Stress Intensity Factor K, MPa.Vm FIGURE 11.13 SCC growth rate curve.

controlled by the rate of chemical reactions at the crack tip, and (iii) rapid crack growth to fracture as stress intensity approaches KIC, the critical stress intensity for rapid brittle fracture in air.

Growth rates vary depending upon the cracking mechanisms involved. For dissolution-type processes, moderate crack growth rates (ca. 10-3 mm/s) are limited by the rate at which metal can enter solution and be transported away from the crack tip. For hydrogen embrittlement, crack growth rates are much higher as hydrogen often needs only to diffuse a short distance ahead of the crack tip, and consequently rates of 1 mm/s may be encountered.

It should be noted that evaluation of SCC cracking behavior based on pre-cracked specimens provides no information on the early development of SCC cracks, often known as the initiation stage. Caution should therefore be taken when extrapolating SCC linear elastic fracture mechanics (LEFM) data to small defects on smooth engineered surfaces as it is recognized that cracks can develop at stress intensities below the KISCC threshold level.

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