Nucleation, stress corrosion cracking

As a second example, we show in Fig. 27 the nucleation of intergranular stress corrosion cracks in sensitized Type 304SS as observed by Diercks and Dragel at the Argonne National Laboratory (see [56]). The cracks were found to nucleate from pits or at intergranular penetrations (e.g., intergranular attack, IGA). In other cases, cracks are found to nucleate from MnS inclusion but, in this case too, the primary event is probably the formation of a pit which acts as a stress riser. Examination of many micrographs indicates that the critical nucleus is of the order of 30-1 (X) pm deep. 

The failure time, however, incorporates both the time required for crack initiation and a period of slow crack growth so that the separate effect of the environment on each of these stages cannot be ascertained. (Some of the difficulty stems from the lack of a precise definition for crack initiation.) This difficulty is underscored by the results of Brown and Beachem [1] on SCC of titanium alloys. They showed that certain of the alloys that appeared to be immune to stress corrosion cracking in the traditional (smooth specimen) tests are, in fact, highly susceptible to environment-enhanced crack growth. The apparent immunity was explained by the fact that these alloys were nearly immune to pitting corrosion, which was required for crack nucleation in the same environment [1]. 

Prior to the 1960s, stress corrosion cracking and corrosion fatigue were principally under the purview of corrosion chemists and metallurgists, and the primary emphasis was on the response of materials in aqueous environments (e.g., sea/salt water), particularly for SCC because of the relative ease of experimentation. Much of the attention was devoted to the understanding of electrochemical reactions that are associated with metal dissolution, crack nucleation, and time-to-failure under a... 

Stres -corrosion cracking is quite widely accepted to be the creation and propagation of cracks in an area where there is tensile stress and an environment which causes electro-chemical dissolution of the metal, initially at the nucleating point and subsequently at the crack tip. Stress-corrosion cracking can occur even though the environment alone may not cause general corrosion. 

Stress corrosion cracking (SCC) represents a process of crack nucleation in metals under the simultaneous action of corrosive medium and tensile (external or internal) stresses. A few typical examples of corrosive media for some metals and alloys are given in Table 4.1. 

The stress corrosion cracking of a-titanium alloys occurs by nucleation of hydride (by the interaction of absorbed hydrogen with the metal). 

Other parameters that play an impcatantrole in SCC testing are surface condition and residual stress. The nucleation of stress-corrosion cracks strongly depends... 

Most ceramics (as we have seen) contain flaws holes and cracks left by processing, cracks caused by thermal stress, corrosion or abrasion. Even if there are no cracks to start with, differences in elastic moduli between phases will nucleate cracks on loading. And most of these flaws have a size which is roughly that of the powder particles from which the ceramic was made. If the flaw size can be reduced, or if samples containing abnormally large flaws can be detected and rejected, the mean strength of the ceramic component is increased. 

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