Near-Threshold Propagation of Mode II and Mode III Cracks L. Holáň 1 , R. Pippan 2 , J. Pokluda 1 , J. Horníková 1 , A. Hohenwarter 2 , K. Slámečka 1 1 Brno University of Technology, Faculty of Mechanical Engineering, Technická 2, 616 69 Brno, Czech Republic, y101840@stud.fme.vutbr.cz 2 Erich Schmid Institute of Material Science, Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria ABSTRACT. Two prototype experiments allowing a simultaneous mode II and mode III fatigue crack propagation in a single specimen are described and the differences in related to growth are discussed. The cylindrical specimens made of austenitic and ferritic steels with circumferential V-notch were prepared. The specially manufactured loading setups enabled to assure a pure remote shear mode II at both the top and the bottom sites of the specimen, whereas a pure mode III operated at front and back sites. Differences between the mechanisms of crack propagation were assessed by means of the 3D fractographical analysis based on the stereophotogrammetry in SEM. The stress intensity factors K II and K III were determined by a numerical method based on the ANSYS code and compared with asymptotically computed values of K III . The threshold values ΔK IIth (R = 0.1) were found to be of 1.2 MPam 1/2 in ferrite and of 3.5 MPam 1/2 in austenite, whereas ΔK IIIth ≈ 2.0 .in ferrite and ΔK IIIth ≈ 4.7 MPam 1/2 in austenite. In both steels, the near-threshold crack growth rate under the mode II was found to be much higher than that under the mode III. INTRODUCTION While the principal micromechanisms of fatigue crack growth under modes I and II are well known, there is a lack of any plausible interpretation in case of a pure mode III crack propagation [1]. Most of experiments allowing mode II and mode III crack propagation were performed in a pure torsion or asymmetrical 4 point bending (e.g. [2- 8]). Except for the paper by Nayeb-Hashemi et al. [4], no detailed examinations of the micromechanism of shear mode crack growth were reported in these studies. Therefore, the investigation of shear crack growth mechanisms constitutes a rather big challenge. A careful fractographical observation of mode II and III growth under pure shear remote loading in the region of very low cycle fatigue were performed by Pokluda et al. [9]. These investigations indicated that the microscopic mode of the pure remote mode II crack extension was rather a mixed I+II mechanism. Similarly, the mode II and the combined mode I+II were dominating microscopical fracture micromechanisms also during the remote mode III loading. The crack growth rate in mode II was found to be about two times higher than that in mode III. As it is shown hereafter, however, the real ratio of the crack tip opening displacements is CTOD II /CTOD III ≈ 0.5 while the ratio CTOD II /CTOD III ≈ 1 was presumed in that paper. Therefore, the mode II crack growth rate was, in fact, more than five times higher than the mode III one. 585