13.4: Polymer Stress-Strain Curve
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Stress-strain curves show the response of a material to an applied (usually tensile) stress. They allow important information such as a material's elastic modulus and yield stress to be determined. Accurate knowledge of these parameters is paramount in engineering design.
Polymer stress-strain curves are produced by stretching a sample at a constant rate through the application of a tensile force. By using a constant rate of testing the strain-rate dependency of polymer behaviour is not allowed to dominate. However it should be appreciated that polymers have a marked inherent time-dependence in their response to deformation, which sets their behaviour apart from other classes of material.
The following interactive object shows an idealised polymer stress-strain curve and allows you to explore the different stages in the curve as the strain is increased.
Considère's construction is a useful way of considering the phenomenon of necking and cold drawing.
As well as having significant time dependence, polymer behaviour is also temperature dependent. For example the plot below shows schematically different types of polymer stress-strain behaviour, which occur for different types of polymers and at different relative temperatures. The blue curve is for a glassy or semi-crystalline polymer below its glass transition temperature, the red curve is for a semi-crystalline polymer above its glass transition temperature and the green curve is for a rubber.
The exact form of the stress-strain curve depends on the polymer under investigation. Indeed not all polymers exhibit necking and cold drawing. Polycarbonate is an example of a brittle polymer.