Issue34

J. Pokluda et alii, Frattura ed Integrità Strutturale, 34 (2015) 142-149; DOI: 10.3221/IGF-ESIS.34.15 148 A fulfilment of the deflection (branching) criterion (3) is demonstrated in Tab. 2. The criterion was not fulfilled for the ARMCO iron where no mode I branching was observed (deflection α II ≈ 19°). Medium-sized deflection angles in titanium correspond to approximately equal sides of the criterion. For other materials the criterion was fulfilled and considerable deflections towards local mode I were observed to various levels. For nickel and ferritic-pearlitic steels, both the deflection angles and the mode mixities revealed an almost balanced contribution of local modes I and II. Therefore, values of the mode II threshold calculated using both Eqs. (1) and (2) are displayed in Tab. 1. On the other hand, almost pure local mode I loading was detected in pearlitic and stainless steel. For these materials, consequently, only threshold values calculated according to Eq. (2) are shown in Tab. 1. Material 1.15 Δ K IIeff,th ≥ Δ K Ieff,th deflection α II mode mixity Δ k IIeff /Δ k Ieff ARMCO iron 1.7 < 2.7 19° 1.88 Titanium 2.0 ≈ 2.0 39° 0.71 Ferritic-pearlitic steel 3.1 > 3.0 50° 0.40 Nickel 3.3 > 2.3 52° 0.36 Pearlitic steel 3.1 > 3.0 60° 0.19 Stainless steel 2.9 > 2.3 67° 0.06 Table 2 : Fulfillment of the mode I branching criterion and comparison with the real tendency to mode I crack growth measured on fracture surfaces of the six tested materials in terms of deflection angles α II and local mode mixities according to Eq. (4). S UMMARY ffective thresholds and fracture surface morphology in 3D were measured for mode II and mode III cracks in the ferritic-pearlitic and the pure pearlitic steel. Both the deflection angle and the mode II threshold were much higher in the ferritic-pearlitic and pearlitic steels than in the ARMCO iron. After the local mode I growth was triggered in the pearlitic grain, the crack propagation continued with a large component of mode I also in the rest of the material. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations. It was not fulfilled in materials with low deflection angles from the maximum shear plane (ARMCO iron, titanium) and was fulfilled in materials with a high component of local mode I (nickel, ferritic-pearlitic, pearlitic and austenitic steels). In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. Identification of the effect of microstructure of multiphase materials contributes to the research of shear-mode crack growth micromechanisms. A CKNOWLEDGMENTS he authors acknowledge the financial support of this work by the Czech Science Foundation in the frame of the project No. P108/12/0144 and by the European Regional Development Fund (CEITEC CZ.1.05/1.1.00/02.0068). R EFERENCES [1] Vojtek, T., Pokluda, J., Hohenwarter, A., Pippan, R., Three-dimensional Morphology of Fracture Surfaces Generated by Modes II and III Fatigue Loading in Ferrite and Austenite, Engng Fract Mech, 108 (2013) 285-293. [2] Vojtek, T., Pippan, R., Hohenwarter, A., Holáň, L., Pokluda, J., Near-threshold Propagation of Mode II and Mode III Fatigue Cracks in Ferrite and Austenite, Acta Mater, 61 (2013) 4625-4635. E T