Fatigue failure mechanisms of short glass-fiber reinforced nylon 66 based on nonlinear dynamic viscoelastic measurement

The fatigue behavior of short glass-fiber reinforced nylon 66 under stress controlled fatigue tests was studied on the basis of the nonlinear dynamic viscoelasticity measurements. In order to analyze the effect of nonlinear v iscoelasticity on the fatigue behavior, quantitative measurements of nonlin ear viscoelasticity have been carried out based on Fourier analysis. It was found that the nonlinear viscoelastic behavior that was closely related to the irreversible structural change appeared markedly during fatigue proces s. The failure models in fatigue process were proposed based on the cross-s ection morphology under optical microscopic observation before final failur e of the specimens. The fatigue behavior could be classified into the two f ailure mechanisms, depending on whether the fatigue test was carried out be low or above glass transition temperature of the matrix nylon 66. The fatig ue process proceeded with the following steps:(l) the damage started with v oid formation at fiber ends; (2) the microcracks propagated around the fibe r ends (T less than or equal to T-g) or the microcracks propagated bring ac companied with debonding along the fiber sides and also, forming the crack walls(T > T-g); (3) the cracks propagated between the fiber ends(T greater than or equal to T-g) in a brittle manner, or the crack walls dominantly re mained being connected by bridges(T > T-g) in a ductile manner; (4) the fas t crack propagation occurred, after the crack reached to a critical size, a nd finally, the specimen failed. (C) 2001 Elsevier Science Ltd. All rights reserved.