Computational Methods for Debonding in Composites

216 Ernest T.Y. Ng and A. Suleman
bound of Zhao and Weng’s results. Note that in any event, our results have shown
to be stiffer when compared to Ju and Lee and Zhao and Weng’s numerical models
for the same reasons as described previously. However, as shown in Fig. 10.2, our
results show good agreement when compared to Llorca’s experimental result.
When compared to Denda, Weng and Zheng’s secant model, our results (von-
Mises, Gurson-0.02 and Gurson-0.08) have shown that we have a lower overall yield
stress on the composite. Also, the overall stress-strain curve predicted by our models
are stiffer than Denda, Weng and Zheng’s secant model.
In the case of Ju, Liu and Sun’s model, we compared our results using prolate
spheroidal shape fibers with ζ = 3.0 and spherical shape fibers. In general, our
results lie between their upper bound (perfectly bonded case) and lower bounded
(porous case). Again, our results show a stiffer overall stress-strain curve. Note that
the only difference between Ju, Liu and Sun’s model and Ju and Lee’s model is the
interfacial damage model and they both employed the Weibull statistics governing
equation for the evolutionary damage process. Consequently, it is expected that they
have similar results. On the other hand, our results have shown a good agreement
between our models and Papzian and Alder’s experimental results.
In summary, we have the following observations from the model validation and
verification.
1. The TFA-GPM model tends to give stiffer overall stress-strain curves. The
possible reason is the approximation of the continuous plastic strain fields by
piecewise uniform plastic strain fields and the Mori-Tanaka scheme.
2. The TFA-GPM in combination with Gurson yield criterion with initial porosity of
2% reduces the overall stiffness of the elastoplastic curves in the uniaxial tension
case. This clearly confirms that porosity and the evolutionary damage due to
porosity are significant factors that affect the overall elastoplastic behavior of the
composite.
3. Initial yielding of the overall composite materials is sandwiched between the
perfectly bonded case and the porous case in the case of Ju and Lee, Denda,
Weng and Zheng and Ju, Liu and Sun regardless of the yield criteria used.
10.5.3 4-phase Composite Material
In this section, we will consider a more complex model in order to show the full
capabilities of the proposed model to simulate a 4-phase Carbon/Glass Fibrous
composite. For simplicity, we assume the following holds true for this analysis:
1. The total fiber volume fraction is assumed to be 40%.
2. The graphite fibers occupy 60% of the total fiber volume fraction.
The initial distribution of the percentage of different phases given are shown in
Table 10.2.