The fracture surface of stress corrosion cracking of tempered SNCM 8 steel in a 3.5% NaCl solution environment was observed by means of scanning electron microscopy and the X-ray diffraction technique. A quantitative nature of the topography and the residual stress of the fracture surface were found to be more closely related to the crack growth rate than to the stress intensity factor. The relation between the areal fraction φ of prior-austenite grain boundary fracture in the fracture surface and the growth rate da/dt of stress corrosion cracks was obtained as
da/dt=exp(A-Cφ)
where A is a constant dependent on tempering temperature while C is independent. The residual stress σR measured on the fracture surface by the X-ray method was tensile and changed as a function of the areal fraction of grain boundary fracture:
σR=(1-φ)σR
The mechanical field near the tip of a stress corrosion crack with microbranching was modeled by a blunt crack with the equivalent root radius ρeq. The equivalent stress intensity factor given by
Keq=√ρl/ρeqK
was proposed to be a fracture mechanics parameter for characterizing the near-tip field, thus the crack growth rate. The value of ρeq is twice the microbranching width of the crack and ρl is the limiting root radius for a sharp crack. The concept the equivalent stress intensity factor may explain the finding that the fractographic feature is more closely related to the crack growth rate than to the stress intensity factor.
