CHANGES IN MECHANICAL-BEHAVIOR DURING FATIGUE OF SEMICRYSTALLINE THERMOPLASTICS

The tensile fatigue behavior of two engineering thermoplastics (polyac etal and nylons(6,6)) were studied by measuring changes in the dynamic viscoelastic response together with changes in potential energy densi ty, strain energy density, and irreversible work. The results show tha t both stress softening and hardening can occur in controlled load cyc lic conditions. At high stress levels and/or frequencies, both the pol yacetal and nylons(6,6) show evidence of thermal softening as characte rized by changes in their dynamic viscoelastic properties and decrease in storage modulus with corresponding increases in loss modulus and l oss tangent. This effect is supported by observed decreases in the ove rall crystallinity as measured in DSC experiments. At lower stress lev els (the mechanically dominated region), all results indicate that, al though fatigue crack propagation (FCP) is one of the mechanisms govern ing the fatigue life, its contribution is minor and crack initiation t ime constitutes the majority of the fatigue life. Also, during the ini tiation stage, both materials become less viscoelastic and more elasti c. This phenomenon is evidenced by overall reductions in the loss modu lus, loss tangent, and irreversible work densities while the storage m odulus is maintained. (C) 1995 John Wiley & Sons, Inc.