PSI - Issue 39

Georg Schnalzger et al. / Procedia Structural Integrity 39 (2022) 313–326 Author name / Structural Integrity Procedia 00 (2019) 000–000

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4.2. Fracture surfaces The fracture surface analysis shown in Fig. 12 proves that the aligned structure leads to an overall reduction of the local surface roughness. These microstructural and fractographic investigation also yield a qualitative explanation for the experimental results showing higher crack growth rates even at lower load levels for the pre-deformed material: The higher crack growth rates of the pre-deformed material are indicative of a lower threshold of stress intensity factor range. The reduction of the threshold of stress intensity factor range has been often observed for material subjected to severe plastic deformation including rail steels (Leitner et al. (2019)). The reason is primarily connected to a reduction of contact and geometric shielding effects. Even though this literature describes global Mode-I loading conditions, it is fair to assume that these mechanisms also play an important role for Mode-II crack growth, for example when crack face interaction is taken into account. The higher crack growth rates can again be correlated to results gained from Mode-I experiments. It is known that the microstructural modifications lead to a dramatic reduction of the fracture toughness, i.e. static crack growth resistance in this material or in rail steels in general due to the afore-mentioned microstructural modifications (Hohenwarter et al. (2011)). Also crack propagation experiments in Mode-I show higher crack growth rates (Leitner et al. (2019)). Even though in the present case the loading case is different, similar broad trends can be expected.

Fig. 12. Fracture surfaces: (a) undeformed ( γ = 0) and (b) pre-deformed ( γ ~ 3.5) R260.

5. Conclusion Within the current study the fatigue crack growth behavior under cyclic Mode-II and static Mode-I loading of the pearlitic rail steel R260 is investigated considering an undeformed and pre-deformed state. A modified anvil design is developed to produce severely deformed specimens using high pressure torsion. In a first step, a shear strain γ of 3.5 equaling a von Mises strain ε vM of 2 is achieved. In the fatigue tests, different crack growth behaviors with respect to crack paths and resulting propagation modes are observed depending on the material deformation state: (i) In the undeformed material, the crack initially propagates in Mode-II. After around 130 µm a bifurcation starts to form with a branch angle of 50-70° turning the crack into a mixed-mode with significant Mode-I proportion, see Fig. 13 (a). (ii) In the pre-deformed specimen, the crack propagates under Mode-II without forming bifurcations at a lower stress intensity factor range. Even the final fracture occurred along the aligned microstructure, see Fig. 13 (b).