Cyclic viscoelastoplasticity and fatigue fracture of polymer composites

Observations are reported on isotactic polypropylene/nanoclay hybrids with various concentrations of filler ranging from 0 to 5 wt.% in cyclic tensile tests with a stress-controlled program (ratcheting between minimum stresses σ ^min and maximum stresses σ ^max). A pronounced effect of filler is demonstrated: reinforcement of polypropylene with 1 wt.% of nanoclay results in reduction of maximum and minimum strains per cycle by several times and growth of number of cycles to failure by an order of magnitude. To rationalize these findings, a constitutive model is developed in cyclic viscoelastoplasticity of polymer nanocomposites. Adjustable parameters in the stress-strain relations are found by fitting experimental data in relaxation tests and cyclic tests. It is demonstrated that the model correctly predicts growth of maximum and minimum strains per cycle with number of cycles and can be applied for evaluation of fatigue failure of nanocomposites.