A Fractographic Study of Gray Cast Iron Fatigue Specimens

Tuesday, September 14, 2021: 8:20 AM
240 (America's Center)
Mr. Daniel W. Brinkley, PE , Accident Reconstruction Analysis, Raleigh, NC
Prof. Harvey West, PE , NC State University, Raleigh, NC
Tensile and fatigue specimens were machined from a large gray cast iron casting having a pearlitic microstructure. The fatigue specimens were broken using a combination of high stress, low cycle and low stress, high cycle fatigue conditions under cyclic axial tension. The obtained fatigue strength data was in agreement with previously published data. The specimen fractures were examined with a scanning electron microscope using both secondary electron images and backscattered electron images. The obtained images confirmed the “debonding” theory proposed by others showing a large portion of the fracture involved separation between the graphite and the mating as-solidified metal surfaces. This debonding produces fracture on multiple planes resulting in a rough textured fracture whether the fracture was produced by tensile overload or by fatigue. There was essentially no difference between fracture features of the tensile specimens and the high stress, low cycle fatigue specimens. Possible striation features were observed on the low stress, high cycle fatigue specimen, but the spacing of these striations were comparable to the spacing of the pearlite lamella.