Older, ordinary structural steel sections are often inhomogeneous in their microstructure, having a low-toughness "core" material and a higher toughness surface material. The overall behaviour of such structural components has been found to be more ductile than predicted by "weakest-link" arguments. This behaviour is analysed by extending the theory of the globally elastic fracture of layered sandwich materials to accommodate non-linear behaviour and mixed toughness. The extended theory has been applied to the behaviour of full scale beams typical for bridges.