A single-network elastomer fails at the crack tip. Stress concentrates at the point of the crack, bonds break there, and the crack advances. The damage localizes and the material tears.
Double-network elastomers contain a sacrificial network embedded in an elastic matrix. Under stress, the sacrificial network breaks — but not at the crack tip. The bond-breaking is delocalized: it happens ahead of and uncorrelated with the crack front, scattered across broad regions of the material. The early, widespread micro-failures dissipate energy before catastrophic failure can nucleate.
The through-claim: controlled, distributed failure is a form of structural integrity. The material that breaks everywhere a little doesn't break anywhere completely. Toughness at the macroscale emerges from fragility at the microscale. The sacrificial network's job is to break — that's its function, not its failure.
In single-network materials, damage is information about where the crack is going. In double-network materials, damage is energy dissipation distributed away from where the crack is going. The same physical process (bond rupture) serves opposite functions depending on whether it's correlated with the crack front or decorrelated from it.
The design principle is counterintuitive: to make a material that resists catastrophic failure, build in components that fail easily. The ease of sacrificial failure is precisely what prevents the kind of failure that matters. If the sacrificial bonds were stronger, they wouldn't break early and widely enough to protect the structural network. Their weakness is their contribution.