Napkin ring adhesion tests over a broad range of experimental conditions suggested a de-bonding mechanism for glassy thermosets associated with orun-awayo nonlinear viscoelasticity. Finite element analyses of these tests using a high fidelity, nonlinear constitutive equation were used to identify a single, scalar metric that consistently predicted the initiation of de-bonding, a critical value of the maximum principal strain in the ointerphaseo zone. In principle, such a de-bonding metric enables evaluation of design margins in practical components.