PSI - Issue 7

B.M. Schönbauer et al. / Procedia Structural Integrity 7 (2017) 492–496 B.M. Schönbauer et Al./ Structural Integrity Procedia 00 (2017) 000–000

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3. Results and discussion 3.1. Smooth specimens

It has been shown in Schönbauer et al. (2017d) that the tension-compression fatigue limit of smooth specimens, σ w0 , can be well predicted using the simple estimation σ w0 = 1.6∙ HV , with the Vickers hardness HV = 352 of the material. Under torsional loading, the fatigue limit, τ w0 , is in good accordance with the von Mises criterion: τ w0 = σ w0 / √ 3 = 1.6∙ HV / √ 3. The crack initiation sites under tension-compression loading were non-metallic inclusions located at the surface or in the interior of specimens (see Fig. 2). In contrast, all specimens that were tested under torsional loading failed from the surface, and initial cracking occurred under Mode II/III followed by crack brunching and subsequent Mode I crack growth (see Fig. 3(a)). No inclusion or other defects were observed at the location of crack initiation as shown in Fig. 3(b). This clearly shows an influence of the loading condition on the crack initiation mechanism.

Fig. 2. (a) Fracture surface observed after tension-compression testing at σ a = 714 MPa and N f = 3.69×10 5 cycles and (b) σ a = 575MPa and N f = 6.14×10 8 cycles.

Fig. 3. (a) Fatigue crack on the surface of specimen and (b) fracture surface after torsional testing at τ a = 400 MPa and N f = 2.34×10 6 .

3.2. Defect containing specimens In previous investigations (Schönbauer et al. (2016); (2017a)), it has been shown that the defect tolerance of 17-4PH under uniaxial loading can be well characterised using the √ area parameter model by Murakami and Endo (1986). In the presence of small defects, the fatigue limit under fully reversed loading, σ w , could be calculated by: