PSI - Issue 7

A. Giertler et al. / Procedia Structural Integrity 7 (2017) 321–326 A. Giertler et Al./ Structural Integrity Procedia 00 (2017) 000–000

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orientation along the rolling direction. A statistical analysis of the martensitic microstructure on the basis of a reconstruction of the former austenite grains by the software ARPGE revealed a mean grain size of 14μm , Cayron 2006. For this purpose, the orientation relationship of Kurdjumov-Sachs {111} A ||{011} M <110> A ||<111> M has been used for the reconstruction of the martensitic structure on the former austenite grains. In addition to a RUMUL 100kN resonance testing machine with a test frequency of f =95Hz, an ultrasonic testing machine of the type BOKU Vienna with a test frequency of f =20,000Hz was used for the fatigue tests. The sample surfaces have been mechanically and electrolytically polished to allow an observation of the fatigue damage on the sample surface during the fatigue test. For the specimens from the material with the lower hardness of 37 HRC, a shallow notch has been inserted into the gauge length with a notch factor of 1.2. The surface of the notches is observed during the fatigue experiments using a high-resolution digital microscope type HIROX and a high-resolution thermographic camera type Infratec ImagerIR 8380hp. An early detection of the fatigue damage within the microstructure is possible by its heat signature, due to the high optical resolution of the thermal imaging lens with a pixe l size of 5μm and a - field of view of 3200μm to 2600μm. The fatigued specimens have been investigated using high-resolution scanning electron microscopy (SEM) in combination with automated electron backscattering diffraction (EBSD) and focussed ion beam (FIB). 3. Results Fig. 1 shows the fatigue life S-N diagrams for the two strength levels of 37HRC and 57HRC. All fatigue tests have been carried out at a stress ratio of R =-1, at room temperature and in laboratory air atmosphere. Figure 1a) shows the S-N data for the hardness of 37HRC. Only fatigue failure from the surface on all fractured specimens was observed. Furthermore, no fractures above 10 7 load cycles occurred. In order to determine the fatigue strength, a knee point has been assumed for 10 6 cycles. This results in a fatigue strength of σ FL =490MPa for the test frequency of 95Hz and for the test frequency of 20,000Hz a fatigue strength of σ FL =680MPa was determined. The fatigue strength of σ FL =490MPa has also been confirmed by thermometric measurements with the aid of a high-resolution thermographic camera in load increase tests. By monitoring the sample surface during the fatigue tests, a first local plastic deformation within the microstructure could be observed by means of its heat signature with the aid of high-resolution thermography camera at an remote stress amplitude of σ a = 396 MPa. This value is below the fatigue strength of σ FL =490MPa of the material condition of 37HRC and shows that local plastic deformation has to be taken into account even far below the fatigue strength.

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b)

Fig. 1. Wöhler S-N Diagram showing the fatigue life for the steel 50CrMo4 at two testing frequencies of f =95Hz and f =20kHz for a) the 37HRC hardness condition and b) for the 57HRC hardness condition.

The difference in the fatigue strength of Δ σ =200MPa for the 37HRC hardness condition can be attributed to an influence of the test frequency. By increasing the test frequency from 95Hz to 20,000Hz, the time to reach the maximum load has been reduced. This results in an increase in strain rate. The influence of the strain rate has been widely investigated in the past for many materials in static as well as cyclic tests. It was found that the flow stress