Application of Rate Sensitivity Random Process Statistics to the of Plane-Strain Fracture Toughness

THE usefulness of the K c and Kic indices of fracture toughness in evaluating high-strength alloys is now generally recognized. Since there were problems with the high-speed alternator rotors of the late 1950's, ~'2 there has been a persistent effort to extend the "Fracture Mechanic" measurements of KIc to medium- and low-strength steels. The rotors were so large that safety by virtue of conventional fracture mode transition temperature could not be assured. A decade later, similar questions are posed with regard to the construction of very thick-walled pressure vessels for nuclear power plants, this time fortunately without the precedence of catastrophic fracture failures, s Such considerations have motivated Kic testing of heavy sections near ambient temperatures.~'2 '4's Most available data, however, are for smaller, less costly specimens embrittled by low testing temperatures, e.g., Refs. 6 through 9. In all, some two dozen different low- and medium-strength steels have been studied. It is fair to say that in most cases the scatter of the Kic data has been far greater than desired, often an order of magnitude greater than expected from experience with high-strength alloys. Variations in KIc as large as a factor of two are observed between ostensibly valid, replicate measurements. It seems unlikely that sources of variation such as inhomogeneity of the material or differences in testing methodology could be responsible for the fluctuations in observed fracture strength. This uncertainty is naturally a source of concern in evahiating the fracture safety of thick mild steel structures. One possible explanation of the scatter emerged from a 1965 study by Krafft s of a gas transmission line pipe steel. It suggested that the Kic value is dependent in an extremely sensitive yet reproducible way upon the crack loading rate, dKI/dt or/~. This contention was based on data at temperatures far below those of ordinary usage. Subsequent work by Crosley and Ripling 9 on larger specimens of an A302-B pressure vessel steel permitted the loading rate sensitivity of Kic to be examined around room temperature. Analysis of these results indicated persistence of the same pattern and degree of Kic rate sensitivity as observed in the line pipe steel at low temperature. To investigate the generality of this effect, a third set of speed scanning