A unified fracture kinetics representation of the three regions of stress corrosion cracking

Abstract

Environmentally assisted crack propagation (or stress corrosion cracking, SCC) is a recurring problem in many aspects of engineering practice -witness the large number of publications appearing in both scientific and engineering journals.Numerous authors agree that SCC is controlled by thermal activation, e.g., [1][2][3][4][5][6], and it is recognized that the process is very complex indeed. The experimental results are often presentedinthe log velocity stress intensity factor KI, or log velocity-crack driving force G_ coordinate system: Fig. i is the sche-I matic representation of the three regions and the threshold zone that is always obtained in SCC experiments for all materials, in any environment and at any temperature.Attempts to describe the processthat is, to derive a mathematical description of the processes -have been made for some time; as a result, a great deal of information has become available.In Regions I and II corrosion reaction takes place in the crack tip zone, followed by the bond breaking steps. The rate of the crack propagation depends on the stress-enhanced chemical reaction, but the two regions are controlled by different mechanisms.In its simplest form, the corrosion reaction occurs in two consecutive steps: step one is the flow of the corrosive material to the crack tip, and step two is the chemical reaction between the reactant and the crack tip zone matrix.Since the process is consecutive, the overall rate is determined by the slower of the two steps.In Region I the rate of the chemical reaction determines the crack velocity; in Region II it is the rate of the reactant transport to the crack tip, because at intermediate stresses the rate of the corrosion reaction increases [2]. Region III is a parallel process [3,[5][6][7], and consequently the fastest mechanism controls the overall velocity; here the stresses are very high, near the critical stress, and the bonds break before any corrosion reaction can take place.Following Charles and Hilling [8,9] it was shown by Wiederhorn [2] that, at least in glass-water environment, the crack propagation velocity associated with the corrosion process can be described by an equation of the type v~ exp(_ AG~=-°AV + YVm/~ ) kT where v is the crack velocity, AG ~ is the bond strength in the corroded zone, a is the applied stress, AV is the activation volume, V is'the molar volume of the glass, and 0 is the radius of curvature o~ the crack tip. Further refinements of this equation were discussed by Wiederhorn [2], and detailed analysis of the kinetics of these regions were developed by Brown [3]. The kinetics analysis has led to the following general equation for the crack velocity Int Journ of Fracture 23 (1983)