Bonded layers of dissimilar materials form the basis for a number of engineering applications, such as thin films and coatings, layered microelectronic devices, and a variety of solid freeform fabrication or layered manufacturing processes. In many such applications, differential expansion stresses can give rise to initiation of debonding at free-edges and subsequent interface crack extension. As such, methods are required for designing "debond-resistant" bimaterial systems. To this end, design guidelines have recently been developed based on both initiation of debonding at free-edges and subsequent steady-state interface crack extension2-3. However, a gray area exits between the two approaches, in which a short interface crack exists in the vicinity of the free-edge. Depending on the bimaterial configuration and crack length, such cracks may or may not be susceptible to subsequent delamination.
In this study, the susceptibility to debonding of short interface edge-cracks is investigated for the general global problem configuration of a bimaterial strip with a uniform edge load applied to the top layer (a general model of differential expansion). The goal of this study is to determine the critical crack length L below which interface crack extension is inhibited, or more specifically, for which the mode I component of the interface stress intensity factor is negative. This is equivalent to a maximum allowable flaw size, which is of substantial interest to both designers and inspectors of bimaterial systems. The critical crack length L has been extracted from parametric finite element analyses of the global problem configuration over a wide range of bimaterial combinations using the commercial software package ABAQUS. The numerical results indicate that L increases with both the relative stiffness and thickness of the top layer, and may represent a significant inspectable flaw size for a variety of practical bimaterial configurations.
Keywords: interface crack, free-edges, bimaterial layers, finite element analysis
1 This work has been supported by the Wright State University Research Council. The authors also wish to thank
Jack Beuth and Paul McKeown for their thoughts and insights related to this research.
2 Klingbeil, N.W. and Beuth, J.L., "On the Design of Debond-Resistant Bimaterials, Part I: Free-Edge Singularity
Approach," Engineering Fracture Mechanics, Vol. 66, 93-110.
3 Klingbeil, N.W. and Beuth, J.L., "On the Design of Debond-Resistant Bimaterials, Part II: A Comparison of Free-Edge
and Interface Crack Approaches," Engineering Fracture Mechanics, Vol. 66, 111-128.