PSI - Issue 33

Sabrina Vantadori et al. / Procedia Structural Integrity 33 (2021) 773–780 S. Vantadori, C. Ronchei, D. Scorza, A. Zanichelli / Structural Integrity Procedia 00 (2021) 000–000

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graphite nodules) dispersed in a metallic matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). Over the last years, the production of DCI mechanical components with structural function has strongly increased. This aspect is mainly due to the interesting combination of mechanical (high tensile strength and toughness, good wear resistance and ductility) and technological (low melting temperature and shrinkage, high fluidity) DCI properties together with relatively low manufacturing costs. Therefore, DCIs are suitable for the production of large castings with complex geometries, such as wind turbine, big engine blocks, parts of hydraulic presses, tractor parts. Regarding large casts (about tens of tons), the DCI final microstructure cannot be efficiently controlled and, consequently, material intrinsic defects cannot be completely avoided (Benedetti et al. (2017)). Among the casting defects, we can recall the degenerated graphite, poor graphite nodularity, large micro-shrinkage porosities, non metallic inclusions, solidification cavities and undesired segregation (Di Cocco and Iacoviello (2017)). As is well-known, these material intrinsic defects not only negatively affect the static mechanical properties (e.g. the ultimate tensile strength and the elongation at failure) but also are preferential crack initiation sites under cyclic loading (Borsato et al. (2018)). The influence of the above defects and, more in general, of the DCI microstructure on fatigue strength is a research topic worth of investigation and still open (see, for instance, the interesting experimental work by Bellini et al. (2019)). Since DCI components with structural functions are frequently subjected to cyclic loading histories, the present paper aims to theoretically estimate both the fatigue strength and lifetime of DCI specimens subjected to uniaxial (tension and torsion) and biaxial (combined tension-torsion) loading in high/medium-cycle fatigue regime. To such an aim, a critical plane-based multiaxial fatigue criterion (Vantadori et al. (2020a)) formulated in terms of stresses is hereafter applied to several experimental fatigue data available in the technical literature (Cengiz, (2012), Tovo et al. (2014)). Note that the above data are related to both infinite life tests (characterised by a number of loading cycles to failure between 1 and 100 million) and finite life tests performed on smooth specimens made of commercial DCIs. Then the theoretical results are compared with the experimental ones and, for finite life fatigue tests, with those derived through the Crossland criterion (Suresh (2008)).

Nomenclature 1 2 ˆ , ˆ , 3 ˆ averaged principal stress directions C

shear stress component on the critical plane slope of the S-N curve under fully reversed normal stress slope of the S-N curve under fully reversed shear stress normal stress component perpendicular to the critical plane

m * m

N

cal N calculated number of loading cycles to failure e xp N experimental number of loading cycles to failure 0 N reference number of loading cycles under fully reversed normal stress 0 * N reference number of loading cycles under fully reversed shear stress R loading ratio w S stress vector related to the critical plane uvw local reference system on the critical plane w normal to the critical plane XYZ fixed reference system  phase shift between tensile and torsional loading 1 af ,   fatigue strength for fully reversed normal stress eq,a  equivalent uniaxial stress amplitude u  material ultimate tensile strength 1 af ,   fatigue strength for fully reversed shear stress