Influence of Precracked Specimen Configuration and Starting Stress Intensity on the Stress Corrosion Cracking of 4340 Steel

Abstract

The results of a Langley Research Center (LaRC) investigation on the effects of precracked specimen configuration and initial starting stress intensity on crack growth rate and threshold stress intensity for onset of cracking and for crack arrest are presented. The study was part of an interlaboratory (round robin) test program coordinated by ASTM Committees G-1 and E-24 to evaluate the usefulness of precracked specimens and analysis for stress corrosion characterization. The objectives of the LaRC effort were to determine if crack growth rates determined at various levels of stress intensity were independent of specimen configuration and initial starting stress intensity, and whether the threshold levels for onset and arrest were comparable. Tests were conducted on AISI 4340 steel (heat treated to a yield strength of 1240 MPa) in 3.5% sodium chloride (NaCl) solution using two specimen configurations: a single-edge-cracked specimen tested in cantilever bending (SE(B)) under constant load, and a modified compact specimen bolt loaded (MC(Wb)) to a constant deflection. Initial starting stress intensities varied from 40 to 55 MPa m1/2. The results indicate that for this material and environmental system the threshold stress intensity value determined was independent of the specimen configuration if the stress intensity value associated with the MC(Wb) specimen is taken where the discontinuous break occurs in the velocity-stress intensity curve. The threshold value determined at this discontinuous break for the MC(Wb) specimens agreed well with the lowest stress intensity at which onset of crack growth occurred for the SE(B) specimens. The crack growth rates determined for the two specimen configurations were consistent, although they could not be directly compared because of differences in the stress intensity range over which the tests were conducted.