Tensile testing of polyethylene
In this experiment samples of polyethylene are subjected to a tensile stress. They are stretched at a constant rate until they fail. The sample is dumbbell-shaped so that it can be firmly held by each set of grips.
The test is carried out using a tensometer. The photograph below is of the Hounsfield H20K-W apparatus, which was used to perform the experiment.
First the sample is secured in place. The strain rate for the test is programmed into the machine, after which the test may begin. After the sample has failed and the test is complete the data is transferred to a computer, where it can be treated and presented as a stress-strain curve.
The test is performed at different strain rates in order to demonstrate the time-dependence of the sample's behaviour.
You can view videoclips of a typical test run displaying necking, drawing and fracture. The video of the whole test has been speeded up by a factor of 48 - in reality these thick polyethylene samples took around 30 minutes to break at a strain rate of 10 mm/min. By dragging the video position marker to and fro it become easier to see certain features such as the decrease in the amount of the sample of original width as necking proceeds. Note the position where fracture occurs.
The figure below shows applied force plotted against extension for two polyethylene samples that underwent a tensile test until failure. The blue line is for a sample stretched at 10 mm/min, the pink is for 20 mm/min.
The blue line shows necking and an extensive period of cold drawing, which are characteristic features of the stress-strain behaviour of a semi-crystalline polymer above its glass transition temperature (Tg). For polyethylene, Tg = 148 K and the tests were carried out at room temperature (approx. 293 K). The pink line has a very different form, without the long flat section due to cold drawing seen in the blue line. At the higher strain rate (pink line) almost no cold drawing was able to take place before the sample snapped. This is a good demonstration of the time-dependent nature of the properties of polymers i.e. the observed behaviour is dependent on the strain rate.
The results presented here show the difference in behaviour of polyethylene as a result of a different applied strain rate. All other variables were the same for both tests. No cold drawing is observed at the higher strain rate as it is not slow enough for the polymer chains within the sample to disentangle and reorganise themselves parallel to the direction of applied stress; at the lower strain rate there is enough time for cold drawing to occur.
These experiments have highlighted the effect of changing the strain rate on the response of polyethylene to an applied tensile stress. A series of similar experiments could also be performed in order to compare the behaviour of a number of different polymers.