The tensile behaviour of common engineering polymers

Abstract

A polymer is a macromolecule made of long chains of carbon atoms. This structure gives to the polymers special properties such as rubber elasticity. This investigation focuses on the rubber elasticity of the polymers. Different common polymers are tested on a machine that stretches the polymer at different speeds. The testshows that some polymers stretch whereas others break instantly due to the tensile force. The structure of the polymer determines whether it will stretch or break. Speed is also an important variable: the slower the polymer is stretched the longer it will be able to be stretched.

Introduction

Polymers are commonly used in nowadays industry. As a result, it is very important to know whichpolymer fits best for every use. Each polymer has its specific properties and different tensile behaviours at different temperature and therefore a specific range of application. The goal of this experiment is to examine the tensile properties of some common polymers, to understand how the structure of the polymer influences the tensile properties. The testing speed will also be investigatedto determine how it affects the tensile properties.

Materials and methods

The test used in this experiment is the standard ISO 527 test. A polymer test specimen is placed between grips as describe in Figure 1. Once the specimen is firmly held in place the machine will start pulling up the specimen stretching it as much as possible until it breaks. Then the machine records the yieldstress, the elongation of the specimen and the stress at break. The specimen used for this test is the ISO multipurpose test specimen (ISO 3167), it is 150 mm long, with the centre section 10 mm wide by 4 mm thick by 80 mm long. The machine is connected to a computer which prints all the data as well as a graph of yield force versus extension for each tested polymer.

[pic]

Figure 1

ResultsAll the data acquired by the machine and printed by the computer are given from page 6 to 12.

Tensile test of different polymer at a speed of 100mm/min

|Polymers |Yield Stress (MPa) |Failure stress (MPa) |% Elongation at failure |Results Characterise |
|Polycarbonate |61.7 |0.569 |9.45|Tough, Strong, Hard |
|Nylon |56.91 |0.6055 |24.76 |Tough, Strong, Hard |
|Polystyrene |43.25 |0.577 |5.21 |Brittle, Strong, Hard |
|High density polyethylene |24.90|0.566 |10.46 |Brittle, Weak, Soft |
|(HDPE) | | | | |
|Low density polyethylene |11.728 |0.6173 |43.69 |Tough, Weak, Soft |
|(LDPE)| | | | |

The yield stress (?) is a function of the elastic modulus (E). The equation is ? = kEn, where k and n are constants. K and n can be found by plotting ln(?) versus ln(E). Indeed ln(?) versus ln(E) gives a linear relationship : ln(?) = n × ln(E) + ln(k).

|Polymers|E (N/m2) |? (N/m2) |ln(E) |ln(?) |
|LDPE |2E+08 |11730000 |19.11383 |16.27766 |
|HDPE |7.8E+08 |24900000 |20.4748 |17.03038 |
|Nylon…