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

Section 4
4.5. The relation between the local conditions for void initiation and global mechanical
properties
The average values of the maximum principal stresses in the particles are calculated for all
specimens and inserted in Tables 4.8 and 4.9. For calculating the mean values, the near-zero
CODvi-particles have been excluded. In Figure 4.29, the average σ max p fr - and σ max interf dec-
values are plotted against the yield stress of the composites. One can see that these values are
far from being equal in all specimens. The average of values of the maximum particle stresses
and maximum stresses at the interface increase with increasing yield stress of the composites.
The large differences in the average σ p max-values of about 400 MPa cannot be explained by
methodogical uncertainties and computational inaccuracies of the analyzing procedure. This
leads to the question: “What might be a reason for such a behavior?”
A possible explanation could be that the onset of plastic deformation has a decisive influence
on the particle fracture. Assume that void initiation would occur at the onset of gross plastic
yielding in the matrix (not at the onset of local plastic yielding near some corners or edges of
the particles). This would require that, considered at the meso-scale, the equivalent stress of
the composite around the considered particle, σ c eq , reaches the composite yield stress, σy. For
σ max [MPa]
1600
1200
800
400
0
0 100 200 300 400
σ y [MPa]
64
Al2O3 particles
Al2O3 interface
SiC particles
Fig. 4.29. The dependence of the average σ max interf dec- and σ max p fr-values on the composite
yield strength, σy, for all investigated MMCs.