Delamination of sandwich composed of three structured materials is considered. A crack is located near or within the layer. Under tension applied at infinity, the mode I fracture is implemented. For analytical examination of fracture process, the modified Leonov-Panasyuk-Dugdale model is applied together with the Neuber-Novozhilov approach. Analytical description of plotting the fracture diagram of quasi-brittle sandwich for the plane stress state is given.