PET, also called poly (ethylene terephthalate) or polyester, is one of the most used thermoplastics. In 2020, 510 million tones was produced. That is the same as 5.1e10 kg, or 3.7e7 m3. That sounds like a large amount, but just how much is it? If it was used to create a single cube of PET then the cube would have a side length of 333 m, or a cube of 15 cm for each person on earth. Another way to say it is that the largest of the great pyramids in Egypt has a volume of 2.6e6 m3, to there is more than one pyramid of that size that is produced every month! In this article I will discuss how to predict the response of a PET bottle under load.
60% of all PET is used in synthetic fibers, and 30% is used for bottles. PET is certainly an important material!
Properties of PET
PET is stiff thermoplastic material with can be either amorphous (and transparent) or semi-crystalline. The stress-strain response of PET is often characterized by strain-rate dependent yield stress and stress softening after yielding. These behaviors can easily be modeled using the already calibrated PolyUMod-PET material model that is part of the PolyUMod Material Database. The ID of this model is 2002, and the predicted stress-strain response at different strain rates in monotonic tension is shown in the figure to the right.
Inflating a PET Bottle
To demonstrate the usefulness of the PolyUMod-PET material model I decided to simulate the deformation response of a PET soda bottle exposed to gradually increasing internal pressure. For this demonstration I used Ansys. The analysis is very easy to setup. Here are the 3 easy steps.
1. Read in the CAD file of the bottle into Ansys Workbench
2. Specify the PolyUMod Library PET Material Model
The material parameters can be exported from MCalibration, or just typed in directly in Mechanical.
3. Apply the Loads and Boundary Conditions
In this example, I’m holding the top of the bottle fixed, and apply a uniform pressure on the entire inside surface of the bottle. The interior pressure is ramped up from 0 to 0.8 MPa in 10 seconds. The temperature is room temperature.
The following 2 images show contours of the total deformations and the Mises stress. The max Mises stress is about 59 MPa, which is a very large stress value for PET. The figures also show that there are significant stress concentrations and deformations at the bottom of the bottle. That is interesting, and not what I would have guessed.