Effect of microcracking on the micromechanics of fatigue crack growth in austempered ductile iron J. ORTIZ*, A. P. CISILINO and J. L. OTEGUI Welding and Fracture Division—INTEMA, Faculty of Engineering, Universidad Nacional de Mar del Plata—CONICET, Mar del Plata, Argentina Received in final form 27 April 2001 ABSTRACT The effect of microcracking on the mechanics of fatigue crack growth in austempered ductile iron is studied in this paper. The mechanism of fatigue crack growth is modelled using the boundary element method, customized for the accurate evaluation of the interaction effects between cracks and microcracks emanating from graphite nodules. The effects of nodule size and distribution and crack closure are considered, with deviation bounds of computed results estimated through weight-function analyses. A continuum approach is employed as a means of quantifying the shielding effect of microcracking on the dominant propagating crack, due to the reduction of stiffness of the material in the neighbourhood of the crack tip. Although the results obtained may not yield actual numbers for real cases, they are in accordance with experimental observations and demonstrate how the main factors affect the crack growth of the macrocrack. Keywords austempered ductile iron; boundary element method; fatigue crack growth; microcracking; numerical modelling. NOMENCLATURE A c =surface of the process area at macrocrack tip A nod =cross-sectional area of nodules ADI =austempered ductile iron a =length of macrocracks c =length of microcracks emanating from nodules C =constant in fatigue-crack propagation law d =average minimum distance between nodules da/dN =crack propagation rate K m =crack-tip stress intensity factor for microcracks K max , K min , K op =maximum, minimum and opening level of crack-tip stress intensity factor K tip =crack-tip stress intensity factor for macrocracks E =Young’s modulus M=number of microcracks per unit volume m =exponent in fatigue-crack propagation law N area =area nodule count N vol =volume nodule count N =load cycles r =average radius of graphite nodules r c =radius of penny-shaped microcracks S =strain density factor Correspondence: A. P. Cisilino, Welding and Fracture Division— INTEMA, Faculty of Engineering, Universidad Nacional de Mar del Plata—CONICET, Av. Juan B. Justo 4302 (7600), Mar del Plata, Argentina. E-mail: cisilino@fi.mdp.edu.ar *On leave from Universidad Privada del Norte, Trujillo, Peru ´. © 2001 Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 24, 591–605 591