  Dry adhesives rely on short-ranged intermolecular bonds, hence requiring a
low elastic modulus to conform to the surface roughness of the adhered
material. Under external loads, however, soft adhesives accumulate strain
energy, which release drives the propagation of interfacial flaws prompting
detachment. The ideal adhesive is then soft but rigid. The solution to this
controversial requirement is a bi-material adhesive having a soft tip, for
surface conformation, and a rigid backing, for reduced strain energy release,
hence, better adhesive strength. This design strategy is widely observed in
nature across multiple species. However, the detachment mechanisms of these
adhesives are poorly understood and quantitative analysis of their adhesive
strength is still missing. Based on linear elastic fracture mechanics, we
analyze the strength of axisymmetric bi-material adhesives. We observed two
main detachment mechanisms, namely (i) center crack propagation and (ii) edge
crack propagation. If the soft tip is sufficiently thin, mechanism (i)
dominates and provides stable crack propagation, thereby toughening the
interface. We ultimately provide the maximum theoretical strength of these
adhesives obtaining closed form estimates for the detachment stress independent
of the crack size, rendering the interface flaw tolerant.
