numero25

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108 ; DOI: 10.3221/IGF-ESIS.25.15 102 Special Issue: Characterization of Crack Tip Stress Field Pearlitic ductile cast iron: damaging micromechanisms at crack tip F. Iacoviello, V. Di Cocco, A. Rossi Università di Cassino e del Lazio Meridionale, DiCeM, via G. Di Biasio 43, 03043 Cassino (FR), Italy iacoviello@unicas.it M. Cavallini Università di Roma “La Sapienza”, DICMA, via Eudossiana 18, Rome, Italy A BSTRACT . Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steels with analogous performances are the main DCIs advantages. In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests (“in situ” tests) and identifying different damaging micromechanisms. In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (by means of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, and considering loading conditions and damaging micromechanisms, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role. K EYWORDS . Ductile cast irons (DCIs); Fatigue crack propagation; Graphite nodules; Damaging micromechanisms. I NTRODUCTION uctile cast irons (DCIs) have been relatively recently developed and they are characterized by the presence of free graphite with a nodule shape (instead of lamellae as in grey cast iron): this allows to combine the more peculiar cast iron property (castability) with mechanical properties that are similar to those of carbon steels [1] (first of all, toughness). DCIs are used in the form of ductile iron pipes (for transportation of raw and tap water, sewage, slurries and process chemicals), in safety related components for automotive applications (gears, bushings, suspension, brakes, steering, crankshafts) and in critical applications as containers for storage and transportation of nuclear wastes [1- 2]. Matrix controls mechanical properties and matrix names are used to designate spheroidal cast iron types. Many different DCIs grades are commercially available. Among them, ferritic-pearlitic DCIs offer a wide range of mechanical properties, with ferritic grades that are characterized by good ductility and a tensile strength (more or less equivalent to a low carbon steel), pearlitic DCIs that show higher strength values, good wear resistance and moderate ductility and, finally, ferritic–pearlitic grades properties that are intermediate between ferritic and pearlitic ones, at least considering tensile strength (Fig. 1). In fact, considering the fatigue crack propagation resistance (Fig. 2), the ferritic-pearlitic DCI seems to be characterized by the best behaviour, at least for higher  K and R values. D