Focused ion beam four-slot milling for Poisson's ratio and residual stress evaluation at the micron scale

A novel method is presented for the assessment of the Poisson's ratio and residual stress on the micron scale, based on focused ion beam (FIB) two-step four-slot micro-milling and in-situ digital image correlation (DIC) analysis of the induced relaxation strains at the specimen's surface. The methodology has been fully validated through modelling and experiments on three different materials, namely, physical vapour deposition (PVD) chromium nitride (CrN), and as-deposited and annealed Cu thin films. The cases of non-equibiaxial stress state and non-isotropic elastic behaviour are discussed in detail, and a quantitative evaluation of the accuracy, error sensitivity and the effects of microstructure and anisotropy is presented. This method represents a substantial improvement over the existing state-of-the-art. It is suitable for application on amorphous and nanostructured materials, and provides a breakthrough in micro-scale Poisson's ratio analysis of thin films and small structures and components.