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. Discussion The study has explicitly shown a change of the FCG behavior under cyclic Mode-II loading of the pearlitic steel due to pre-deformation. The differences concern the crack path, resulting loading mode, and load required to propagate the crack. For the discussion of the results, additional microstructural analysis and fractographic investigations are presented. Additionally, the numerical predictions and experimental observations are compared. 4.1. Crack paths and propagation modes In the undeformed pearlitic steel the Mode-II dominated coplanar FCG spans a crack length of about 126 µm after tension-tension pre-cracking. Then the crack started to form bifurcations with a branch angle of approximately 50 70°, see Fig. 9 (a). The experiment was stopped after this bifurcation formation, as according to the numerical results in Fig. 6 the further propagation under mixed-mode exhibits a significant Mode-I component. The work at hand intended to prevent bifurcations with the superposition of static axial stresses achieving a load situation expected for RCF cracks in rails. The compressive axial stresses decreased the fatigue propagation rate, however, could not prevent the bifurcation. Nevertheless, further experiments with compressive static Mode-I loading are planned to come closer to real track conditions. The numerical simulation performed for the undeformed pearlitic steel assuming isotropic material provide an explanation for the bifurcation formation from a continuum mechanical point of view. The small angular deviations from Mode-II predicted already after pre-fatigue (Fig. 7) lead to a curved crack that tends to turn more and more into a mixed-mode crack finally forming a bifurcation and propagating with a significant Mode-I component. The micrograph in Fig. 10 (a) shows the microstructural aspect of the crack propagation in the undeformed material. The pearlitic colonies are randomly oriented so that on the micro-scale the crack will locally propagate along preferential propagation planes. The resulting local tortuous crack path can also be seen inspecting the Mode-I pre fatigue crack path in Fig. 9. Under Mode-II loading a similar crack path could be expected when the crack would globally propagate in a coplanar fashion over large distances and when crack flank interaction effects would not reduce the crack driving force. The present experimental observations coincide with findings reported in literature for similar materials and loading conditions, after crack growth in Mode-II over some hundred micrometers, the crack bifurcates and turns into a mixed-mode, see for example Doquet and Pommier (2004), Vojtek et al. (2013). The pre-deformed R260 showed a significantly different FCG behavior with respect to the propagation rate and crack path. The crack propagates at lower shear stress amplitudes in Mode-II without forming bifurcations, see Fig. 8 (a) and (c). This behavior is attributed to the microstructural alignment and refinement imposed by the HPT process. The microstructural evolution of pearlitic steel during HPT-deformation has been thoroughly described in literature, see for example Hohenwarter et al. (2009), Leitner et al. (2018), Leitner et al. (2019). The work at hand repeats the most significant features as far as they are necessary to understand the effect of deformation on the FCG behavior in present experiments. Fig. 10 (a) illustrates the undeformed pearlitic structure (as-rolled R260) consisting of a lamellar structure of ferrite (dark) and cementite (bright). The lamellae are arranged within pearlite colonies within a more or less parallel arrangement, the colonies themselves appear to be randomly oriented. The Vickers-microhardness in the undeformed state amounts to 275 HV. The imposed shear deformation during HPT gradually aligns the pearlite colonies to the shear plane, reduces the lamellae spacing and leads to a substructure formation within the ferrite phase. Fig. 10 (b) shows the alignment for a shear strain γ of around 3.5. The hardness of the pre-deformed R260 at this deformation state amounts around 400 HV. The microstructural alignment can be also inferred from the resulting crack path in Fig. 9 (b) with a more or less coplanar crack growth. Very similar to the Mode-II regime also the fatigue pre-crack generated in Mode-I shows a relatively straight crack path in comparison to the undeformed material, see Fig. 9 (a).