Issue34

J. Pokluda et alii, Frattura ed Integrità Strutturale, 34 (2015) 142-149; DOI: 10.3221/IGF-ESIS.34.15 144 Eq. (2). It was suggested that in this material a different crack growth mechanism was predominant [2]. In the model of this mechanism the crack propagates by absorption of antishielding dislocations by the crack tip and when dislocations come very near to the crack tip (a few lattice spacings) a decohesion occurs. This mechanism is efficient only when the crack kinks to the opening mode. The shear mechanism (emission of dislocations) is hard to realize here which results in the dominance of the decohesion mechanism. This article focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. Moreover, a simple criterion for mode I deflection from the mode II crack-tip loading will also be discussed with respect to the fractography observations [3]. A linear elastic fracture mechanics condition for mode I deflection is [e. g. 4]: Δ k I = 1.15 Δ K IIeff,th ≥ Δ K Ieff,th , (3) where Δ k I is the local stress intensity factor for an elementary mode I branch. Evaluation of this criterion is in a good agreement with the observed deflection angles α II and the related mode mixities, summarized in Tab. 2. Mode mixities can be calculated according to Eq. (4) using local stress intensity factors Δ k I and Δ k II [3]: IIeff II Ieff II 3cos 1 3sin k k       (4) E XPERIMENTS xperiments were done on ferritic-pearlitc steel (ISO C60E4, 0.45 w% C) and a pearlitic steel for rails (R260) using three different specimens: the CTS specimens for pure mode II loading, the cylindrical specimens with circumferential crack loaded in torsion for pure mode III and cylindrical specimens with circumferential crack loaded in a special device by pure shear for a combined loading of modes II, III and II + III. A more detailed description of the specimens can be found in [2] and the microstructure of both investigated steels is depicted in Fig. 1. Precracks were made at the notch roots of the specimens by cyclic compressive loading. This procedure eliminates extrinsic shielding (friction and residual stress) at the beginning of the shear-mode experiments and enables measurements of effective values of the stress intensity factor range at the threshold for pure modes II and III loading [2, 3]. After the experiments the specimens were fractured in mode I. Threshold values were determined directly from the dependence of crack growth rate on stress intensity factor range. Measured angles [°] Effective thresholds [MPam 1/2 ] Deflection α Mode II Twisting β Mode III Mode II calculated Mode II measured Mode III measured ARMCO iron 19 ± 13 16 ± 12 1.4 * 1.5 a) 2.6 a) Titanium 39 ± 21 36 ± 20 1.7 * 1.7 a) 2.8 a) Nickel 52 ± 24 36 ± 22 2.4 * , 3.1 ** 2.9 a) 4.3 a) Ferritic-pearlitic steel 50 ± 14 31 ± 18 3.2 * , 2.5 ** 2.7 4.4 Pearlitic steel 60 ± 8 41 ± 18 2.5 ** 2.7 4.5 Stainless steel 67 ± 5 39 ± 16 2.5 ** 2.5 a) 4.2 a) a) [3], * Eq. (1), ** Eq. (2) Table 1 : Summary of the experimentally obtained values of deflection angles of mode II loaded cracks, twist angles of mode III loaded cracks and the effective thresholds in mode II and mode III for 6 metallic materials. Effective mode II thresholds calculated using Eq. (1) is shown for comparison. For materials with a large local mode I component the values according to Eq. (2) are shown. Fracture surfaces were reconstructed in 3D using the stereophotogrammetry in the scanning electron microscope (SEM) [5]. Quantification of the 3D data was done by a profile analysis – see Figs. 2 – 5. The coordinate l passes along the line E