PSI - Issue 4

David Simunek et al. / Procedia Structural Integrity 4 (2017) 27–34 D. Simunek/ Structural Integrity Procedia 00 (2017) 000 – 000

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ΔK

Stress intensity factor range

F

Geometry factor Bending moment

M b N A

Number of accelerated cycles

N CA N D OL R OL

Number of load-cycles at constant amplitude loading

Number of delay cycles

Overload

Overload factor

1.1. 1:3-scale specimens

Testing at realistic conditions is indispensable for research and for determination of material parameters. Thereby, the shapes of the specimen geometries, loading conditions as well as load sequences need to be similar to real conditions. Testing of real components or similar specimens of the same size is usually costly and time consuming. For this reason, 1:3 scale rotating bending specimens are manufactured to build a link between small scale SEB specimen and full-scale axle tests and to optimize transferability of the assessment methods (for the results on SEB and full-scale specimens see the companion articles on EBFW 3 in this volume). The 1:3 specimens (as well as the SEB specimens) are manufactured out of railway axle blanks with a diameter of 190 mm. In case of the 1:3 specimens, the axle is cut into segments with a length of approximately 300 mm. These axle segments are divided into four parts according to Fig. 2 for subsequent manufacturing by turning. The position of starter notch – to be eroded later – is 20 mm below the primary surface of the axle (45° from midpoint of the original axle – see also Fig. 2) to guarantee similar material properties and microstructure for each specimen. After manufacturing the round specimens, a semi-elliptical notch with a depth (length of the minor semi-axis) of a 0 = 1 mm and a length of the major semi-axis c 0 = 1.25 mm is eroded, which acts as crack initiation point in the subsequent fatigue tests. The surface of the specimens near the notch is polished to achieve optimum quality for the optical crack length measurement during testing.

Fig. 2 Manufacturing of 1:3 scale specimens from a railway axle section

The specimens are tested in a four-point rotary bending test machine. The bending moment is applied by a pneumatic cylinder and coupling rods (Fig. 3). An optical crack length measurement system with a high-resolution camera and a specific telecentric lens is applied. For the crack length measurement on the surface, the machine is stopped after each 5.000 to 10.000 load cycles. The final switch-off criteria for a test are the maximum deflection of the specimen, vibrations during testing due to imbalances at large crack lengths, and a specified crack length on the surface.