It has long been desirable to conduct Stress Relaxation measurements over
shorter test times. The problem has been to achieve a short well-characterised loading
phase and to obtain the first stress relaxation reading soon after the completion of
loading. Although it is possible to programme a standard computer-controlled
mechanical tester to acquire readings down to very short time periods, uncertainty of
achieving the correct deformation at very high loading rates restricts its use. The
Hounsfield Test Equipment company in collaboration with the Loughborough
University has produced a compression tester intended to overcome the problem.
This new machine is capable of giving compressive strain rates from 0.1
nun/min upto 250 mm/min (SLOW MODE) and one extremely high compressive strain
rate of 2 m/sec (120,000 mm/min - FAST MODE)
The machine was employed initially to produce sets of stress relaxation curves
to investigate possible machine errors. The main error found was incorrect
compression produced by the tester; it varied from 20% to 27% instead of the desired
25%. After this and some smaller errors were identified, several precautionary actions
were taken to compensate for them.
The compensation techniques permitted progress to a study of three different
Natural Rubber compounds at three different temperatures using several different
compressive strains in both slow and fast modes.
A clear difference in the residual forces between fast mode test curves and slow
mode test curves has been identified and the possible reasons for differences are
It has been shown that the relaxation curves in slow mode can be represented
by two linear portions representing two different relaxation processes. On the each
relaxation process the effect of the rate of compressive strain and the temperature of
the material is discussed.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.