Single-sample decohesion test: mechanics and implementation

Abstract

The Single-Sample Decohesion Test (SSDT), developed by the authors, eliminates shortcomings of current interfacial fracture toughness testing methods. In this approach, a highly stressed super layer is used to drive delamination and create any mode mix at the crack tip. SSDT uses the change in crack surface area to vary the available energy per unit area for crack growth and thus to bound the steady state interfacial fracture toughness. Therefore, this technique uses a single sample to measure the interfacial fracture toughness and maintains the advantages of the original decohesion test: (1) SSY conditions, (2) micro-electronics and MEMS representative interfaces, and (3) ability to generate any mode mix. Critical to the usefulness of this test is the availability of a mechanics based analytical model to calculate the test site energy release rate. The test sites are cantilevered thin film strips, which undergo large, nonlinear deflection, consist of multilayered materials, are subject to in-plane biaxial loading, have varying in-plane dimensions, and can be loaded from pure shear to pure peel. In this paper, one dimensional and linear and nonlinear classical laminate plate theories are compared to a finite element representation of the SSDT test site. The regions of applicability of linear and nonlinear analytical models are discussed and nonlinear effects such as the importance of capturing the bifurcation of curvature are highlighted. Application of the nonlinear plate theory model to the test is shown and interfacial fracture toughness results are provided for a Ti/Si interface at several mode mixes.