EFFECT OF MICROSTRUCTURE ON FATIGUE CRACK-PROPAGATION IN IRON-CARBON ALLOYS

The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of crack growth was ductile, the crack growth rate da/dN could be related to the cyclic stress intensity factor ΔK by an equation of the form da/dN = (ΔK)m where and m are constants. The constant m was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack tip by slip processes. The constant m was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies. The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase. © 1979 American Society for Metals and the Metallurgical Society of AIME.