# How Soft Polymers Cope with Cracks and Notches
## 2 Experimental Tests on Macroscopically Flawed Thin Plates
### Plates Containing Edge Crack-Like Flaws  
The last set of experiments that we review is related to plates containing edge crack-like defects (Figure 2b), where we consider the effects of the relative crack size \(a/W\) and of the applied strain rate. A first group of edge-cracked plates (specimens EC1-EC4 in Table 3) has been prepared with the commercial silicone RTV 615 (Elantas Europe Srl). From tensile tests on sound specimens, the initial Young modulus of the material is equal to \(E=1.50\) MPa and the Poisson ratio is equal to \(\
u=0.42\). The plates are subjected to tensile loading along the \(y\)-axis up to failure, applied with three different  
Figure 5: Images of the precracked specimens at the initial stage (left column), and with maps of the Green–Lagrange strain \(E_{yy}\) at an intermediate stage (central column) and final stage at incipient failure (right column). The remote stretch is shown on the top of each plot. (**a–c**) Specimens type CC1 (\(a/W=0.179\)); (**d–f**) specimens type CC4 (\(a/W=0.446\)). In (b,e), the length scale is expressed in cm. Figure 6: Green–Lagrange strain \(E_{yy}\) distribution along the cracked section of the plate, at an intermediate (continuous lines) and ultimate values (dashed lines) of the remote applied stretch. (**a**) Specimens type CC1 (\(a/W=0.179\)) and (**b**) specimens type CC4 (\(a/W=0.446\)). strain rates: \(\dot{\epsilon}_{0}{}^{(1)}=1.9\cdot 10^{-3}\) s\({}^{-1}\), \(\dot{\epsilon}_{0}{}^{(2)}=0.48\cdot 10^{-3}\) s\({}^{-1}\), and \(\dot{\epsilon}_{0}{}^{(3)}=0.16\cdot 10^{-3}\) s\({}^{-1}\). The response during the experimental tests is monitored by means of the DIC technique. A second group of edge-cracked plates (EC5-EC8 in Table 3) has been prepared using a different silicone rubber (Elite Double 32 by Zhermack Dental), with an estimated Young modulus equal to \(E=1.36\) MPa and the Poisson ratio is equal to \(v=0.42\). The plates are subjected to tensile loading along the \(y\)-axis up to failure; such a loading is applied with a strain rate \(\dot{\epsilon}_{0}=3.8-4.2\cdot 10^{-3}\) s\({}^{-1}\). Four series of specimens have been prepared for each group, with different values of the relative crack length \(a/W\), while the plate aspect ratio is kept constant at \(L/W\approx 1.5\). The results obtained from the DIC analyses in the specimens EC2-EC4 are illustrated in Figure 7, specifically, the Green-Lagrange strain \(E_{yy}\) (parallel to the loading direction) at incipient failure for the highest strain rate among the three considered. From a qualitative observation, it can be noticed that crack tip blunting is limited (compared, for instance, to the case of an internal crack in Figure 5). The results show that specimens with only one ligament zone are more sensible to defects with respect to the ones containing a central crack (CC1-CC4). The eccentric load effect, which increases \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen ID** & **\(W\) (mm)** & **\(a\) (mm)** & **\(t\) (mm)** & **\(a/W\)** & \(\lambda_{U}\) \\ \hline EC1 & 26.3 & 1 & 2.3–3.5 & 0.038 & 1.26 \\ \hline EC2 & 26 & 2 & 2.3–3.2 & 0.077 & 1.18 \\ \hline EC3 & 26 & 5 & 2.3–2.9 & 0.192 & 1.11 \\ \hline EC4 & 26 & 8 & 2.5–2.9 & 0.308 & 1.07 \\ \hline EC5 & 25 & 1.8 & 4.2 & 0.072 & 1.42 \\ \hline EC6 & 25.5 & 3 & 4.2 & 0.118 & 1.36 \\ \hline EC7 & 26 & 4 & 4.3 & 0.154 & 1.28 \\ \hline EC8 & 25 & 5 & 4.6 & 0.202 & 1.26 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Geometric characteristics of the edge-cracked plates. The rightmost column contains the ultimate remote stretch before failure. In specimens EC1–EC4, the fastest rate \(\dot{\epsilon}_{0}{}^{(1)}=1.9\cdot 10^{-3}\) s\({}^{-1}\) is considered. Figure 7: Map of the Green–Lagrange strain \(E_{yy}\) at incipient failure, at the applied rate \(\dot{\epsilon}_{0}{}^{(1)}\). The remote stretch is shown on the top of each plot. (**a**) Specimens type EC2 (\(a/W=0.077\)), (**b**) type EC3 (\(a/W=0.192\)) and (**c**) type EC4 (\(a/W=0.308\)). In (a), the length scale is expressed in cm.