Crack growth behavior of 4340 steel under corrosion and corrosion fatigue conditions

Crack initiation and growth behavior in 4340 steel under static and cyclic loads in various environments are examined using the two-parameter approach developed by the authors. It is shown that environmental effects can be time dependent, stress dependent, or both, depending on the amplitude, peak stress, and aggressiveness of the environment. It is shown that the tests under vacuum are required to establish the reference state for the quantification of the chemical driving forces involved in crack initiation and growth in aggressive environments. Failure diagrams are proposed by extending the Kitagawa-Takahashi diagram for fatigue. It is shown that these can be used to quantify the mechanical and chemical internal stresses required to cause both initiation and growth of an incipient crack in a smooth as well as notched specimens. Conditions for propagating and nonpropagating incipient cracks that are initiated under stress concentrations are also discussed, taking the example of the 4340 steel data available.