Issue 41

T. Vojtek et alii, Frattura ed Integrità Strutturale, 41 (2017) 245-251; DOI: 10.3221/IGF-ESIS.41.33 245 Focused on Crack Tip Fields On the connection between mode II and mode III effective thresholds in metals Tomáš Vojtek, Stanislav Žák, Jaroslav Pokluda Central European Institute of Technology (CEITEC), Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech Republic tomas.vojtek@ceitec.vutbr.cz , stanislav.zak@ceitec.vutbr.cz , pokluda@fme.vutbr.cz A BSTRACT . Closure-free long cracks under the remote mode III loading grow in a more complicated way than those under the remote mode II. For bcc metals, a coplanar in-plane spreading of tongues driven by the local mode II loading components at crack-front asperities prevails while twisting of crack-front segments to mode I, often leading to factory-roof morphology, is typical for other materials. In bcc metals, therefore, the formulation of a quantitative relationship connecting effective thresholds in modes II and III demands to calculate the local mode II components of stress intensity factors at typical asperities of a crack front loaded in the remote mode III. Therefore, a numerical model of a serrated crack front was created and the results were c ompared with experimentally determined ratio of mode II and III effective thresholds for the ARMCO iron. Although the calculated crack-front roughness needs an experimental verification, the preliminary results indicate that the model can provide a quantitative explanation of the experimentally observed ratio of mode II and mode III effective thresholds in bcc metals. K EYWORDS . Modes II and III; Effective threshold; Micromechanism; Finite element method; ARMCO iron. Citation: Vojtek, T., Žák, S., Pokluda, J., On the connection between mode II and mode III effective thresholds in metals, Frattura ed Integrità Strutturale, 40 (2017) 245-251. 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 espite the applied (remote) shear-mode II, III or II+III loading, the fronts of long cracks in metallic materials are, particularly in the small-scale yielding case, always loaded in a local mixed mode I+II+III due to their 3D microscopic tortuosity and frictionally induced mode I (e.g. [1], [2]). The contact of asperities in the crack wake (friction stress) induces a rather high crack tip shielding level which, as a rule, makes the extrinsic component of the resistance to the crack growth higher than the intrinsic one [3]. After a certain crack extension, such cracks start to deflect from the plane of the maximum shear stress to reduce the extrinsic resistance (the friction stress) by increasing the mode I loading component [4 – 9]. Consequently, the shear-mode cracks usually rather quickly become opening-mode cracks and the investigation of shear-mode crack-growth mechanisms and the related intrinsic resistance is very difficult. However, one can still experimentally uncover the crack growth mechanisms and the intrinsic resistance (effective thresholds) for D