PSI - Issue 23

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

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PSB formation comes along with the growth of extrusions and intrusions, which act as additional stress raisers, eventually leading to fatigue crack initiation. Such a situation is shown in Fig. 4, where the surface PSBs (Fig. 4a) were cut by means of FIB milling in the SEM. The cross section (Fig. 4b) reveal crack initiation below the PSB. However, it is worth to be mentioned that in the case of the 37HRC condition of the steel 50CrMo4 such cracks seem to be non-propagating cracks, which do not cause fatigue failure (at least up to 10 9 cycles). Obviously, the martensite paket/block boundaries are efficient microstructural barriers and the steel in the medium strength condition reveals a real fatigue limit (cf. Fig. 3a).

PSB

FIB section



a b Fig. 4. Formation of surface protrusions during fatigue of the steel 50CrMo4 (1.5 · 10 8 cycles, 

a =640MPa, 20kHz): (a) SEM surface micrograph

and (b) FIB section through a protrusion

The situation is different in the case of the high-strength condition 57HRC of 50CrMo4 and 16MnCr7 7, respectively. At numbers of cycles exceeding 10 7 (VHCF), the crack initiation site shifts to non-metallic inclusions in the material's interior. The "fish eye" shape (Fig. 5a) of the fracture surface can be correlated to the long-fatigue-crack propagation regime, while a fine-granular area (FGA) around the inclusion in the center of the fish eye is a further characteristic feature of internal VHCF crack initiation (Fig. 5b).

1 µm

2 00µm

2 0µm

a

b

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Fig. 5. Fracture surface of a fatigue specimen of 16MnCr7 7 (9 · 10 7 cycles,  a =625 MPa): (a) overview with fish eye and (b) crack initiation site at non-metallic inclusion surrounded by FGA [8]; (c) FIB section through a non-metallic inclusion with adjacent nano-crystalline FGA (50CrMo4, 57HRC,  a =750MPa, 3.5 · 10 8 cycles) [2]. According to damage mechanics analyses, e.g. in Grad et al. (2012), Krupp et al. (2017), the following mechanism for FGA formation can be suggested: (i) local stress concentration at the inclusion leads to plastic slip. (ii) Accumulation of small-scale plasticity results in dislocation patterning and polygonization to nano grains. (iii) Due to the nano grain size, the stress intensity threshold  K th,FGA is exceeded in the vicinity of the inclusion, leading to crack initiation. (iv) This mechanism proceeds until the crack reaches the condition of the macroscopic threshold  K th and long fatigue crack propagation within the fish eye sets in.