Cracking modes in layered hyperelastic structures

Stretchable devices composed of layered hyperelastic materials are under rapid development to interact with human body, and merge the gap between human and machines. These devices are required to sustain large deformation without mechanical failure over a long time, which remains a challenge. Compared to the fast movement in functions design, the failure mechanism of layered hyperelastic structures has been much less explored. Here we study the cracking modes in layered hyperelastic structures made of relatively brittle films bonded to tougher substrates. An initial surface crack in the film is considered. It is found that the surface crack always tends to penetrate through the thickness of the film at first. With the increase of stretch, two cracking modes – crack channeling and interface debonding – are observed one after the other. The sequence is determined by the ratio of interfacial debonding energy to fracture energy of the film, and a dimensionless cracking number which depends on the film-to-substrate modulus ratio, thickness ratio and dimensionless fracture energy of the film. We establish the critical conditions for different cracking modes and build the phase diagram. This work provides a pathway to design cracking-resistant stretchable devices.