Comparison of Fracture Mechanics and Nominal Stress Analyses in Stress Corrosion Cracking

Abstract

Abstract Since the development of the fatigue cracked cantilever beam stress corrosion specimen, the fracture mechanics approach has been widely used to analyze the stress corrosion behavior of materials. However, there has been only limited experimental verification of the fact that propagation of flaws in corrosive environments is characterizec by the crack tip stress-intensity factor, KI, rather than by the nominal stress, σN. Therefore, an investigation was conducted to compare the fracture mechanics (KIc, KIscc) and nominal stress (σNf, σNscc) analyses of fatigue cracked cantilever beam specimens. The results indicate that both KIc and KIscc are material properties and independent of specimen size. However, the nominal stress counterparts to KIc and KIscc (σNf and σNscc) represent qualitative stress levels for the 18Ni (250) maraging-steel used in the investigation were 67 and 36 ksi √inch, respectively, regardless of specimen size. However, the (σNf and σNscc values varied by about a factor of 3 for the same order of magnitude change in specimen depth, W. Because the crack propagation rates for some materials in synthetic sea water may be very slow, high strength, highly alloyed steels should be tested for approximately 1000 hours under constant load conditions to measure KIscc values. In general, the results of this investigation have shown that the fracture mechanics approach is a quantitative method of assessing load thresholds, independent of specimen size, for the onset of fracture or of environmentally induced crack growth in notched specimens. Furthermore, KIc and KIscc values appear to be applicable for use in structural design and associated inspection because they properly account for the trade-off between design stress and flaw size.