Stress relaxation testing with a novel machine using rapid application of load

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 discussed. 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.