Effect of nano-fibers on the stress-strain behavior of semi-crystalline poly(ethylene terephthalate) at different strain rates

Abstract

Uniaxial compression tests were performed on amorphous poly(ethylene terephthalate) (PET), amorphous poly(ethylene terephthalate)- glycol (PETG), semi-crystalline PET, and semicrystalline PET with various amounts of nano-fibers added. The stress-strain behavior for each material at several strain rates is presented. A typical stress-strain curve consists of a relatively steep rise in stress at small strains where the slope is the Young's Modulus. Then, the stress reaches a maximum before decreasing. This stress is called the yield stress. It is the stress at which plastic deformation begins. As the strain continues to increase, the stress begins to rise gradually because of strain hardening. The results show that the initial addition of 1% nanofibers by weight to semi-crystalline PET increases the yield stress by nearly thirty percent. As the amount of nano-fibers increases, the yield stress remains relatively unchanged. This could be due to aggregation of the nano-fibers at higher weight percent as seen in micrographs. For each material, the yield stress was found to be rate-dependent, scaling with the log of the strain rate at low rates and transitioning to a different rate dependence at high rates. The slope of the yield stress versus In(strain rate) for each material was approximately parallel. This shows that the addition of nano-fibers does not affect rate sensitivity. The semi-crystalline PETs with nanofibers did not show any signs of strain hardening up to the strain magnitude tested and needs to be further explored. The maximum stiffness was found to be in the amorphous PET followed by the semi-crystalline PET with 2% by weight nano-fibers blended in. The semi-crystalline PET alone has a much lower stiffness than amorphous PET, but this stiffness increases as nano-fibers are added up until they reach 3% by weight at which the stiffness decreases.