Dynamic delamination of patterned thin films

We present here a novel experimental/numerical protocol to extract the fracture toughness of the thin film/substrate interface. The testing method involves using a laser-induced acoustic stress wave to load the film/substrate fracture plane in tension at very high strain rate (~10^7/s) leading to the inertia-driven interface delamination. A weak adhesive layer is selectively introduced at the interface to serve as a pre-crack exploiting the inertial effect to obtain stable crack growth. The kinetic energy imparted to the weakly bonded region of the film is converted into fracture energy as the thin film delaminates in a controlled fashion. To support the dynamic experiment in extracting the interface fracture toughness values, we develop a numerical scheme based on the combination of spectral representation of the elastodynamic solutions for the substrate and finite element model for the thin film. Cohesive elements are introduced along the bi-material interface to capture the decohesion process. The important role of the inertia on the crack extension and the mixed-mode failure are demonstrated by observing the traction stress evolutions at various points along the bond line. To speed up the simulation process we develop a numerical scheme based on the combination of a nonlinear beam model to capture the elastodynamic response of the thin film and a cohesive failure model to simulate the interface. Numerical results are then validated with experimental measurements of the interface crack evolution history using resistance gage technique. The fracture toughness values measured from the dynamic tests are finally validated with results obtained by using four-point bending technique