Issue 41

T. Vojtek et alii, Frattura ed Integrità Strutturale, 41 (2017) 245-251; DOI: 10.3221/IGF-ESIS.41.33 249 be of 23°. This value must be, of course, verified by the measured linear roughness of these precrack fronts. Nevertheless, it seems to be plausible and sufficiently small to allow the local mode II mechanism operate at real micro-tortuous crack fronts without sharp protrusions. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0 5 10 15 20 25 30 35 40 45 ratio   r cal = k 2 / K III asperity angle   α [°] maximum average experimental ΔKIIeff,th/ΔKIIIeff,th Figure 5 : Dependences of r cal,max and r cal,av on the asperity angle α calculated for the respective points of the maximal value (stress concentration at point 3 in Fig. 3) and of the averaged value of the ratio r cal along the half tooth (points 2 to 8). The horizontal dashed line represents the experimental ratio of effective thresholds Δ K IIeff,th / Δ K IIIeff,th . The model of local mode I/II crack growth was already able to explain the existence of crystallographic facets oriented nearly parallel with the shear stress direction. It was also used to propose the formula for prediction of the intrinsic mode II threshold [15], which was found in a good agreement with experiments for many metallic materials. This study represents an important further step on the way to achieve an appropriate quantitative description of the effective mode III thresholds too. The results obtained by relatively complicated numerical calculations made to determine the local mode II SIFs of the serrated crack front seem to be very promising in terms of finding such a relationship. C ONCLUSION his study represents an important step on the way to quantitatively describe the effective mode III thresholds in metallic materials with the bcc crystal lattice. In these materials, the micromechanism of propagation of mode III cracks was found to be a coplanar (in-plane) spreading of tongues driven by the local mode II loading component at asperities of microtortuous crack fronts. Since the in-plane 2D modelling of the tortuous crack geometry is sufficiently relevant here, the finite element analysis of the local mode II component for a precrack with serrated front loaded in the remote mode III was performed. Comparison of the calculated results with experimentally determined ratio of mode II and III effective thresholds revealed that, for the ARMCO iron as a case study, the characteristic angle α of the asperities (saw teeth) at the precrack front should be of 23°. Although this value must be verified by the measured linear roughness of the precrack fronts in the ARMCO specimens, it seems to be plausible and sufficiently small to allow the local mode II mechanism operate at real micro-tortuous crack fronts without any sharp protrusions. Thus, the result achieved is promising in terms of finding a physically justified formula for mode III effective thresholds in metallic materials with bcc lattice. A CKNOWLEDGEMENTS he authors acknowledge the financial support of this work by the Czech Science Foundation (GA CR) in the frame of the Projects No. 17-15716Y and No. 17-18566S, and by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601). T T