A simple model for elastic fracture in thin films

A simple model for the cracking of thin films has been developed and investigated. The film is represented by a network of bonds and nodes which initially form a triangular lattice in which each node is at the junction of six bonds. The nodes are also attached to a rigid substrate by bonds which are not broken but have a relatively small force constant. Cracking is simulated by a sequence of thermally activated bond breaking events followed by mechanical relaxation. If the stretching force constant associated with the bonds in the surface layer is large, linear cracks propagate rapidly and, at a later stage, are connected by secondary cracks which become less and less linear as the strain in the surface layer is relaxed by the cracking process. Many of our simulations exhibit a distint “initiation” period in which the bond breaking rate is low, followed by a period of rapid crack propagation in which a large fraction of the surface strain energy is released. This period is then followed by a second period of relatively slow bond breaking. During the first period isolated defects are formed, followed by linear cracks in the second period. During the third period non-linear cracks connect the linear cracks, and structures which resemble shear bands are formed. Results for the effects of finite relaxation rates for the elastic network (visco-elastic effects) are also presented.