Issue 38

M. de Freitas et alii, Frattura ed Integrità Strutturale, 38 (2016) 121-127; DOI: 10.3221/IGF-ESIS.38.16 121 Focussed on Multiaxial Fatigue and Fracture Comparison between SSF and Critical-Plane models to predict fatigue lives under multiaxial proportional load histories Manuel de Freitas, Luis Reis IDMEC, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal mfreitas@dem.ist.utl.pt , luis.g.reis@ist.utl.pt Marco Antonio Meggiolaro, Jaime Tupiassú Pinho de Castro Pontifical Catholic University of Rio de Janeiro, PUC-Rio, R. Marquês de São Vicente 225, Rio de Janeiro, 22451-900, Brazil meggi@puc-rio.br , jtcastro@puc-rio.br A BSTRACT . Materials can be classified as shear or tensile sensitive, depending on the main fatigue microcrack initiation process under multiaxial loadings. The nature of the initiating microcrack can be evaluated from a stress scale factor (SSF), which usually multiplies the hydrostatic or the normal stress term from the adopted multiaxial fatigue damage parameter. Low SSF values are associated with a shear-sensitive material, while a large SSF indicates that a tensile-based multiaxial fatigue damage model should be used instead. For tension-torsion histories, a recent published approach combines the shear and normal stress amplitudes using a SSF polynomial function that depends on the stress amplitude ratio (SAR) between the shear and the normal components. Alternatively, critical-plane models calculate damage on the plane where damage is maximized, adopting a SSF value that is assumed constant for a given material, sometimes varying with the fatigue life (in cycles), but not with the SAR, the stress amplitude level, or the loading path shape. In this work, in-phase proportional tension-torsion tests in 42CrMo4 steel specimens for several values of the SAR are presented. The SSF approach is then compared with critical-plane models, based on their predicted fatigue lives and the observed values for these tension-torsion histories. K EYWORDS . Multiaxial fatigue life prediction; Critical-plane approach; Polynomial stress scale factor approach. Citation: de Freitas, M., Reis, L., Meggiolaro, M. A., de Castro, J.T.P., Comparison between SSF and Critical-Plane models to predict fatigue lives under multiaxial proportional load histories, Frattura ed Integrità Strutturale, 38 (2016) 121-127. Received: 11.05.2016 Accepted: 10.06.2016 Published: 01.10.2016 Copyright: © 2016 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 nitiating microcracks under multiaxial fatigue loadings can be sub-divided into shear or tensile types [1]. The dominant fatigue mechanism in so-called shear-sensitive materials is Mode II microcrack nucleation in shear, so the cracks initiate in directions that maximize the ranges of the shear components, with the normal components only I