Abstract
ABSTRACT. 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.
Highlights
O bservation of crack paths on a microscopic level enables us to reveal local loading modes at the crack tip under various kinds of remote loading
Identification of the effect of microstructure of multiphase materials contributes to the research of shear-mode crack growth micromechanisms
After the local mode I growth was triggered in the pearlitic grain, the crack propagation continued with a large component of mode I in the rest of the material
Summary
O bservation of crack paths on a microscopic level enables us to reveal local loading modes at the crack tip under various kinds of remote loading. In pure ferrite (ARMCO iron) the dense set of slip planes of the bcc lattice enables a selection of the slip systems oriented almost parallel to the applied shear stress This is probably the reason why the shear cracks in this material propagate almost coplanarly (small deflection angle) and the intrinsic mode II threshold is the lowest one of all tested materials. The mode II intrinsic threshold was high here, which can be related to a low Schmid factor in the highly deflected planes This good agreement between the deflection angles and the values of thresholds (see Tab. 1) is reflected by a successful use of a simple physical-based relationship for intrinsic mode II threshold, Eq (1), for singlephase metals.
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