Microcracks form by a mixture of local thinning and intergranular fracture in a 170-nm-thick Cu film that is well bonded to a polyimide substrate and is stretched to a strain of 30%. Details can be found in this paper. A related forum topic can be found

In recent development of deformable electronics, it has been noticed that thin metal films often rupture at small tensile strains. Here we report experiments with Cu films deposited on polymeric substrates, and show that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded Cu films can sustain strains up to 10% without appreciable cracks, and up to 30% with discontinuous microcracks. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture. The films rupture at large strains when the localization is retarded by the adherent substrates.