Issue34

S. Henschel et alii, Frattura ed Integrità Strutturale, 34 (2015) 326-333; DOI: 10.3221/IGF-ESIS.34.35 330 nucleation was necessary and the strain was localized in a narrow shear zone. Such an effect of non-metallic inclusion clusters was also already observed in the case of tensile loading [20]. In that study, the shear zones were analyzed by means of thermography. Consequently, these zones were detectable by a local increase in heating rate due to localized deformation. Figure 3: Crack tip blunting in a region with locally low inclusion content. Cast C, T = 20 °C, static loading. Figure 4: Large non-metallic inclusion cluster directly at the fatigue crack tip. a) Cast A, b) Cast B, dynamic loading. The amount of crack path deflection was measured by means of stereoscopy enabled by the beam tilting of the SEM. Fig. 5 shows the blunting of the precrack and the deflection due to the non-metallic inclusions. The observations revealed that crack path deflection superposed the crack tip blunting. The amount of superposition depended on the distance between the inclusion cluster and the fatigue crack tip. Line 1 in Fig. 5b exhibited an intense crack tip blunting due to the distance of approximately 100 µm between the fatigue crack tip and the cluster. In contrast, line 2 in Fig. 5b exhibited a considerable smaller crack tip blunting due to the smaller distance between the fatigue crack tip and the inclusion cluster. Hence, the characterization of the amount of blunting by the measurement of stretch zone width and stretch zone height can result in erroneous values if only one fracture surface is analysed. The formation of a shear zone and the crack path deflection was a consequence of the inclusion clusters near the crack tip. Hence, the local decrease of toughness due to the non-metallic inclusions was the reason for the crack path deflection. The crack path deflection, therefore, was not assumed to increase the toughness. Locally advancing crack fronts can also be found in other materials with non-metallic inclusion. Dynamic loading of a similar G42CrMoS4 in [21] resulted analogously in an inhomogeneous crack front development. Consequently, the crack tip was locally unloaded, which caused an interrupted crack growth. The surrounding material was further loaded and exhibited crack initiation and growth at higher loads.