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The slow constant extension rate (CER) test using fatigue precracked fracture specimens offers advantages over the conventional static (fixed displacement and constant load) tests. It generally requires fewer specimens and shorter test times to determine the threshold for crack initiation. However, apparent anomalies in the subcritical crack growth kinetics have been observed, which complicate the interpretation of the CER test measurements. Oscillations in the crack velocity occur resulting in macroscopic intermittent crack growth and periodic fluctuations in the applied load. An analysis of the CER test applied to environment-assisted cracking (EAC) is made and a phenomenological model is proposed to describe the mechanics of macroscopic intermittent crack growth. As a result of the analysis, the oscillations in the crack velocity are found to depend on the elastic behavior of the specimen and loading system and the aggressiveness and activity of the environment at the crack tip.
constant extension rate test, environment-assisted cracking, intermittent or discontinuous crack growth, crack growth resistance curves
NRC Research Associate, Naval Research Laboratory, Washington, D. C.,
Metallurgist, Naval Research Laboratory, Washington, D. C.,