Polyethylene Tensile Strength Test Specimen

These tensile test specimens are made up of thin strips of white polyethylene, which are wider at each end and pinch to a narrow, straight neck in the middle. The dimensions of the test specimens are standardised, to allow for machine compatibility and easy comparison between other materials. Tensile tests like this are destructive; the material is pulled apart until it breaks. These test specimens are, clearly, yet to be tested. 

 

The test specimens are designed to be used in a tensile test to measure how materials behave in tension. In the experiment, the wider sections of the specimen are gripped on each side by a machine, which slowly pulls the material apart lengthways. The instrument measures the force required to pull the material apart by a certain distance. With these data, a stress-strain curve may be plotted, and characteristics like the material’s ultimate tensile strength, Young’s Modulus, Poisson’s ratio and yield strength can be determined. These materials properties are vital for materials scientists, designers and engineers to choose the best material for a particular purpose or predict how materials will perform in use, and can be crucial for safety and performance.

 

Plastic materials like polyethylene tend to be quite elastic, so will stretch when put under tension, making the sample longer but the cross-section narrower. These samples would probably stretch many times their own length before breaking. Eventually, one section will become much thinner – called necking – and it’s here that the sample usually breaks. Fibres might appear at the fracture surface of polyethylene, a result of the polymer molecules aligning during necking. Different plastics will perform differently in these tests; a more brittle plastic like Perspex would show much less elongation and necking. PVC performs somewhere between the two.