Issue34

S. Henschel et alii, Frattura ed Integrità Strutturale, 34 (2015) 326-333; DOI: 10.3221/IGF-ESIS.34.35 329 Irrespective of the loading rate, the final crack length after the test ( a f = a 0 +  a ) was marked by heat tinting. Finally, breaking open of the specimens enabled the measurement of a 0 and a f by means of light microscopy. Fractography Fracture surfaces were investigated by means of scanning electron microscopy (SEM) with secondary electron contrast and energy dispersive X-ray diffraction. The beam tilting mode of the SEM (MIRA3 XMU, TESCAN) enabled the determination of the fracture surface topography in 3D [17]. Hence, the crack tip blunting process was characterized by the parameters stretch zone width (SZW) and stretch zone height (SZH). R ESULTS AND D ISCUSSION Loading rate dependent crack initiation ig. 2 shows the crack growth resistance curves for the different casts under quasi-static and dynamic loading conditions. 0.0 0.1 0.2 0.3 0.4 0.5 0 50 100 150 Excl. line Cast A Cast B Cast C J d / N/mm Crack extension / mm b) Blunting line Figure 2: Static (a) and dynamic (b) crack growth resistance curves. No significant difference between different casts due to scattering. Open symbols represent the calculated curves of the type: J = A + B (  a ) C . At both loading rates, no significant difference between the different casts was observed. Furthermore, the scattering at the dynamic tests was larger than at quasi-static loading. This was attributed to the testing of multiple specimens under dynamic loading conditions. The increase of loading rate by approximately four orders of magnitude did not result in a considerable deterioration of the toughness behavior. However, a slightly lower toughness was observed at small crack extensions. Furthermore, the slope of the crack growth resistance curve was slightly larger under the dynamic loading conditions. The low sensitivity of the material to the increase of loading rate was attributed to the superposition of two effects: the embrittlement due to the higher strain rate [18] and the decreased flow stress due to adiabatic heating within the plastic zone [19]. These observations are in accordance with Krabiell and Dahl [10], who observed only a slight effect of the loading rate on the fracture toughness at ambient temperature in the case of a quenched and tempered steel. Fracture surfaces Fig. 3 shows the blunting of the crack tip in a region with low inclusion content. The blunting of the precrack was characterized by an intense plastic deformation. Due to the very low content of non-metallic inclusions in this region, void nucleation was inhibited. However, the surrounding area exhibited non-metallic inclusions that nucleated voids. It can be seen in Fig. 3 that the relatively large amount of non-metallic inclusions in the lower right part of this figure lead to a considerable deviation of the crack path from its original plane. Hence, the agglomeration of non-metallic inclusions favors the crack path deflection at the expense of a minimum area of the fracture surface. Non-metallic inclusion clusters in the vicinity of the fatigue crack tip resulted in considerable crack path deflection as shown in Fig. 4. Furthermore, crack tip blunting was only observed at regions of the fatigue crack tip that had a distance of at least 50 µm to the inclusion cluster. The deflected crack path was characterized by a nearly flat appearance. This observation can be explained by a shear zone which developed between the original crack (evolved from the fatigue crack) and the voids which formed at the inclusion cluster. These voids coalesced and formed a crack in front of the main crack. It was assumed that these cracks connected analogously to the void sheet mechanism. Hence, no additional void F 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 Cast A Cast B Cast C J / N/mm Crack extension / mm Blunting line Average J-  a curve Scatterband a)