Investigation of buckling transition from straight-sided to telephone-cord wrinkles in Al films

Aluminum films 150 nm thick have been deposited on polymethylmethacrylate (PMMA) substrates by a magnetron sputtering process. Parallel straight-sided wrinkles are induced by axial compression. The evolution from straight-sided to 'telephone-cord' wrinkles is experimentally investigated using optical microscopy. Finite element modeling of instability is introduced for analyzing the secondary bifurcation or buckling of straight-sided wrinkles. Imperfections based on a random combination of buckling eigenmodes are introduced in the finite element method (FEM) analysis to take into account secondary bifurcation. Because the perturbation of the imperfection is totally random and small, it is a more realistic approach for taking into account the roughness and flatness of the film. Moreover, the perturbation method can be applied more widely in some complex patterns of telephone-cord wrinkles. Good agreement is found between experimental observation and FEM calculations.