Toward understanding the stress oscillation phenomenon in polymers due to tensile impact loading

Though stress oscillation phenomena can be triggered by static and dynamic tensile loading in amorphous (co)polyesters, they have nothing in common. Static loading-caused stress oscillation appears to be controlled by strain-induced crystallization and related heat release and thus can be observed only in crystallizable polymers. On the other hand, dynamic loading-induced stress oscillation is a more universal phenomenon as related to the natural frequency of the test set-up. The latter was established by determining the natural frequency of the instrumented anvil via fast Fourier transformation using a crystallized poly(ethylene terephthalate) (cPET) specimen failing brittlely. When the instrumented tensile impact response of amorphous PET (aPET) and commercial copolyesters (Eastar® PCTG and A150, respectively) has been filtered by considering the natural frequency of the test configuration, the related fractograms no longer showed stress oscillation. It was concluded that samples undergoing adiabatic-type multiple shear banding and prone to extensive cold-drawing are the best candidates to demonstrate the onset of dynamic stress oscillation. The latter is favored by low entanglement density of copolyesters.