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

Section 8
15-RT Specimen decrease slightly with increasing distance, r. A slight decrease is also
observed for particles lying at a large angles, θ > 50 °. The particle volume fraction has no
simple effect on the σ p max-values.
For the different aging conditions and particle volume fractions, the computed average values
of the maximum principal stresses in the particles for fractured particles and at the interface
for debonded particles show a roughly linear dependency on the composite yield strength.
This can be explained, if it is assumed that the void initiation occurs at the onset of gross
plastic yielding in the matrix at the location of the first particles in front of the crack tip. A
rise of the σ max interf dec-values might be also related with increasing volume fraction of the
spinel phase with increasing aging time at the particle/matrix interface, leading to enhanced
interfacial bonding.
It is shown that the distributions of strengths of ceramic reinforcements in the cast and PM
MMCs is not in contradiction to the Weibull distribution. The values of the Weibull modulus,
m, determined from the values of the particles strength, are in good accordance with values
given in literature.
A good correlation is found between the local Ji-values evaluated from the average of the
local CODi-values and the global Ji-values determined from the fracture mechanics tests for
all investigated materials, excepting the PM MMCs. This is explained when it is assumed that
the fracture of one particle might trigger the fracture of the neighboring particles within a
certain region so that the area of local crack extension is larger than the resolution of the
potential drop technique. The larger particle size (100 µm, compared to 10 µm for the cast
MMC) facilitates this effect.
The proposed procedure is applied to the MnS-inclusions in the mild steel St37 to study the
effect of the inclusion size on the local fracture properties and local conditions for void
initiation, as well. It is shown that the CODvi-values strongly decrease with increasing
diameter of MnS-inclusions. A clear reduction of the σ interf max-values with increasing diameter
of the MnS-inclusions is found, as well.
It is shown that equal channel angular pressing (ECAP) leads to an increase of the uniformity
of the particle distribution in PM MMCs with small particle sizes and clustered particle
distribution. The model proposed by Tan and Zhang [23] about the estimation of the critical
value of the particle size which is re quired for a homogeneous particle distribution in the
matrix is extended to the case of a combination of extrusion and ECAP. A good accordance
between the modified Tan and Zhang model and the quantitative analysis of the particle
distribution by the quadrat method is found.
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