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MATERIALS SCIENTISTS ARE CONCERNED WITH QUANTITATIVE MECHANISMSof toughening and hope to predict the life expectancy of toughened thermosets. This endeavor should enable chemists to develop new polymers or tailor existing polymers to meet new requirements.

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 hut 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 ofconditions studied here. The fracture behavior at various temperatures and loading rates could be characterized to a first approximation by a master cyrve of fracture energy us. 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.

The fracture behavior of carboxyl-terminated butadiene-acrylonitrile elastomer-modified epoxy polymers in bulk, as adhesives, and as matrix polymers in composites is described. The dramatic increase in fracture energy over that of unmodified epoxies is discussed in terms of the elastomer being dispersed as small (1-5-μm) particles that increase the crack-front deformation zone size. Differences in fracture energies when these materials are used as adhesives are explained in terms of constraints on the deformation zone. The frac ture energy of the modified epoxies is shown to exhibit a time-tem perature dependence far greater than the unmodified epoxies, in bulk and as adhesives. In composites these toughened epoxies increase delamination resistance under quasi-static loading. However, the de tailed mechanisms are not fully known, especially the effect of the rate dependence of these polymers on impact resistance. THE FRACTURE BEHAVIOR OF EPOXY-BASED POLYMERS in bulk, as adhe sives, and as matrix resins in fiber-reinforeed composites has considerableimportance because these materials are used for structural components in aircraft, automobiles, ships, and housing. In many instances the component is load-bearing so that its strength, stiffness, and toughness are critical to the function and reliability of the structure.

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