dissertation global and local fracture properties of metal matrix ...

Section 4
Table 4.6. The values of the plastic zone size at angles θ =0°, 70° and the distance between
particles and the crack tip at COD = 0.2 µm
Material ω70 [µm] ω0 [µm] r[µm]
cast Al2O3-10-RT 40 9.1 10…50
PM SiC-10-24h 12 2.7 50…300
maximum extension of the plastic zone at near-zero values is not comparable with the
distance to the nearest SiC particle. So no near-zero CODvi-values can be observed for this
material.
4.4. The maximum particle stresses at the moment of void initiation
The stress tensors for both the particles and the matrix were calculated for investigated MMCs
according to the procedure described in Section 4. In Table 4.7, the results of calculation for
Particle 4 of Specimen Al2O3-10-RT (Fig. 9.1, Tab. 9.1) are presented as example. Figures
4.23-4.25, the maximum particle stresses at the moment of the void initiation are plotted
against the angle to the crack plane, θ, and the distance to the crack tip, r, for the cast and PM
MMCs. In comparison to the very large scatter of the CODi- and CODvi- values (Figs. 4.15-
4.20), the scatter of the σ p max-values is much lower. The stresses vary between 800 and 1400
MPa for the alumina particles in the cast MMCs and between 700 and 1100 MPa for SiC
particles in the PM MMCs. The σ p max-values for alumina particles are nearly independent of
the angle to the crack plane, θ, (Fig. 4.23). A slight decrease may be found for particles which
lie at angles larger than θ ≈ 55°. A slight decrease of the σ p max-values with increasing r is
observed for the cast MMCs in Figure 4.24.
For the PM MMCs, the stress tensors were calculated only for specimens SiC-10%-15 min.
and SiC-10%-200h. In other specimens, the particle fracture occurs in the elastic region,
where the mean-field approach fails and cannot be used to determine the stress states in
constituent phases.
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