Issue 41

M. Vormwald et alii, Frattura ed Integrità Strutturale, 41 (2017) 314-322; DOI: 10.3221/IGF-ESIS.41.42 314 Focused on Crack Tip Fields Variable mode-mixity during fatigue cycles – crack tip parameters determined from displacement fields measured by digital image correlation Michael Vormwald, Yigiter Hos Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Str. 3, D-64287 Darmstadt, Germany vormwald@wm.tu-darmstadt.de , hos@wm.tu-darmstadt.de José L.F. Freire, Giancarlo L.G. Gonzáles, Jorge G. Díaz Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rua Marquês de São Vicente, 225, Gávea - Rio de Janeiro, Brasil jlfreire@puc-rio.br, gonzalesglg@aaa.puc-rio.br , jorgegdiaz@aluno.puc-rio.br A BSTRACT . This paper focusses on discussing equivalent stress intensity factors and kink angles after a change of mode-mixity from one cycle to the next and when the mode-mixity changes continuously during the fatigue cycles. Thin-walled tubes with through-wall cracks have been loaded by proportional and non-proportional tension and torsion. In the experimental investigation, the region of fatigue crack growth was observed by applying the digital image correlation technique. Data on the variations of the displacement and strain fields during the cycles were acquired and used to determine mixed-mode variations of stress intensity factors associated with opening modes I, II and III. For each specific specimen the crack path was observed in order to relate its curvature – both kinks and continuously developing warped cracks – with the variations of the displacement field and associated stress intensity factors. K EYWORDS . Mixed-mode; Fatigue crack growth; Digital image correlation; Crack kinking. Citation: Vormwald, M., Hos, Y., Freire, J. L. F., Gonzáles, G.L.G., Díaz J. G., Variable mode-mixity during fatigue cycles – crack tip parameters determined from displacement fields measured by digital image correlation, Frattura ed Integrità Strutturale, 41 (2017) 314-322. Received: 28.02.2017 Accepted: 03.05.2017 Published: 01.07.2017 Copyright: © 2017 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 horough reviews on crack-path evolution rules and on fatigue crack-growth under non-proportional mixed-mode loading were presented by Mróz et al. [1] and Zerres et al. [2]. Schöllmann et al. [3], Richard and co-workers [4-6] and Yang et al. [7] presented, discussed criteria for the prediction of crack growth, and summarized findings for 2D and 3D components subjected to multiaxial loading. Analysis of the literature review revealed that it is still an open question the determination of equivalent stress intensity factors, SIF, as well as crack growth rules and the prediction of crack path tangent for complex opening-mode problems, mainly if opening mode III is present. The influence of several variables such as: structural component geometry and material, load mode-mixity, presence of all three opening modes, nominal load range ratio R, local crack tip effects including plasticity and 3D corner effects [8], crack flank roughness, crack closure and crack blunting, make the understanding of the fatigue crack growth an extraordinary problem. A T