Fatigue threshold-ductile iron.

Abstract

An experimental investigation on the fatigue threshold of ductile iron was carried out. The influence of microstructure of the cast metal and stress ratio ranging between 0.10 and 0.50 was examined. Ductile iron, ASTM A 536 805506, was poured in the laboratory as Yblocks and heattreated to obtain ferrite, pearlite, or a combination of the two in the matrix. Precracked compacttension specimens were tested for threshold fatiguecrack propagation on a 25ton INSTRON servohydraulic testing machine interfaced to a computer, following the ASTM Kdecreasing schedule. Online cracklength and crackclosure stress measurements were performed using a crackmouthopeningdisplacement gauge. Threshold stress intensities-both applied and effective-are highest in pearlitic ductile iron and decrease with reduction in pearlite content in the microstructure. Threshold values of ductile irons, both ascast and heattreated, are comparable to those of steels. Applied threshold stress intensities are higher at low stress ratios (0.10) than at high stress ratios (0.50). The dependence of threshold on stress ratio is significant for pearlitic irons (7 to 14 MPam) and ascast irons (6 to 13.3 MPam). This influence of stress ratio has been attributed to crack closure, which has been demonstrated experimentally. The effective threshold stress intensity of a ductile iron with a particular microstructure is unique over the stressratio range studied. Effective threshold values are not significantly influenced by microstructure (4.7 to 5.4 MPam). Optical and electronmicroscopy studies have contributed to a better understanding of crackgrowth characteristics near the threshold region of ductile iron. The threshold crackgrowth characteristics of ductile iron facilitate damagetolerant design of critical, cyclically loaded components.