EARLY STAGES IN THE DEVELOPMENT OF STRESS-CORROSION CRACKS

Processes involved in the growth of short cracks and stage I of long stress corrosion cracks have been identified and evaluated. There is evidence that electrochemical effects can cause short stress corrosion cracks to grow at rates which are either faster or slower than those of long cracks. Short cracks can grow at faster rates than long cracks for a salt film dissolution growth mechanism or from reduced oxygen inhibition of hydrolytic acidification. An increasing crack growth rate with increasing crack length could result from a process of increasing crack tip concentration of a critical anion, such as Cl-, with increasing crack length in a system where the crack velocity is dependent on the Cl- or some other anion concentration. An increasing potential drop between crack tip and mouth would result in an increased anion concentration at the crack tip and hence an increasing crack velocity. Stage I behavior of long cracks is another early development stage in the life of a stress corrosion crack which is poorly understood. This stage can be described by da/dt = AK(m) where da/dt is the crack velocity, A is a constant, K is the stress intensity and m ranges from 2 to 24 for a variety of materials and environments. Only the salt film dissolution model was found to describe this stage quantitatively; however, the model was only tested on one material and its general applicability is unknown.