Issue 48

C. Santus, Frattura ed Integrità Strutturale, 48 (2019) 442-450; DOI: 10.3221/IGF-ESIS.48.42 442 Focussed on “Crack Paths” Initial orientation of the fretting fatigue cracks in shrink-fit connection specimens Ciro Santus DICI – Department of Civil and Industrial Engineering, University of Pisa, Italy ciro.santus@ing.unipi.it, http://orcid.org/0000-0003-0859-626X A BSTRACT . The bending configuration of shrink-fitted connection fretting fatigue tests causes a fluctuation in the contact pressure and in turn a more favourable condition for shear, rather than tensile, crack initiation. According to the classification type I (shear) and type II (tensile) fretting cracks, type II was only observed in the tests with no surface enhancement, which experienced the lowest fretting strength and thus a reduced pressure fluctuation effect. The crack initial direction was predicted as the critical plane orientation undergoing the maximum Fatemi-Socie (FS) stress parameter, for the shallow type I shear cracks. A significant result is that if the critical plane is searched at the fretting hot-spot, an incorrect outward direction is obtained, since the orthogonal direction, with the same shear stress amplitude, experiences a more compressive maximum normal stress. Conversely, with the application of the Point Method, taking the stress location at half the critical distance and by following the potential critical plane, a higher shear stress along the inward direction is obtained. The more compressive stress below the contact can therefore be compensated, and consequently the inward shallow direction is correctly predicted. K EYWORDS . Fretting Fatigue; Initial crack orientation; Multiaxial fatigue criteria; SEM observations. Citation: Santus, C., Initial orientation of the fretting fatigue cracks in shrink-fit connection specimens, Frattura ed Integrità Strutturale, 48 (2019) 442-450. Received: 28.11.2018 Accepted: 28.01.2019 Published: 01.04.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION retting fatigue damage is experienced by those elements forced to stay in contact with a fluctuating loading, such as blade dovetail connections of compressor rotors, riveted joints, and shrink-fitted shafts, as summarized by Baietto- Dubourg and Lindley [1]. Fretting fatigue can be found at the edge of the contact, where the high contact pressure induces shear traction superimposed on a longitudinal cyclic (bulk) stress on a region which also experiences partial sliding. This loading condition implies complex multiaxial fatigue, usually with inaccurate knowledge of the effective coefficient of friction. Some local micro-wear can also be observed, though the contact interface is usually assumed to be undamaged, i.e. the nominal geometry. F