Predicting cohesive failure in thermosets

Modeling of stresses in epoxies used as adhesives, coatings, or encapsulants in electronic packaging can guide an engineer to more robust designs and material selections. However, stresses by themselves allow evaluation of qualitative trends only. Quantitative assessment of design margins requires some knowledge of when stresses become excessive and failure is imminent. In this study, stresses were predicted accurately in a wide variety of tests, and the state of stress and strain was examined at the point of experimental failure to extract a single scalar metric that design engineers could use to correlate with the observed initiation of cracking. A value of the maximum principal strain of roughly 40% satisfactorily matched data encompassing different geometries, modes of deformation, and test temperature and is apparently linked to a physical mechanism of failure arising from “run‐away” nonlinear viscoelasticity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011