is assumed that stress is distributed homogeneously throughout the entire system, even after failure of sacrificial bonds. Hence, stress can never concentrate within the MS model. In the RSN model stress concentration is possible and therefore it can be used to study the nucleation and propagation of failure in double networks. It is fascinating that this fundamental difference between the MS model and the RSN model does not result in big differences in macroscopic failure or the fraction of broken sacrificial bonds.

Interestingly, a second look at the type of sacrificial bonds that break does reveal a striking difference between the MS model and the RSN model caused by the concentration of stress. By construction, in the MS model sacrificial bonds break in the order of their strength, from weak to strong, until a certain threshold $\lambda_{S}^{*}$ is reached and macroscopic failure occurs (see Fig. \[fig:StressConc\](a)). At this threshold, $\lambda_{S}^{*}$, the force required to break a (matrix-reinforced) sacrificial bond is equal to the force required to break a matrix bond. However, in the RSN model we see that sacrificial bonds stronger than $\lambda_{S}^{*}$ also break (see Fig. \[fig:StressConc\](b) and (c)). In particular, in Fig. \[fig:StressConc\](d) we plot the fraction of 