Fatigue Crack Propagation in SGI and CGI

Fatigue properties of spheroidal graphite cast iron (SGI) and compacted graphite cast Iron (CGI) were studied in this research. Specimens in form of strips (94x9x3.8 mm) were subjected to alternating bending fatigue tests. S-N curves and fatigue limits for both materials were determined. From the crack growth rate vs. average crack length curves, two behaviors were observed, namely short crack growth rate and long crack growth rate. Mathematical models were suggested describing both types of cracks, thus enabling the calculation of fatigue life of SGI and CGI. From da/dN versus stress intensity variation experiments, the Paris equation da/dN = C (ΔK)P was derived. The smaller the value of p the better is the crack growth resistance of the material. For SGI, p = 1.1855 and for CGI, p = 1.2434 indicating inferior crack growth resistance of the CGI.