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.

In structural (or load bearing) applications of adhe sives, the system may be required to support loads over a wide range of temperatures for many years and thus even a small de pendence on temperature and rate (or time) can be very impor tant. For elastomer-toughened adhesives the high fracture energy is associated with a large crack-tip deformation zone and the optimum conditions are obtained when the bond thickness is approximately equal to the zone size. Increasing the loading rate or lowering the temperature shifts the optimum bond thickness to smaller values with little effect on the maximum fracture ener gy. Consequently, the fracture energy increases, decreases, or re mains relatively unchanged with increasing loading rate or de creasing temperature if the bond thickness is less than, greater than, or about equal to the optimum values, respectively. This result makes it possible to predict at least in a qualitative way the mode I failure properties of adhesive bonds as a function of load ing rate and temperature.

No abstract

No abstract

No abstract