Real time development of stress and birefringence in PET during uniaxial stretching as detected by spectral birefringence technique

Mukerrem Cakmak

Abstract

The birefringence and stress development of PET films has been measured by our new stretching system that couples the Spectral Birefringence Technique with the stress strain measurements. The tensile stretching machine which is specifically designed such that both clamps of the samples move away from each other. This allows for keeping the midsection of the sample stationary. The birefringence and width of the sample is measured at this location. The width of the sample is measured using laser micrometer through which true strain is obtained real time. It has been found that the birefringence development starts before strain-hardening point. The system is fast enough to follow the deformation at high rates of strains. The increased strain rates results in higher birefringence levels through all stages of deformation (orig.)