Local crack extension criteria in highly constrained polymer layers

The resistance to fracture of thin ductile polymer layers used to bond stiff elastic substrates together is an important design parameter in a variety of applications involving thin ceramic, glass and metal substrates. This work in particular addresses the local elasto-plastic fracture processes of pressure-sensitive polymers used as adhesive layers in silicon wafer bonded systems. Stress and deformation fields under mode I conditions are obtained using a new pressure-sensitive rate-independent constitutive material model for polyimide. Detailed computational analyses of a typical mode I delamination test specimen are performed to determine the highly constrained local deformation of the thin bonding layer. An investigation of the competing failure mechanisms revealed that a likely failure mode will consist of interfacial debonding at the site which develops the maximum traction. The low levels of hydrostatic pressure within the polymer suggest that no triaxiality-induced cavitation is likely to occur in pressure sensitive adhesives obeying associated plastic flow. A relation is proposed which gives the fracture strength of the bond (or critical energy release rate for interface failure) in terms of the thickness of the bonding layer.