Tomáš Vojtek

Although 2D fracture mechanics solutions have been successfully applied, many analyses are affected by the wrong presumption of the 2D plane stress state at the free surface intersected by a crack front. The significance of this presumption and the related confusion have grown rapidly with the progress of strain field measurement techniques, such as the digital image correlation. The article demonstrates this wrong presumption on two of the most commonly measured and studied entities, which are the plastic zone in front of the crack tip and the crack tip opening displacement. 3D stress and strain solution is necessary to be performed. A realistic crack front curvature is required to be considered. Only such solutions can be used for comparison with the experimentally obtained strain fields. Other solutions are wrong and can bring confusion, wrong conclusions about material behavior, and slow down the progress in the field of fracture mechanics due to the incompatibility of the presumed stress state and the stress state present in experiments.

The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. 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. 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.

An analysis of the electric state of air shows the presence of various ion sorts. The therapeutic effect of negative high-mobility ions of proper concentration is known. This positive effect was observed in caves that are used for speleotherapy. This article presents the capability of methods for measuring ion concentration and for ion spectral analysis.