PSI - Issue 28

Stepan Major et al. / Procedia Structural Integrity 28 (2020) 561–576 Author name / Structural Integrity Procedia 00 (2019) 000–000

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4. Results and discussion In this paragraph we will look on the results of fracture surface reconstruction. A global view on the spatial orientation of the surface elements is shown in Fig. 6. These graphs are the result of reconstruction in which the square net with a 99 = 99 μm was used. The figure shows the inclination of the elementary surface expressed as the deviation of the normal vector from the z -axis (deviation angle α ) by a different color. Due to the large number of points (elementary areas) here the color of the area expresses only a certain trend. In the case of bending loads, it has a very small informative value, basically shows only the fact, that the fracture surface is from a macroscopic point of view perpendicular to the axis of the sample.

Fig. 6. Global view of the spatial orientation of surface elements (Virgin specimens). Inclination of fracture area (general trend of angle α) is expressed by the color of the area concerned: (a) - bending; (b) -torsion.; see Fig. 4 (a) and (e).

We will now look at a comparison of the results obtained from five areas studied (numbered from 1 to 5, see Fig. 5). For these five areas both square nets (side length a 99 = 99 μm and a 05 = 5 μm) were used for comparison. The results of the analysis of the spatial orientation of the five studied areas, are shown using histograms in Fig. 7. These histograms are representing the relative frequency of individual angles of inclination of surface elements (angles were divided into groups in the range of 10 o ). The results are shown only for two extreme loading conditions, i.e. pure bending (i.e. L R = 0 ) and pure torsion (i.e. L R = 1 ), primarily due to the limited content of this work. In the case of a 05 and simple bending, it is clear from Fig. 7 (a) that the highest frequency is shown by the angles of inclination in the interval 10º to 15º. This fact shows, that the formation of facets significantly deviated from the macroscopic plane of the fracture is less probable, than the formation of facets, which practically lie in this plane. This plane is roughly perpendicular to the axis of the sample in both samples (samples loaded by pure bending and also by pure torsion). When we compare, the case of a square network a 99 with the results observed for the network characterized by length a 05 (see Fig. 7 (c) and (a)), significant increase in the frequency of angles in the interval 0º to 10º with subsequent rapid decrease in frequency was observed, while for angles higher than 50º their frequency decreases to virtually zero. This trend can be explained by the fact, that in this case, the characteristic dimension of the network is larger than the characteristic dimension of the facets and steps. Therefor the deviation angle α does not differ much from the global value of the angle of inclination of the macroscopic fracture plane. Similarly, for the case of torsional stress