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Experiences with stress corrosion cracking (SCC) test methods based on linear elastic fracture mechanics principles suggest that factors such as loading procedures, hold times, overloads during fatigue precracking, and electrochemical potential variances may substantially influence test results. However, the relative importance of each factor on SCC behavior has not been determined unequivocally and much remains to be explained.
Initial experiments to determine the baseline SCC properties (in terms of KIscc) of the two HY-130 steel plates used in these studies on the effect of cyclic load or potential changes on SCC behavior showed that a thickness-independent KIscc is obtainable from a specimen with a minimum 25 mm thickness for both the freely corroding (FC) and zinc-coupled (Zn) conditions. The step-loading procedure appears to be adequate for the determination of acceptable KIscc values. The KIscc values were used to guide the selection of load and potential in subsequent experiments.
Experiments on the effects of two types of cyclic loading, tension-compression and tension-zero, on SCC behavior of precracked specimens were conducted utilizing the cantilever test method. The period of load change was about 100 h and the waveform was approximately square. Cycling per se does not appear to influence the so-called incubation time for SCC initiation under tension-zero cycling; and for this loading mode a characteristic delay time of approximately 100 to 300 h elapsed prior to detecting the first indication of stable crack growth. In contrast, tension-compression load cycling caused a several-fold increase in the time required to initiate SCC on maintaining the load after several tension-compression cycles. The observations suggest that if the tension-compression cyclic loading pattern were sustained, initiation of crack growth could have been deferred for several thousand hours, if not indefinitely.
Studies on the kinetics of initiation or arrest of SCC show that these processes respond more rapidly to load changes than potential changes. The observed effects associated with changes in potential on crack growth response are suggestive of a hydrogen-influenced mechanism. These studies also showed that long hold times (10 000 h) will not initiate or reinitiate SCC if the applied KI is below the threshold (KIscc) for the material and environmental condition. No evidence of different threshold values associated with arrest and initiation of SCC was noted.
stress corrosion, hydrogen, HY-130 steel, cantilever test, overload, initiation and arrest
Naval Research Laboratory, Washington, D.C.