PSI - Issue 28

S. Henschel et al. / Procedia Structural Integrity 28 (2020) 1369–1377

1375

S. Henschel et al. / Procedia Structural Integrity 00 (2020) 000–000

7

0°

45°

Fatigue precrack

Stretch zone

Unstable crack growth

Fig. 12. Fractographic analysis of crack tip blunting and further crack extension.

Table 2. Stretch zone width ( S ZW ) and stretch zone height ( S ZH ) at di ff erent loading angles ( α ). α/ ◦ 0 15 30

45

S ZW / µ m

22.7

23.7

22.2

21.5

S ZH / µ m

8.7

6.1

6.3

4.2

S ZW / S ZH

2.6

3.9

3.5

5.1

toughness decreased while the mode II fracture toughness increased with increasing loading angle. This behavior can also be displayed in a di ff erent way. A limiting curve as an approximate fracture criterion is shown in Figure 11b. With this curve, the material will not fracture.

3.5. Fractographic analysis

The macroscopic features of the fracture surface were already discussed in Section 3.2. Figure 12 shows micro scopic features of typical fracture surfaces at di ff erent loading angles. The analyses revealed the crack tip blunting of the fatigue precrack tip that was followed by predominantly ductile fracture. Nevertheless, the crack extension was unstable. Minor amounts of cleavage fracture facets were also observed (not shown). The blunting of the precrack tip was quantified. Table 2 shows the results. It can be seen that S ZW was independent of the loading angle, i.e. the K I / K II ratio. In contrast, S ZH decreased with increasing α . Consequently, there was a flattening of the stretch zone with increasing mode II component.

4. Conclusions and Outlook

The mixed-mode fracture toughness behavior of the high-strength cast steel 42CrMo4 was measured. Since the original geometry of the compact tension shear specimen did not fit the testing machine and the aimed material behavior, the geometry was modified. Consequently, the formulas for calculating the stress intensity factors were derived from a finite element analysis. Additionally, the stress intensity factors were measured experimentally by using strain gauges. A linear-elastic material behavior was observed. However, the specimens had an insu ffi cient thickness. Conse quently, no plane-strain fracture toughness was determined. The deformation around the crack tip was analyzed by strain gauges. This analysis was supported by a prior finite element analysis. However, these measurements did not lead to su ffi cient exact stress intensity factors. Other strain