The effects of cure temperature and time on the bulk fracture properties of a structural adhesive
The purpose of this investigation is to determine experimentally the possibility of optimizing the room temperature bulk fracture properties of structural adhesives with respect to cure temperature, time and cool-down conditions. The model adhesive, Metlbond, is a solid film modified nitrile epoxy resin supplied in two forms: Metlbond 1113 (supported with synthetic fabric carrier cloth) and Metlbond 1113–2 (without carrier cloth). The effects of carrier cloth on the bulk fracture properties are investigated as well. The uniaxial tensile strength and rigidity values were determined over a wide range of cure temperatures and times with fast and slow cool-down conditions during a previous investigation by the authors. For the present investigation, the fracture toughness of the model adhesives, subjected to opening mode failure, are experimentally determined, with the use of single-edge-cracked specimens, for different cure and cool-down conditions. It is found that the optimum fracture toughness values are obtained at low temperature-long time cure conditions in the absence of carrier cloth when slow cool-down condition is employed. Using the elastic-plastic material behavior assumption, it is shown that an average crack tip plastic zone radius can be determined using the fracture toughness and tensile strength values obtained corresponding to a given cool-down condition. These average plastic zone radii values are used along with the available tensile rigidity values to evaluate the optimum fracture energies of the model adhesives for a number of cure schedules. It is found that the optimum fracture energy levels are obtained at high temperature-short time cure conditions, using slow cool-down in the absence of carrier cloth.