Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 584  = -1 tests. Shear fatigue caused both the highest  ’-martensite contents and the highest fatigue lives, whereas loading with  = -0.5, -0.1, and 0.5 resulted in the lowest  ’-martensite contents as well as the lowest fatigue lives. Figure 3 : Correlation of  ’-martensite content of the steel PM 16-7-6 at fatigue failure vs. a) von Mises equivalent strain amplitude Δ  vM /2 and b) number of cycles to failure N f for uniaxial and biaxial tests at different strain ratios  .  Microstructure Fig. 4a shows a typical microstructure of PM 16-7-6 steel after cyclic deformation obtained by scanning electron microscopy in back-scatted electron contrast. The  ’-martensite formed as lenses within deformation bands of a certain width, preferentially at intersection points of two deformation bands which is in good agreement to the literature e.g. [10, 16]. Thin deformation bands contained no  ’-martensite nuclei which is in accordance to [10]. Transmission electron microscopy and electron channeling contrast imaging revealed that deformation bands consist of a high density of stacking faults which result in a hexagonal crystal structure, called  -martensite, see e.g. [10]. Electron backscatter diffraction (EBSD) measurements were done after cyclic deformation on vibrational polished specimen surfaces. Phase maps showing austenite (fcc),  -martensite (hexagonal) and  ’-martensite (bcc) are presented in Fig. 4b to 4d for  = 1,  = -1, and  = 0.5 at a von Mises equivalent strain amplitude Δ  vM /2 = 0.4 · 10 -2 . The bcc phase was always found inside the hexagonal regions. Thus, phase transformation from austenite into  ’-martensite occurs via  -martensite as an intermediate structure. Comparing different loading conditions, higher volume fractions of  ’-martensite and  - martensite of about 20 % were observed after shear loading. In contrast, only small  ’-martensite and  -martensite regions were found after equibiaxial cycling. The most intensive  -martensite formation was obtained in the specimen after biaxial fatigue loading with  = 0.5, although the  ’-martensite regions were small. Moreover, martensitic transformation around cracks was observed for all investigated conditions (not shown here) due to intensive plastic deformation in the plastic zone around the crack tip. This is under consideration for further detailed investigations. The EBSD results showing local phase distributions within an area of 0.53 mm x 0.4 mm were consistent with the martensite volume fractions obtained by the ferrite sensor (compare Fig. 3a). Fatigue Life Fig. 5a shows the fatigue lives under uniaxial and biaxial loading of the powder metallurgical steel (PM 16-7-6) in comparison to data of cast steel 16-6-6 [9] by using von Mises equivalent strain hypothesis. The lowest fatigue lives of all investigated loading conditions of PM 16-7-6 were observed for biaxial loading with strain ratios  of 0.5, -0.1 and -0.5. However, they ranged in the scatter band of factor two of the uniaxial fatigue lives as well as the equibiaxial (  = 1) fatigue lives. Thus, the uniaxial Basquin–Manson-Coffin (BMC) relationship is conservative for biaxial loading of the investigated TRIP steel. In contrast to the present study, Itoh et al. [3] observed fatigue lives for  = 0.5, 0 and -0.5 between those of equibiaxial (  = 1) and shear (  = -1) tests on steel AISI 304 at 650 °C.