The testing is carried out to internal procedures that conform to BS EN ISO 899-1. The tests are normally performed on tensile dumbbells which can be machined or moulded. There are two ways of representing the creep performance of polymer materials:
- Creep strain
- Creep rupture
Creep strain involves the application of a constant tensile load on the test specimen - while continuously measuring the resulting strain. From this it is possible to calculate the creep modulus.
The stress is applied using a lever system with dead weights and the strain is measured using precision extensometers which allow the strain to be recorded to computer. The tests can be carried out in air over a range of temperatures up to 80°C.
Creep rupture is a measure of failure time as a function of applied stress. During this test the strain is not recorded. A range of applied stresses are used to generate failure times typically from 100s to 106s from this a plot of stress versus log failure time can be generated. A minimum of 20 data points are required to produce a creep rupture plot. Plots can be generated over a range of temperatures up to 80°C.
If plots of fatigue versus log failure time are produced over at least three different temperatures, then it is possible to predict longer failure times at the lower temperature by time-temperature superposition.