PSI - Issue 28

Sabrina Vantadori et al. / Procedia Structural Integrity 28 (2020) 1055–1061 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The matrix microstructure can be ferritic, pearlitic, ferritic-pearlitic, martensitic,ausferritic etc., depending on both chemical composition modifications and different heat treatments (Iacoviello et al. (2019), Fernandino et al. (2020)). The graphite particles may nucleate on small inclusions during the solidification, where the nodularization process is achieved due to the nodularising effect of some elements such as magnesium, calcium, cerium and others from the rare earth group. The most commonly used nodularising element is the magnesium, generally used in the form of alloy with other components (Fe-Si-Mg alloys) (Vincente (2019)). The properties of a DCI notably depends on the matrix microstructure, alloying elements and the peculiarities of the graphite elements (nodularity, graphite volume fraction, element distribution etc.). DCI structural components are employed in several applications, as structural material in engines and suspension components in automotive industry, wind turbines, canisters for terminal storage of nuclear waste, and in other structural and non-structural applications (Di Cocco et al. (2017)). This wide use is expected to expand at high rate next years. In such applications, the increasing use of ductile cast iron is justified by the good combination of mechanical properties (such as high tensile strength, good wear resistance and ductility) and technological properties (low melting temperature and shrinkage, high fluidity). Another advantage, not negligible, is the low production cost. Note that, even in components casted under optimised conditions, the designer has to take into account the presence of metallurgical defects, such as shrinkage, cavities, porosity, slag inclusions, degenerate graphite. Under cyclic loading condition, such defects tend to behave as cracks. It was experimentally observed that, although the fatigue strength of DCI depends on several parameters (i.e. graphite shape, size and distribution, matrix microstructure, defects distribution), its endurance is mainly controlled by the growth of fatigue cracks from casting defects. Since multiaxial fatigue is the stress condition frequently occurring in real applications, this paper focuses on fatigue behaviour of DCI specimens under such stress conditions, according to a total life approach. More precisely, the goal of the present paper is to estimate lifetime of ductile cast iron smooth specimens under multiaxial fatigue loading by using the multiaxial critical plane-based criterion proposed by Carpinteri and others (Carpinteri et al. (2017) and Vantadori et al. (2020)). The specimens examined were extracted from a very large structural component, consisting in a large crossbar of a hydraulic press for ceramic tiles (Tovo et al. (2014)).

Nomenclature a C

amplitude of the shear stress component , a m N N amplitude and mean value of the normal stress component , eq a N equivalent normal stress amplitude 0 N reference number of loading cycles m slope of S-N curve under fully reversed normal stress * m slope of S-N curve under fully reversed shear stress t time instant T period of the cyclic loading cal N calculated number of loading cycles to failure exp N experimental number of loading cycles to failure W weight function



off-angle defining the critical plane

, ,    principal Euler angles ˆ , ˆ , ˆ    averaged values of principal Euler angles , 1 af   fully-reversed normal stress fatigue strength u  ultimate tensile strength