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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As determined by EBSD measurements, slip band formation takes place mainly along the martensitic blocks. Crack initiation takes place predominantly on the surface. A cross-section of the slip band found on a run-out specimen using the FIB technique shows that a microcrack has formed below the surface, cf. Fig. 2b. However, the crack was blocked due to the barrier effect of the surrounding microstructure up to a load number of N=2 ‧ 10 8 number of cycles at a stress amplitude σ a =480MPa. Therefore, a real fatigue strength can be assumed up to a number of cycles of N=10 9 cycles for the material in the 37HRC hardness condition, considering of a constant stress amplitude. 3.2. Type-II Fatigue behavior Examination of the fracture surfaces of the specimens from the fatigue tests for the 57HRC hardness condition revealed that for both test frequencies all fatigue cracks have been initiated at nonmetallic inclusions. Only inclusions of the type Al 2 O 3 as crack initiation sites were found. In addition, the onset of a FGA formation with decreasing stress amplitude was observed. A fracture mechanical based evaluation of the inclusions as well as of the FGAs was carried out using the eq. 1 from the model developed by Murakami for internal crack initiation, Murakami 1989. ( ) i a area K π σ 0.50 max = (1) For this purpose, the projected area √ area of the inclusions as well as of the FGAs have been measured according to Fig. 3a. By using the data for the area and the remote stress amplitude σ a , values for the stress intensity factor K max can be calculated for each inclusion and the respective FGA and plotted vs. the corresponding number of cycles for each specimen, Fig. 3b. Here again, a distinction is made between the test series of 95Hz and 20,000Hz.

a)

b)

Fig. 3. a) Internal crack initiation at a non-metallic Al 2 O 3 inclusion and FGA formation on a fracture surface after N f =3.49·10 8 number of cycles loaded with a stress amplitude of σ a =750MPa and b) calculation of the stress intensity factor K max for the non-metallic inclusions and the corresponding FGAs based on the model according to Murakami. The calculation of the stress intensity factor K max for the non-metallic inclusion is based on the assumption that these have broken or detached from the surrounding matrix and thus represent a crack nucleus. According to Fig. 3b, the stress intensity factor K max decreases with increasing number of cycles. This trend can be explained by the statistical size distribution of the nonmetallic inclusions within the stressed volume in the gauge length of the specimens. With decreasing stress amplitude, the stress intensity at the inclusion ΔK Inc is not high enough to initiate a fatigue crack. By forming the FGA, the local stress intensity factor ΔK FGA increases up to the final size of the FGA, which can be observed on the fracture surface of broken specimens.