Cyclic fatigue of polymer nanocomposites

This paper presents an experimental study on cyclic fatigue of two polymer nanocomposites in two common failure modes: mechanical failure in epoxy nanocomposites and thermal softening in polyamide (PA, nylon) 6 nanocomposites. For epoxy nanocomposites, the effects of hard (silica) and soft (rubber) nano-particles on un-notched samples under constant cyclic stress amplitude fatigue were studied. Hard particles were shown to increase but soft particles decrease the fatigue life of nanocomposites compared to unmodified epoxy. At the same stress amplitude, the extent of fatigue crack growth prior to fast fracture was largest in rubber nanocomposites and least in pure epoxy, reflecting the differences in their fracture toughness values. Ternary nanocomposites with both hard and soft (silica and rubber) particles were also investigated and their fatigue performances were compared to the binary nanocomposites. Further, the stress (σ_a) versus life (N_r) test data of pure epoxy and its binary and ternary nanocomposites are well described by Basquin's law. PA6 nanocomposites exhibited fatigue failure due to thermal softening when the maximum local temperature of the specimens subjected to cyclic loading reached the glass transition temperature, T_g, of the material. Critical stress (σ_a) versus frequency (ω) envelopes for design against thermal failure were obtained for PA6/organoclay, PA6/POE-g-MA and PA6/pristine clay. Experimental results compared favorably with theoretical predictions.