PSI - Issue 23

Ulrich Krupp et al. / Procedia Structural Integrity 23 (2019) 517–522 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

520

4

20000 Hz) at uniaxial loading with a stress ratio of R=-1 up to a maximum number of 10 9 cycles. VHCF damage was monitored in situ by means of high-resolution thermography (Infratec ImageIR 8380, resolution: 1,9mm) and light optical microscopy. In addition to this, the ultrasonic testing device can be implemented within a scanning electron microscope (SEM), Söker et al. (2016). To allow microscopic observation and correlation of the microstructure with the evolution of VHCF damage, the cylindrical samples were given a shallow notch that limits the area of natural crack initiation to only a few mm². After cycling, the specimen's surfaces and fracture surfaces were further analyzed using analytical SEM in combination with focused ion beam (FIB) milling. 3. Results and Discussion Table 2 summarizes the mechanical properties of the studied materials. Here, the thermomechanically treated 16MnCr7 7 material with self-annealed martensite shows a particularly high impact energy A v , which obviously correlates with a higher fatigue limit  FL as compared to the bainitic and coarse-grained counterpart. Fig. 3 shows the S/N data of the 37HRC (Fig. 3a) and the 57 HRC condition (Fig. 3b) tested at 95Hz and 20kHz, respectively. The difference in fatigue strength of  100MPa (Fig. 3a) can be attributed to the occurrence of a high strain rate during ultrasonic testing (cf. Krupp et al. (2017)). High strain rates shift the thermally activated Peierls contribution of the critical shear stress towards higher temperatures, i.e., the yield strength increases with increasing strain rate (cf. Bach et al. (2016)). In the case of the high-strength condition, this effect vanishes due to the much higher athermic contribution of dislocation strengthening. In-situ monitoring of the specimen surface during cyclic deformation reveals an increasing temperature within banded structures (Fig. 3c). The banded structure was correlated with the occurrence of segregation bands; localization of cyclic plasticity is limited to Cr-depleted bands, where at a later state the formation of persistant slip bands (PSBs) sets in (see arrow in Fig. 3c).

Table 2. Mechanical properties of the studied steel materials. material R p0,2 [MPa]

R m [MPa]

A 5 [%]

A v [J]

 FL [MPa]

50CrMo4 37HRC 50CrMo4 57HRC 16MnCrV7 7 bain. 16MnCrV7 7 mart.

992

1095 2128 1197 1370

11,5

-- --

 500  600  390  610

1561

4

885

16 14

17

1000

104

a c Fig. 3. Wöhler S/N diagram for the steel 50CrMo4 for two different test frequencies (full symbols: 20kHz, open symbols: 95Hz) for (a) the 37HRC condition and (b) the 57HRC condition; (c) thermogram of the specimen surface during a fatigue experiment (crack initiation site marked by arrow). b