STUDY OF SURFACE INSTABILITY ABOUT HYPERELASTIC FILMS ON AUXETIC SUBSTRATES UNDER COMPRESSION

When a bilayer structure consisting of a thin stiff film and a thick compliant substrate subjected to compressive deformation, its free surface would be wrinkled to minimize the energy of the system, and different wrinkle patterns may appear for different ratios of the modulus of the film to that of the substrate. In this article, we developed a novel approach to suppress the surface instability of such bilayer materials under severe compression by adjusting the Poisson's ratio of the substrates. This approach is also applicable to the bilayer consisting of a soft substrate and a film with elastic modulus similar to that of the substrate. We developed an analytical approach for surface instability of the bilayer based on NeoHookean model in the case of small deformation, and obtained the critical strain of the bilayer with a semi-analytical method. Then, we used finite element approach (FEA) to illustrate that the instability of the thin film can be delayed if the substrate has a negative Poisson's ratio. We showed that:(1) when the Poisson's ratio of the substrate is positive and close to 0.5 (nearly incompressible), the surface instability may occur to the bilayer system at a very small compressive strain; (2) if the Poisson's ratio of the substrate is negative and close to -1, the film can be compressed up to 46% without occurence surface instability. The approach developed and the results obtained in this article imply a great potential of auxetic materials used to enhance the compressibility of thin films, which can provide guidance for the design of laminate ductile electronic devices.