Atomic force microscopy observations of the fine structure of cracks have been carried out for nickel thin films on polyimide substrates under tensile test. For strains of the order of 10%, the cracking of the metallic thin film induces notable damaging of the polymeric substrate. Post-mortem observations of the substrate and finite element simulations of the stress field at the cracks head have been performed to characterize this unexpected damaging: the crack propagation at the film/substrate interface and inside the substrate.