Critical geometric boundary for the design of wrinkling-free thin film samples in the elastic regime of a uniaxial tensile test

Wrinkling has been observed frequently in the mechanical testing of thin films or membrane structures, which may cause adverse effects on the accuracy of stress and strain measurements. To overcome this problem, a dogbone geometry design is proposed and investigated to eliminate the occurrence of wrinkling when the thin film sample is stretched in a uniaxial tensile test. A numerical experiment is carried out using finite element code to parametrically study the influence of the geometry on the wrinkling phenomenon. Based on numerical experiments, it is found that even though the material property is linear, the geometric nonlinearity occurring in the pre-buckling stage makes the critical buckling load obtained from post-buckling analysis more consistent and reliable than that predicted by eigenvalue buckling analysis. Non-dimensional geometric parameters are used to select the geometry that can prevent the wrinkling in the elastic regime of the uniaxial tensile test. A critical geometric boundary (CGB) is proposed to evaluate the parametric range of the dogbone geometry to prevent the wrinkling. It is found that the CGBs can be achieved by two modes, which are 'wrinkling delay' for the relatively thicker geometries and 'wrinkling elimination' for the relatively thinner geometries. Finally, a function of CGB is determined for the design of wrinkling-free thin film samples in uniaxial tensile tests without reducing the accuracy of stress and strain measurements in a uniaxial tensile test.