Dynamic mechanical analyses and flexural fatigue of PVC foams

Flexural fatigue tests were performed on cross-linked PVC foams of densities in the range from 75 to 300 kg/m(3) at a frequency of 3 Hz and at a stress ratio, R = 0.1. S-N diagrams were generated, and the failure mechanisms were examined. The fatigue behavior was found to be similar to structural materials with a fatigue strength that increased with increased foam density. The final failure event was catastrophic due to crack propagation initiating at the tension side of the specimen. SEM analyses performed on the R300 foam specimen revealed cell wall cracking and densification of the foam during the earlier part of the fatigue life. This densification contributed to stiffening of foam specimens. Viscoelastic parameters of the foams were determined using a dynamic mechanical analyzer over a frequency range of 1-10 Hz. For the virgin specimens it was found that the viscoelastic moduli and damping ratio were quite independent of frequency over this range of frequencies. Viscoelastic parameters were also extracted at intermediate stages of fatigue life and the residual properties were studied with respect to the number of fatigue cycles. It was observed that the variation of dynamic mechanical analysis (DMA) parameters with the number of fatigue cycles was consistent with the variation of residual global stiffness of the material. Experimental details and the analyses of fatigue behavior are presented in this paper. (C) 2002 Elsevier Science Ltd. All rights reserved.