Residual stress pinning of delamination fronts on polymer-metal interfaces

The geometry of delamination fronts between a glassy polymer film and a metal substrate in an asymmetric double cantilever beam geometry is studied. Curved crack fronts are observed when tensile residual stresses are present in the polymer layer. Inward bending of the polymer due to the tensile residual stress locally leads to compressive residual stresses normal to the polymer-metal interface and counteracts the imposed crack opening. This modulates the local toughness at the crack front leading to crack front curvature. It is shown that for subcritical cracks under these conditions the critical energy release rate increases with "age" of the crack tip. It also appears that this ageing depends on the local stress state, and that locations with a higher initial toughness age faster. "Pinning points" with increased energy release rates are found to be associated with crack propagation through the polymer layer and also with local stick-slip movement. It is concluded that on these interfaces that are otherwise homogeneous, a distribution of stresses leads to differing local ageing characteristics and delamination mechanisms. The observations indicate that some macroscopic stick-slip phenomena observed in delamination may be due to inhomogeneous front ageing and movement. (C) 2008 Elsevier B.V. All rights reserved.