FRACTOGRAPHIC ANALYSIS OF FATIGUE CRACKING IN SPHEROIDAL GRAPHITE CAST IRONS

The present paper attempts to describe a fairly intense fractographic analysis which was performed on the fatigue fracture surfaces of two spheroidal graphite cast iron microstructures subjected to fatigue loading at various R-ratio values in an ambient air environment. A significant amount of intergranular failure was evident in both microstructures at low to intermediate DELTA-K levels; this failure mode attained maximas at specific DELTA-K levels which correspond to the condition when the reversed plastic zone size approached the structural parameter (grain) size. The extent or amount of intergranular failure was much higher in the ferritic microstructure than in the predominantly pearlitic microstructure. The maximum extent of intergranular failure in both microstructures was observed at lower stress intensity range, DELTA-K levels, and faster crack velocity values than those recorded for steels. These were the result of the presence of graphite nodules which both enhanced intergranular failure and aided the diffusion of hydrogen through the microstructures. Decohesion between the graphite nodules or spheroids and the matrix was commonplace and a higher number of graphite spheroids (up to about three times) were present on the fatigue fracture surfaces of both microstructures than those observed on a metallographic section. This was the result of an increase in the crack tip stress intensity as the extending fatigue crack approached a "soft" graphite nodule. Two other general fatigue fracture features were commonly observed in both microstructures. They correspond to regions of coarse planar slip, with its characteristic crystallographic appearance, that occurred at low to intermediate DELTA-K levels, and areas or facets of flat, cleavage failure within which regular crack arrest markings were clearly imprinted. In the case of the latter, the arrest markings had an average spacing of 2 to 3-mu-m and were insensitive to DELTA-K level and it was suggested that such features were the result of periodic relaxation of the growing cleavage crack causing plastic deformation lines; this type of cleavage crack growth was the result of the Fridel-Orlov mechanism.