E y12

b where y is the surface energy of material, b is the interatomic spacing, and E is Young's modulus.

Alternatively, hydrogen may diffuse ahead of the crack tip to locations of maximum stress triaxiality. An increase in crack growth can be assumed to be proportional to the amount of hydrogen generated (CH) per load cycle:

dNcf

In this case, f(CH) includes the influence of environmental composition, temperature, potential, and frequency, while g(AK, R) relates to the mechanical driving force, where R is the stress ratio (amjn/amax).

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