Whether or not a stress corrosion crack will grow to reach the critical size for fast mechanical
fracture will depend, among other factors, upon the source of the stress that initiates cracking.
If this is a fabrication stress, such as a residual welding stress, or if it derives from
misalignment of fastener holes, crack propagation may well result in stress relief and the
crack may cease to propagate if the crack tip stress intensity falls below Kscc before Kc is
reached. On the other hand, if the source of the stress that drives the slow stress corrosion
crack is derived from the operating conditions, the crack tip stress is not likely to relax and
catastrophic failure eventually will occur. Fortunately, operating stresses are frequently
below the minimum stress required for stress corrosion cracking and most instances of stress
corrosion failure arise from the presence of stresses of yield stress magnitude left in structures
as the result of fabrication procedures. The exceptions are usually pressure vessels, such as
chemical reactors, high pressure gas transmission lines and steam boilers, and it is probably
not insignificant that the incidence of stress corrosion failure has increased considerably over
the last two decades as engineering design efficiency has improved, involving higher
operating stresses and higher yield strength materials, and as the problems of corrosion
spread relatively uniformly over exposed surfaces have been largely overcome, resulting in
the possibility of more localised forms of corrosion.