# How Soft Polymers Cope with Cracks and Notches
## 2 Experimental Tests on Macroscopically Flawed Thin Plates
### Plates Containing Elliptical Flaws  
The first set of experiments is carried out on elastomeric sheets under tension, containing elliptical flaws of length \(2a\), characterized by a finite notch root radius (Figure 2a). The plates are made of Sylgard(r), a common silicone polymer having the following elastic parameters: initial Young modulus \(E=0.84\) MPa and Poisson ratio \(\
u=0.37\). Three types of specimens have been prepared, containing a centered elliptical flaw with different values of the root radius \(\rho\) and same length \(2a\). The plate aspect ratio is kept constant at \(L/W\approx 2\). The geometric characteristics of the specimens are reported in Table 1. The flawed plates are subjected to tensile loading along the \(y\)-axis, applied at a constant strain rate of \(\dot{\epsilon}_{0}=\dot{\delta}/(2L)=4.8\cdot 10^{-3}\ s^{-1}\). The tests have been interrupted before failure, since an evident notch remodeling was noticed. The response during the experimental tests is monitored by means of the DIC technique.  
Figure 3 shows the initial (undeformed) and the generic stretched shapes of the specimen El1 at two increasing levels of the remote applied stretch. The corresponding strain patterns obtained from the DIC analysis are illustrated in Figure 4, specifically, the Green-Lagrange strain \(E_{yy}\) parallel to the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Specimen ID** & \(W\) **(mm)** & \(a\) **(mm)** & \(b\) **(mm)** & \(t\) **(mm)** & \(a\)/W **(-)** & \(\rho\) **(mm)** \\ \hline El1 & 58.5 & 20 & 5 & 2.0 & 0.342 & 1.250 \\ \hline El2 & 58.5 & 20 & 2.5 & 2.0 & 0.342 & 0.3125 \\ \hline El3 & 58.5 & 20 & 1 & 2.0 & 0.342 & 0.0050 \\ \hline \end{tabular}
\end{table}
Table 1: Geometric characteristics of the thin plates with elliptical notches. The rightmost column contains the values of the initial notch root radius \(\rho=b^{2}/a\).  
Figure 2: **(a)** Sketch of a thin plate containing a centered internal flaw. The enlarged view shows the case of an elliptical notch, with semi-axes \(a\) and \(b\). \(\rho\) is the tip radius of the notch (\(\rho=0\) in the case of a crack-like flaw). **(b)** Sketch of an edge-cracked plate. The plates are subjected to a remote displacement \(\delta\) applied along the \(y\)-axis.  
loading direction and the strain \(E_{xx}\) transversal to such a direction. Three different levels of the applied remote stretch are considered, defined as  
\[\lambda_{0}=1+\frac{\delta}{2L} \tag{1}\]  
where \(\delta\) is the applied displacement.  
The DIC plots show that the material is able to comply with very high deformations, leading to a severe defect remodeling characterized by an evident blunting of the notch. A compressed region just in front and behind the elliptical hole is observed, due to the contraction effect arising in the direction normal to the applied load (see also Figure 3c). This phenomenon has a beneficial effect in terms of the strain concentration, with a sort of augmented notch blunting due to the local flexural instability of the thin plates in the compressed zones [29].  
Figure 4: Strain maps obtained from the Digital Image Correlation (DIC) analyses on the elliptically notched specimen E11, for three increasing values of the applied remote stretch \(\lambda_{0}\). (**a–c**) Green–Lagrange strain \(E_{yy}\); (**d–f**) corresponding maps for the strains \(E_{xx}\). In (a), the length scale is expressed in cm.  
Figure 3: Qualitative view of the (**a**) undeformed and (**b,c**) increasingly deformed configurations, in the flawed specimen E11.