Short Term Stress — Strain Properties

Abstract

The term 'stress-strain' is a convenient one to cover tests that determine the relationship between an appUed force and the resulting deformation, regardless of whether the quantities measured are expressed as stress (force per unit cross sectional area) and strain (deformation as a fraction of initial dimension) or given directly as force and deformation. The so-called 'short term' measurements of stress and strain grouped together in this chapter refer to the various stress/strain or force/deformation measurements where the effects of long times and cycling are ignored. The term 'static stress/strain' tests is also applied to this group to distinguish them from the dynamic or cyclic tests. Neither term is really accurate since deformation must involve movement and a dynamic test might be even shorter, but the meaning is generally understood as referring to tests in which the time period of the deformation is much greater than that required for the molecular network to respond to an applied stress, but short enough to make creep unimportant.Such tests were amongst the first to be studied and standardised and have been of enormous value, particularly in quality control. It must be realized, however, that they are limited as regards complete mechanical characterization, simply because mechanical properties of rubbers are very dependent on time, temperature and test conditions. This can lead to contrasting views of, on the one hand, something of a fanatical belief in, for example, the value of hardness measurements to characterize a compound, and, on the other, dismissal of all simple mechanical tests as virtually useless for understanding material behaviour. The truth is, as one might suspect, somewhere in between. The best value is obtained from the simpler tests if care is taken in choosing the most relevant tests, carrying them out under the most relevant conditions and not expecting the result to be necessarily valid when applied to some different conditions.