Wrinkles and Cracks -- Quantifying the Mechanical Properties of Polymer Thin Films*

To further engineer materials that are mechanically strong and fracture-resistant, direct knowledge of the key mechanical properties such as stiffness, strength, and ductility is a necessity. Previous works have demonstrated the capabilities of a combined wrinkling-cracking test to measure the elastic modulus of ultrathin glassy polymer films. It is shown that the modulus observed often deviate significantly from the bulk, and may undergo subcritical cracking from continuous loading conditions. In this study, the elastic and fracture properties of polycarbonate thin films is measured as a function of film thickness and strain rate. Additionally, using the crack density and channel crack growth, in combination with our numerical model, the strain energy release rate 𝒢 can be calculated to benchmark the failure criterion of the material in terms of a crack driving force that is independent of sample history and geometry.