THERMODYNAMICS, THERMAL EFFECTS AND DILATATION OF NATURAL RUBBER
POLYTECHNIC INST OF BROOKLYN NY
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Of the three different experiments carried out on natural rubber, the force versus temperature measurement have verified that the decay of stress in stretched elastomers is approximately linear with logarithm of time for a considerable period. A viscoelastic mechanism for this decay according to a spectrum of Maxwell terms, can be ruled out as the basis for this form of relaxation because of the nature of the time dependence. As a further difficulty small local defects leading to crack propagation by stress concentration and crystallization of the sample complicate this relatively simple but unexplained form of relaxation. Thus, there is a natural difficulty in the way of determining when the criterion of equilibrium has been adequately fulfilled. In our experiments, the sample was considered fully relaxed when the increment of relaxation over the total span of testing was less than one percent. A new dilatometer was constructed which allows to follow volume changes with stretching. For the first time complications due to swelling could be overcome. Extremely small volume increases were determined during the early exploratory runs, but the dilatometer has to be reconstructed to eliminate metal corrosion.
- Elastomers and Rubber