AbstractThe mechanical response of a polyamide‐6 montmorillonite clay nanocomposite and of a polyamide‐6 was monitored during axial fatigue tests performed at R‐ratios of 0.1 and −1. For both materials, two transitions were usually observed in the evolution of all the stress‐strain‐time parameters studied after similar numbers of loading cycles, suggesting interrelationships between the mechanisms of molecular reorganization. Fatigue test monitoring indicated an initial decrease in the storage modulus and a subsequent trend for this modulus to increase, especially in polyamide‐6. During all tests, a partially recoverable strain was accumulated because of viscoelastic deformation. Nanoparticles reduced this strain in the initial cyclic straining regime but not in the last regime, probably because such particles cannot inhibit viscoelastic events constrained in a volume larger than their interaction volume within the matrix. Based on the accumulated volume variation measured, the nucleation and growth of microvoids can be expected to occur in the last cyclic straining regime. POLYM. COMPOS., 26:636–646, 2005. © 2005 Society of Plastics Engineers