Viscoelastic Fracture of Structural Adhesives

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Abstract
Experiments conducted on several different elastomer-modified epoxy systems indicated that the high fracture energy of most structural adhesives is achieved through crack-tip deformation processes that are viscoelastic. It is essential therefore that the fracture behavior of such materials be determined as a function of temperature and loading history. The linear viscoelastic properties of the model systems were functions of formulation and thermal history but when these parameters were controlled the behavior was thermo-rheologically simple over a wide range of conditions. The fracture behavior was also dependent on formulation and thermal history although the effects of history were quite small in the range of conditions studied here. The fracture behavior at various temperatures and loading rates could be characterized to a first approximation by a master curve of fracture energy vs. reduced time-to-failure. This characterization makes it possible to compare the properties of different formulations and to predict their fracture behavior over a wide range of conditions.