13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 4. Discussion According to the results of the ultrasonic fatigue tests, it is shown that right at the beginning of fatigue loading local plastic deformation is concentrated in the austenite phase, caused by the elastic anisotropy of the duplex microstructure. Slip band formation within the austenite grains, produces strong dislocation pile ups at the phase boundaries. These local stress intensities are predominant sites for crack initiation (cf. Figure 3). The crack propagation process was observed by means of a far field microscope to correlate the crack path with its surrounding microstructure. The α/γ-phase boundary has been identified as an effective barrier against fatigue crack propagation, cf. Figure 7. By knowing the local phase and crystal orientation distribution, e.g., from EBSD measurements, it is possible to predict crack initiation sites in the microstructure by means of FEM. Here, a material model is required that accounts for elastic anisotropy and crystal plasticity. The calculated local shear stresses correlate with initiation sites for slip bands and cracks. As a subject of ongoing work, the 3D data from the diffraction-contrast-tomography experiments will be used for a more precise FEM prediction of slip band and crack initiation sites in the mapped volume. As described earlier, plastic deformation located in the slip band has been found also in the austenite grains even after 108 cycles due to the elastic anisotropy of the material. To model the fatigue crack initiation in the VHCF regime the focus has been placed on the evolution of slip bands, and in particular especially on the slip transmission in the neighboring grains and the crack initiation. The underlying model concept follows the work of Tanaka and Mura [1] on crack initiation along slip bands. It describes the fatigue-crack initiation by the accumulation of dislocation dipoles during strain cycling ( ) ( ) , ( ) where Ni is the number of cycles for crack initiation, G the shear modulus, ws the specific fracture energy per unit area along the slip band, ν the Poisson ratio, d the grain size, Δτ the shear-stress range and τfr the friction stress of dislocation motion. The model was modified for instances by Chan [2], who added the slip band height h and a factor for the cyclic slip irreversibility λ. For the HCF and VHCF regime, where low stress amplitudes are applied, cyclic slip consist of a reversible and irreversible fraction of dislocation movement. The number of cycles Ni for initiation of a crack of length c can be calculated as follows: ( ) ( ) ( ) ( ) . (2) Furthermore, it was shown that slip transmission from one grain to the neighboring grain plays an important role in crack initiation and propagation. This barrier and its effectiveness can be described by the twist and tilt angles of the adjacent slip planes [6, 9]. 5. Conclusion Under fatigue loading, plastic deformation is concentrated in the austenite phase by the emanation of intense slip bands. The formation of slip bands was observed already during the first thousand cycles, leading to local stress intensities at the phase austenite-ferrite boundaries. These areas have been identified experimentally by light microscopy and numerically by the finite element method as critical crack initiation sites. On the other hand, the results are showing that crack initiation is depending to a great extent on the configuration of the tilt and twist angles of the neighboring grains. They control the transport of the local plastic deformation concentrated on the slip bands in the austenite across the phase boundary into the ferrite phase. This effect supports the assumption that VHCF damage depends on the microstructure. Therefore, understanding of the fatigue mechanisms in the VHCF regime requires the knowledge about the microstructure. Anyhow, in case of crack initiation the cracking of the first grain is dominating the lifetime of the specimens. Furthermore, the experimental data from the fatigue experiments shows that phase boundaries play a major role in fatigue crack propagation. They exhibit the ability to slow down or to stop the crack propagation process. This effect is depending on the configuration of the tilt and twist angles of the slip systems of neighboring grains. A close look on the results show that crack initiation and propagation is a three-dimensional problem.
RkJQdWJsaXNoZXIy MjM0NDE=