The Effect of Cross-Link Density on the Growth of Shear Bands in Amine-Cured Epoxies

Massive shear banding is known to be an important mechanism in the toughening of epoxy resins modified with elastomeric particles. Therefore, we propose that a fundamental understanding of shear band propagation will provide more insight into the “toughenability” of cross-linked epoxy resins than the use of simple macroscopic properties such as yield stress or elongation to failure. This investigation studies the effect of cross-link density on the growth rate of shear bands in neat amine-cured epoxy polymers. The model epoxy networks consist of epoxy resins based on the diglycidyl ether of bisphenol A (DGEBA) cured with a stoichiometric amount of 4,4′-diamino diphenyl sulfone (DDS). Cross-link density is controlled by using DGEBA monomers with increasing the molecular weights. Shear bands studies are conducted using notched, rectangular specimens loaded in compression. Constant load and constant load ramp rates are examined and the resulting shear band velocities measured. Results indicate that shear band velocities cannot be normalized to shear yield strengths and that fracture toughness correlates better with shear band growth rates than with yield strengths.