Environmental Crack Driving Force
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IN the call for a recent symposium on stress corrosion cracking (SCC),[1] it was noted that, ‘‘while significant progress has been made in understanding environmentassisted cracking, important fundamental questions like how environment affects the [mechanical] crack driving force (CDF) remain unanswered.’’ In an early attempt to answer this question, West proposed, in a series of publications,[2–4] that the mechanical plus electrochemical ‘‘energy balance’’ criterion for initiation of SCC crack growth is met when the applied mechanical CDF, J, equals or exceeds an environment-affected threshold value of J, J~c : The CDF J can be obtained using Rice’s J-integral analysis method[5] or experimentally using experimental methods first developed by Begley and Landes.[6] In the notation used here, West’s criterion is expressed as J J~c ;
J~c ¼ Jc ð1 þ kÞDcs
½1
where k is a proportionality constant defined by Eq. [4], Jc is the inert-environment value of J and J~c is the environment-affected value of J required to initiate subcritical crack advance of a stationary crack under quasistatic, monotonic loading conditions. Owing to interaction with a corrosive environment, the ideal work of interface separation, cs, is reduced by an amount Dcs. The J-integral is equal to the thermodynamic strain
energy release rate for quasistatic crack advance, both for materials that experience small-scale yielding at the crack tip and linear elastic materials, for which crack growth may be supercritical, once initiated. In 2this limiting linear-elastic case, Jc = K2c /E and J~c ¼ K~c =E, where Kc and K~c are the critical stress intensity factors for initiation of supercritical crack advance in inert and corrosive environments, respectively. Because they are environment-affected, J~c and K~c are in practice often functions of loading rate (crack tip strain rate) as there are competing time- and rate-dependent effects of film disruption, repassivation, and hydrogen diffusion within the fracture process zone (FPZ). Consequently, the experimental value of J~c may be greater than the value as expressed conceptually in Eq. [1]. West used the notation Kscc to designate what is described here as K~c . In the current development, Kscc and Jscc are reserved for describing the thresholds below which SCC crack growth (propagating crack) cannot be sustained in a static load test. These crack growth threshold values may be greater than or less than the stationary crack threshold values K~c and J~c as the crack tip strain rates in the static load SCC test may not match the crack tip strain rates achieved in rising load tests of static cracks. Derivation of Eq. [1], begins with a Griffith[7]–Irwin[8] energy balance type criterion whereby stationary cracks first advance in an inert environment when J equals or exceeds Jc: J Jc ;
M.M. HALL, Jr., Consultant, is with MacRay Consulting, 1366 Hillsdale Drive, Monroeville, PA 15146. Contact e-mail: hallmm63@ comcast.net Manuscript submitted July 13, 2012. Article published online November 27, 2012 1200—VOLUME 44A,
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