Cracking longitudinal

Top/down cracking (longitudinal), ft/mile Thermal cracking (transverse cracking),ft/mile Smoothness (roughness). ... 

SCC has been defined as failure by cracking under the combined action of corrosion and stress (Fig. 9.1). The stress and corrosion components interact S3mergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state. They may run in any direction but are always perpendicular to the principal stress. Longitudinal or transverse crack orientations in tubes are common (Figs. 9.2 and 9.3). Occasionally, both longitudinal and transverse cracks are present on the same tube (Fig. 9.4). Less frequently, SCC is a secondary result of another primary corrosion mode. In such cases, the cracking, rather than the primary corrosion, may be the actual cause of failure (Fig. 9.5). 

Figure 9.2 Longitudinal stress-corrosion cracks in a heat exchanger tnbe the broad gap between the crack faces reveals that high-level residual hoop (circumferential) stresses from the tube-forming operation provided the stress component required for SCC.

Figure 9.4 Both longitudinal and transverse stress-corrosion cracks on a brass heat exchanger tube that had been exposed to ammonia. Note the branching of the cracks.

A routine inspection of the tube bundle during a plant outage revealed fine cracks of the type shown in Fig. 9.11. Scattered longitudinal cracks were observed along the lengths of most tubes. The external surface was covered with a thin film of black copper oxide and deposits. The bundle had been exposed to ammonia levels that produced 14 ppm of ammonia in the accumulated condensate. 

The longitudinal orientation of these cracks reveals that hoop (circumferential) stresses caused by internal pressurization provided the necessary stresses. Ammonia was the specific corrodent involved. 

Figure 9.11 A network of longitudinal cracks along the external surface. (Magnification 7.5x.)...

A total of 13 tubes in the exchanger had suffered cracking of the type illustrated in Fig. 9.14. The cracks are predominantly longitudinally oriented. 

The unit was shut down, drained, and visually examined. Numerous branched cracks of the type shown in Fig. 9.21 were observed. Cracks were oriented longitudinally. 

Intergranular corrosion-fatigue cracks in copper may he difficult to differentiate from stress-corrosion cracking. The longitudinal orientation of the cracks revealed that the cyclic stresses were induced by fluctuations in internal pressure. 

Figure 10.10 Longitudinal corrosion-fatigue crack originating on the external surface.

The longitudinal crack apparent in Fig. 12.21 resulted from stresses from internal pressure that exceeded the tensile strength of the metal in the greatly thinned tube wall. 

Figure 12.21 Cavitation damage on the internal surface of the condenser tube. Note longitudinal crack. The surfaces are covered with orange, air-formed iron oxides that formed subsequently to the removal of the condenser tube.

Figure 14.6 shows the straight, unbranched, longitudinal cracks typical of tubes removed from the unit. The cracked tubes had been located by eddy-current inspection. 

Identification. Weld-root cracks originate at the root of the weld and run longitudinally along the weld, perpendicularly to the base-metal surface and parallel to the axis of the weld. In general, they may be identified visually or by various nondestructive testing techniques such as radiography or ultrasonics. Failures from weld-root cracking may occur soon after start-up or after extended periods of successful service. 

Figure 17.7 Section of longitudinally fractured subterranean cooling water line. Note that the crack runs the entire length of the line.

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Check slip areas for longitudinal and transverse cracks and sharp notches. Check tool Joints for wear, galls, nicks, washes, fins, fatigue cracks at root of threads, or other items that would affect the pressure holding capacity or stability of the Joint. 

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