ON CRACK INTERACTION AND CRACK DENSITY IN STRAIN-INDUCED CRACKING OF BRITTLE FILMS ON DUCTILE SUBSTRATES

Uniaxial straining of a brittle film on a ductile substrate induces the formation of an array of parallel cracks whose density increases with increasing strain. An analytical function is derived for the energy release rate of a crack propagating in the vicinity of already existing cracks. Using this function, a statistical theory is developed for relating the crack density to the population of flaws in the film. In order to assess these theoretical results, a test is proposed which allows the measurement of critical strains for crack propagation in the case of strong crack interaction. This test is applied on a model system consisting of a thin soda-lime glass slide adhesively bonded on an aluminum alloy. The viscoelastic nature of the adhesive determines the critical cracking strain of non-interacting cracks. The cracking strain of interacting cracks varies as a function of crack spacing in agreement with the proposed analytical function. A flaw density distribution function is calculated from the measured dependence of crack density on strain.