from a central flaw. Fig. 9g. 

Fig. 13. which is an early tensile failure model for Kevlar due to Morgan el al. (1982), but also reproduced by Yang (1993), illustrates the problems. Three modes of crack propagation are apparent. In the skin on the left, axial cracks between molecules... 

Here, spiral cracks are dominant at first. When leaching reaches very deep (< 1/3 fiber cross-section) and when the fibers are very thin, mainly axial cracks develop. Crack propagation rates between 1 and 100 mm/s under formation of spiral-shaped cracks have been measured [818]. 

According to the US NRC point of view, a Double Ended Guillotine break (DEG) or an equivalent axial crack must be postulated for the High Energy Fluid stem (HEFS) piping. In contrast, for the Moderate Energy Fluid stem (MEFS) piping, a slit-like defect of D/2 in... 

To simulate an axial crack driven by hoop stress, we consider CL growth in plain strain conditions under constant load cr. The process b ins under conditions ... 

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