# How Soft Polymers Cope with Cracks and Notches
## 2 Experimental Tests on Macroscopically Flawed Thin Plates
### Effects of Intrinsic Material Defects  
The group of experiments on edge-cracked plates has also involved samples with intrinsic defects, in the form of microvoids (D1-D4 in Table 4). For this purpose, we have prepared samples with the same material and sizes of the group EC1-EC4, but following a different treatment during the preparation of the silicone mixture. To prepare the samples, 50 g of component A (matrix) are thoroughly mixed with 5 g of component B (curing agent). At this point, the mixture for the material without defects is carefully degassed in vacuo and subsequently mechanically spread into the custom-made aluminum mold. This stage is followed by a second degassing, then the mixture is cured in oven at 60 \({}^{\circ}\)C overnight and finally mechanically removed from the mold. In order to obtain microbubbles embedded in the material, the silicone mixture is directly cured in oven without the degassing stages.  
A comparison of the materials, with and without the microvoids, is shown in Figure 8a,b. The average void radius is equal to \(r_{v}=0.35\) mm, and its distribution is described by adopting a normal probability function (variance equal to 0.021 mm\({}^{2}\)). The plot of the probability function (Figure 8c) shows a good agreement with the measured distribution, and \(\sim\)80% of the voids have a radius within half of the smallest edge crack length considered.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen ID** & \(\mathbf{W}\) **(mm)** & \(\mathbf{a}\) **(mm)** & \(\mathbf{t}\) **(mm)** & \(\mathbf{a}\)/\(\mathbf{W}\) & \(\lambda\)\(\mathbf{u}\) \\ \hline D1 & 26.3 & 1 & 2.7–3.3 & 0.038 & 1.23 \\ \hline D2 & 26 & 2 & 2.7–3.1 & 0.077 & 1.15 \\ \hline D3 & 26 & 5 & 2.8–3.0 & 0.192 & 1.12 \\ \hline D4 & 26 & 8 & 2.8–3.5 & 0.308 & 1.09 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Geometric characteristics of the edge-cracked plates with intrinsic defects. The rightmost column contains the ultimate remote stretch before failure.  
Figure 8: Images of the elastomeric material employed in specimens EC1–EC4 and D1–D4. Detail of the material (**a**) without and (**b**) with intrinsic defects. (**c**) Distribution of the void size cumulated probability.  
Figure 9 illustrates the failure behavior of edge-cracked specimens EC1-EC4 and D1-D4, in terms of the ultimate stretch vs the relative crack size. As was expected, smaller ultimate stretches are attained for increasing values of the relative crack size _a/W_. The decreasing trend, with a roughly quadratic pattern, confirms that the material is sensitive to the presence of the initial flaws.  
The effect of the strain rate is investigated in Figure 9. The highest strain rate provides the highest ultimate stretch, although the effect tends to reduce at lower ultimate stretches, suggesting that the influence of flaw size on the ultimate stretch decreases if the deformation is applied sufficiently slow. Figure 9 compares the failure in specimens with and without intrinsic material defects. It can be inferred that microdefects tend to anticipate failure, although the results are not so clear, probably due to the uneven distribution of the microbubbles in the samples.