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This paper does not attempt to review all aspects of environment-sensitive fracture testing, but rather is concerned specifically with continuing problems in relation to such tests and with relatively recent developments. The importance of conducting tests in environments related to possible service situations cannot be overemphasized, especially since, in relation to alloy selection, it is well established that alloy additions that provide good resistance in one environment may not do so in another. Similarly, in view of the importance of surface state in corrosion reactions, it is surprising how often laboratory tests involve surfaces that are not representative of those that may obtain in service, with the result that the free corrosion potentials in laboratory tests may differ markedly from those in service, with possibly serious consequences for cracking susceptibility. The problem is partially overcome by the use of potentiostatic control in laboratory tests, but in some, although not all, conditions of exposure composition and potential changes within precracks, crevices, or pits can influence results, and the crack tip potential may not equate to that which is measured at the surface where the crack emerges.
The importance of attempting to simulate service conditions in laboratory tests in relation to the chemistry of the environment and the potential is also the case in relation to stressing conditions, hence the significance of tests involving precracked specimens, the application of slow strain rates, and low-frequency cyclic loading. The last decade or so has seen the widespread introduction of these stressing modes, but there remain queries concerned with such tests, not least in relation to whether they are so demanding that materials which would behave satisfactorily in some service applications are disallowed. Also in the context of such tests there remains for rationalization methods for expressing results. The strain rate dependence of environment-sensitive fracture probably relates to the time dependence of corrosion-related reactions, and it may be worth considering that the parameters currently used for defining thresholds would be more realistically related to stressing rate or the rate of change of the threshold stress intensity factor.
stress corrosion cracking, corrosion fatigue, environment-sensitive fracture, alloy selection, environment composition, electrode potential, surface condition, strain rate effects, precracked specimens, cyclic loading
Professor of Metallurgy, The University of Newcastle upon Tyne, Newcastle upon Tyne,