Biomedical Engineering Reference
In-Depth Information
5.14 Crack propagation as a function of displacement for
microstructure (a) M4 and (b) M6.
owing to the presence of Coulombic interactions, the presence of nanosized
small cracks does not result in appreciable stress concentration. That leads
to the stress profiles for a system with and without crack being almost the
same. A secondary peak can also be observed in all stress-strain curves in
Fig. 5.12(a). This secondary peak may correspond to an internal
reorganization in all the microstructures. However, atomistic visualization
did not reveal any such changes in the structural features. This is also
attributable to the complex hexagonal structure of Si
3
N
4
, which is extremely
difficult to analyze visually. Overall, an important finding from the plots in
Fig. 5.12(a) is that the presence of interfaces results in an increase in the
overall toughness of the material.
In Fig. 5.12(b), stress-strain plots for M3, M4, M5, and M6 are shown. A
direct comparison with the plots in Fig. 5.12(a) reveals that the presence of
second-phase SiC particles results in significant weakening of M1 and M2.
The effect of the initial crack is also found to be significant. This is apparent
by looking at the differences between the curves with initial crack and the
ones without initial cracks. It has been pointed out [21] that high strength
and relatively small sized SiC particles act as stress concentration sites in
Si
3
N
4
matrix, leading to intergranular Si
3
N
4
matrix cracking as a dominant
failure mode. However, as a result of a significant number of nanosized SiC
particles being present in the microsized Si
3
N
4
matrix, the SiC particles
invariantly fall in wake regions of microcracks, which results in significant
structural strength. The observations here confirm the first point - that the
presence of SiC particles will significantly weaken M1 and M2 against
deformation and fracture. A comparison of deformation without initial
crack and with initial crack for microstructures M3 and M4 reveals that, in
