Issue34

S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 585 Figure 4 : TRIP steel PM 16-7-6: a) SEM micrograph (backscattered electron contrast) showing deformation bands and  ’-martensite after cycling with  = -1 (shear loading) at Δ  vM /2 = 0.4 · 10 -2 . EBSD phase maps after cycling with b)  = 1 (equibiaxial loading), c)  = -1 and d)  = 0.5 at Δ  vM /2 = 0.4 · 10 -2 :  The same behavior as for  of 0.5, -0.1 and -0.5 was observed for out-of-phase tests with phase shifts  = 22.5°, 45°, 90° and 135° on TRIP cast steel 16-6-6. The fatigue lives ranged within a factor two scatter band of the equibiaxial tests [9], see Fig. 5a. A phase shift caused no reduction of the fatigue life of the studied TRIP steel which is contrary to results for AISI 316FR at 550 °C [6] and for  -brass at room temperature [7]. The number of cycles to failure of shear tests (  = -1) were up to 7 times and 8.5 times higher than those of uniaxial tests and equibiaxial tests (  = 1), respectively. The differences in the fatigue lives decreased with increasing von Mises equivalent strain amplitude due to increasing plastic deformation. Similar results were observed for the cast steel 16-6-6 under equibiaxial and shear loading as reported in [9]. The observation, that the highest fatigue lives were obtained under shear loading in comparison to uniaxial and other biaxial loading conditions, is in good agreement to the literature of biaxial- planar tests [3–8] and torsional tests [17–19]. The difference between torsional and uniaxial fatigue lives is very pronounced for austenitic steels with a low stacking fault energy [18]. Different crack growth behavior as well as crack directions on the specimen surface were reported in the literature for several straining conditions [1–3]. Therefore, investigations on surface cracks are discussed later in detail. The authors assume that the period of stage I crack propagation in the plane of maximum shear strain (mode II) is much longer under shear loading than under other biaxial conditions. Comparing powder metallurgical and cast steel variants, the fatigue lives of PM 16-7-6 were higher than those of cast 16-6- 6 steel: for uniaxial and  = 1 loading, the factor is 3–7 and for  = -1 loading it is about 2 (in the range Δ  vM /2 of 0.6 · 10 -2 to 0.3 · 10 -2 ). That means, that the factor between fatigue lives of shear and those of equibiaxial loading was lower for PM 16-7-6 than for cast 16-6-6 steel, especially with decreasing von Mises equivalent strain amplitude. Microstructural investigations revealed different grain sizes and types of defects in the materials due to production processes. The grains of the cast steel variant were up to 75 times greater than those of the PM steel variant. Shrinkage cavities in the cast steel [9] had larger dimensions than impurities in the powder metallurgical steel. Furthermore, the yield stress of the PM steel (about 266 MPa) was about 1.18 times higher than that of the cast steel (about 225 MPa). As a conclusion, the powder metallurgical TRIP steel variant (PM 16-7-6) is more tolerant with respect to fatigue failure than the cast TRIP steel variant.