Photoelastic Stress Analysis of Polyethylene Terephthalate

Abstract

Photoelasticity is an experimental stress/strain-analysis method. This technique is based on the photoelastic effect which is a change in the optical indicatrix with mechanical stress. A polariscope is an instrument used for photoelastic stress analysis. In the research performed to write this paper a plane polariscope was used to analyse injection moulded PET samples. The main objective of this work is to investigate the internal stress in PET caused due to thermal contraction (shrinkage) during processing. A new method of data analysis is used where pictures are captured from the samples and then the data from the image is extracted to create a pixel count-intensity graph. Different loads were applied to the samples and the difference in the results between internal and induced stresses were compared. An investigation to verify if thermal healing methods can relieve the concentrations of stress within the material was also performed. The general process was made by inserting the sample into the polariscope, illuminating it with a white light source, capturing an image, saving the image in JPEG file format, converting the JPEG file into text format using a software, and finally analysing the numbers extracted from the digital images. The text illustrates that by extracting data from pictures, it is possible to analyse the stresses by observing the average light intensity in the pixels. The load and thermal healing analysis showed that the change in intensity is indeed depending on the magnitude of stress. Thermal healing also showed to be effective when relieving internal stress. It was also possible to observe the location where the stress is acting.