Rate and temperature effects on crack blunting mechanisms in pure and modified epoxies

The failure mechanisms of several epoxy polymers (including pure, rubber- and particulatemodified, as well as rubber/particulate hybrid epoxies) were investigated over a wide range of strain rates (10^-6 to 10² sec^-1) and temperatures (-80 to 60° C). A substantial variation in fracture toughness, G_Ic, with rate was observed at both very high and very low strain rates. Under impact testing conditions, G_Ic for both pure and rubber-modified epoxies displayed peaks at about 23 and -80° C which appeared to correlate with the corresponding size of the crack tip plastic zone. In order to explain these rate and temperature-dependent G_Ic results, two separate crack blunting mechanisms were proposed: thermal blunting due to crack tip adiabatic heating and plastic blunting associated with shear yield/flow processes. Thermal blunting was found to occur in the pure- and rubber-modified epoxies under all impact testing conditions and temperatures above 0° C. For temperatures below -20° C under impact conditions, the fracture toughness is dependent on viscoelastic loss processes and not thermal blunting. Plastic blunting was predominant at very slow strain rates less than 10^-2 sec^-1 for the pure- and rubber-modified epoxies and at impact strain rates for the fibre and hybrid epoxies. Microstructural studies of fracture surfaces provided some essential support for the two proposed crack blunting mechanisms.