Abstract
Main crack propagation process in polycrystalline materials is very complex due to intercrystalline sliding mechanism effects. Multiple factors make it considerably difficult to analyze the state of stresses in the crack tip within the theoretical scope of linear fracture mechanics (LFM). As a basis for the suggested Bilby-Cottrell-Swinden model, we propose a method to determine the state of stresses in the tip of a propagating crack in microspecimens subjected to tension and measured inside a high voltage transmission electron microscope (HVTEM). Based on the electron microscopy observations of a main crack and the formation of microcracks in the plastic zone, theJ-integral was determined.
Highlights
The complex nature of crack propagation in polycrystalline materials is explained by the intergranular sliding mechanism effect
It is worth to mention that the approximated parameters of the stress curve with an exponential function of α and η, required to determine the stress according to (10), can be obtained by a statistic processing of the experimental curves of the material under tension and the values of the coefficient I which are determined from the graph presented in Figure 7 [2, 17,18,19]. As it was demonstrated by our deformation tests on polycrystalline microspecimens of VT51 titanium the micromechanism which rules fracture is the intergranular sliding
In small-grain polycrystals, with the grain inferior diameter of 1–3 μm, and in polycrystals with the matrix which can be hardened by thermomechanical treatment, the effect of the dislocation sliding mechanism may be totally invoked and the whole path of the main crack is located at the grain boundaries
Summary
The complex nature of crack propagation in polycrystalline materials is explained by the intergranular sliding mechanism effect. The presence of the three modes of fracture, formation of deep microcracks of extended distances along the grain boundaries, crack ramification in grain intersections, and grain fragmentation in the plastic zone of the main crack, caused by the undeniable influence of the interface structure on the kinetics of grain sliding during the crack incubation process, all these characteristics of crack development in polycrystalline materials create a very complex scenario for a detailed analysis of the state of stresses In this case the stresses are being developed in the close area located on the tip of the crack. This requirement is not reached during the development of a fracture because of the intergranular sliding mechanism
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