PSI - Issue 39

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

316

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Table 1. Chemical composition in (wt.-%) and mechanical data of R260 in the as-rolled condition according to DIN Deutsches Institut für Normung e.V. (2011).

C

Si

Mn

S

P

Fe

R m,min (MPa)

A uniform,min (%)

0.62-0.8

0.15-0.58

0.7-1.2

< 0.025

<0.025

in balance

880

10

At first, either tubular FCG specimens or Double Cone (DC) bars are extracted from the railhead to measure FCG in undeformed and pre-deformed material, respectively. The DC specimens are deformed using an adapted High Pressure Torsion (HPT) process prior to the manufacturing of the FCG specimen to simulate the shear deformation prevailing near the rail surface. Afterwards, the FCG specimens are pre-cracked under Mode-I and tested in the FCG experiment under multiaxial loading conditions. Finally, the crack path, microstructure and fracture surfaces are analyzed using an Optical Microscope (OM) and Scanning Electron Microscope (SEM) to investigate the influence of pre deformation on the crack propagation behavior. The complementary FEA provides Stress Intensity Factors (SIF) and mode-mixity ratios. The following sections outline the experimental procedure in more detail. 2.1. Fatigue crack growth experiment Fig. 2 depicts the basic principal of the new non-standard FCG experiment. For the investigation of fatigue crack propagation under cyclic Mode-II and static Mode-I loading the sample geometry shown in Fig. 3 is designed. The tubular specimens exhibit an internal diameter of 14 mm and a gage length of 15 mm. Two notch geometries (notch A and B) and two different external diameters in the notch region of 17 and 20 mm were in use. Notch A is a specially designed spark-eroded notch (see Fig. 3), which is machined at one side of the specimen. Notch B consists of a circular hole with 1.5 mm diameter drilled through the entire specimen and two spark-eroded cuts of 0.75 mm length on both sides of the hole. Consequently, notch A exhibits two notch roots in total and notch B four (i.e., two on the front and back side, respectively).

Fig. 3. Dimensions of the center notched FCG specimen and the two different notch geometries.

The FCG specimen is manufactured from the R260 railhead or DC bar to investigate the undeformed and pre-deformed state, respectively. Prior to the FCG tests the center notched samples are Mode-I pre-cracked on a resonant testing machine (Rumul Russenberger + Müller, Switzerland). The FCG experiment itself is conducted at a servohydraulic axial-torsion machine Instron 8854 at Materials Center Leoben (MCL) under cyclic shear (Mode-II) and static compressive axial stress (Mode-I). During the FCG test, the crack propagation is monitored off-line. In periodic intervals, the experiment is interrupted and micrographs from the specimen surface in the notch region are recorded using the OM Zeiss Axioscope. In general, the experiment lasts until crack branches or bifurcations are detected in the micrographs, as these indicate mixed-mode propagation. After the tests, the specimens are loaded until ultimate fracture to investigate the fracture surfaces, crack paths and microstructure with a SEM.