Kinetics of Subcritical Crack Growth and Deformation in a High Strength Steel

Abstract

The kinetics of subcritical crack growth under sustained loading in a chemically inert environment (dehumidified argon) and the companion deformation kinetics were determined to examine the possible relationship between the crack growth and deformation processes in an AISI 4340 steel tempered at 400 deg F (∼205 degC). Crack growth experiments were carried out over a range of temperatures from 20 to 140 deg C, using the crack tip stress intensity factor K to chacterize the mechanical crack driving force. Deformation kinetics were determined as a function of deformed structure either at constant load or by a strain rate cycling procedure over the same range of temperatures. Detectable crack growth (with rates above 10−5 ipm) in dehumidified argon occurred at K levels exceeding about 70 percent of Kc at room temperature and 50 percent of Kc at the higher temperatures. Crack growth exhibited transient, steady-state and tertiary stages of growth, akin to creep, in agreement with the results of Li, et al. Experimental data indicate that subcritical crack growth in dehumidified argon is controlled by thermally activated processes, with apparent activation energies in the range of 11,000 to 18,000 cal/mole. This range of apparent activation energies is in general agreement with an observed range of 12,000 to 28,000 cal/mole for steady-state creep in this material. The apparent activation energies for steady-state creep were found to be dependent on flow stress and structure. Based on the similarity between the observed crack growth and deformation behaviors and on the order of magnitude agreement between the apparent activation energies, it is reasonable to consider that subcritical crack growth in inert environments is controlled by the time dependent deformation processes occurring at the crack tip. A model for relating steady-state crack growth and steady-state creep is suggested, and is shown to correlate well with experimental data.